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Briggs & Stratton 375
Year: 2005
Model: 10D902
Type: 0129 B1

Lawnmower is slightly smokey on start up but recently is has started to run on reduced power and when I took the speakplug out I found it to be wet and dirty. I have cleaned the plug which was in good condition.

What would have caused the plug to be oily and could my valve stem seals or pistion rings be the likely cause?

Your answer could be directly related to which version of the 50,000 engines that Briggs made. We really can't help you without a model number from the engine.

I like that VW in your avatar. What does the oil in the mower look like when you drain it for an oil change?

ILENGINE said:

Your answer could be directly related to which version of the 50,000 engines that Briggs made. We really can't help you without a model number from the engine.

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Details of engine added in my description now.

tom3 said:

I like that VW in your avatar. What does the oil in the mower look like when you drain it for an oil change?

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1977 Beetle 1200L that is/was my first car and have owned since early 1980's, having bought it from my father.

Lawnmower oil looks black and smells dirty?

I think I would be checking head gasket, as well as valve clearances. Possibly a loose valve seat. Could also be an issue with valve lift due to a camshaft lobe issue.

ILENGINE said:

I think I would be checking head gasket, as well as valve clearances. Possibly a loose valve seat. Could also be an issue with valve lift due to a camshaft lobe issue.

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I had the head off the engine 3 years ago and replaced the head gasket then. Valves and seats seemed fine at the time.

The loss of power happened too suddenly for it to be camshaft lobe issue.

What should the valve clearances be for my engine?

From the operators manual, free download at Briggs website.
Valve clearance with valve springs installed and piston 1/4 in.(6 mm) past top dead center (check when engine is cold).See Repair Manual P/N 270962.Intake0.005 – 0.007 in. (0.13 – 0.18 mm)
. . Exhaust.007 – .009 in. (0.18 – 0.23 mm

Basic Briggs flathead Kool Bore.. I'd guess the cylinder is shot. New rings might help it but this is getting pretty common these days. The chrome plating on the piston wears and the bare aluminum surfaces rubbing together creates a mess. Hope I'm wrong but you'll have to pull the head and check.

tom3 said:

Basic Briggs flathead Kool Bore.. I'd guess the cylinder is shot. New rings might help it but this is getting pretty common these days. The chrome plating on the piston wears and the bare aluminum surfaces rubbing together creates a mess. Hope I'm wrong but you'll have to pull the head and check.

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I will take the head off and see what I find

Shorter Grass said:

I will take the head off and see what I find

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I took the head off and found a notable amount of oil in the cylinder. There is no wear on the cylinder wall and the piston does not have a hole in it and appears to be in reasonably good condition. I am currenlty draining the oil from the mower and have not inspected the oil yet.

Piston rings and bearings - if I am taking the bottom end of the engine apart what all should I replace?

If you have oil in the cylinder check to make sure there is no gas in the oil. The cover on the side of the engine that covers the valve springs is the crankcase breather. There is a one way valve in it . If this valve sticks closed it can pressurize the crankcase and cause high oil consumption. Change the oil with 30wt run engine till warmed up then turn off and let it sit for a few hours then pull the head. If you have oil in the head pull the carb and check for oil in the intake port. Any oil that might come past the exhaust valve guide will get burned in the muffler.if no oil in the intake port leaves rings and cylinder.

Being a side banger oil can only get into the cylinder 2 ways without major damage
1) past the oil scraper ring
2) down the inlet valve guide

neither will be apparent from the crankcase
After that here is a crack in the cylinder, moving valve guide , hole in piston and via the crankcase breather .
There is also no bearings inside the engine.
All shafts run directly on the castings / forgings
Was the bore really good
Was the cross hatching complete with no bare patches indicating wear ?

bertsmobile1 said:

Being a side banger oil can only get into the cylinder 2 ways without major damage
1) past the oil scraper ring
2) down the inlet valve guide

neither will be apparent from the crankcase
After that here is a crack in the cylinder, moving valve guide , hole in piston and via the crankcase breather .
There is also no bearings inside the engine.
All shafts run directly on the castings / forgings
Was the bore really good
Was the cross hatching complete with no bare patches indicating wear ?

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I will check all of the above.

Where am I looking for the cross hatching?

Where am I looking for the cross hatching?

This is what crosshatch would look like in the cylinder,
However with it being that model kool-bore, i don't think they honed them from the factory, as i replaced a short block on one awhile ago and it wasn't crosshatched (honed),

Scrubcadet10 said:

This is what crosshatch would look like in the cylinder,

However with it being that model kool-bore, i don't think they honed them from the factory, as i replaced a short block on one awhile ago and it wasn't crosshatched (honed),

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The bore of mine has not been honed from the factory so there is no cross hatching?

Yes, not honed from the factory, no crosshatch.

tom3 said:

Basic Briggs flathead Kool Bore.. I'd guess the cylinder is shot. New rings might help it but this is getting pretty common these days. The chrome plating on the piston wears and the bare aluminum surfaces rubbing together creates a mess. Hope I'm wrong but you'll have to pull the head and check.

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Chrome plating??? Really? I've never seen a piston with chrome plating.

slomo

slomo said:

Chrome plating??? Really? I've never seen a piston with chrome plating.

slomo

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The piston in my lawnmower is not chrome plated. The head of the poston is not marked or cracked.

I cannot feel any lip in the cylinder wall. I will remove the piston and check it and the priston rings.

Is it worth fixing if it is the cylinder that is the problem?

If the cylinder has any serious wear and scoring, you'll see it with the piston down in the hole.

The engine has sat overnight with the head off and today a pool of oil is in the bottom of the horizontal cylinder which can only be piston, piston rings or cylinder.

Will it be enough to just replace the piston rings?

I cant imagine oil getting to the cylinder with it just sitting. and the head off ...How much oil are you putting in it?
The engine holds approximately 5/8 qt (20 ozs; 0.6 liter).

Shorter Grass said:

The bore of mine has not been honed from the factory so there is no cross hatching?

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That is impossible. All engines are honed from the factory.
That's how the rings seat into the cylinder bore.
If you have no cross hatch, your engine has wear.
Using oil, using more gas to run, harder to start, fouling spark plugs and oil smoke (which is blue in color not white).

Richie F said:

That is impossible. All engines are honed from the factory.
That's how the rings seat into the cylinder bore.
If you have no cross hatch, your engine has wear.
Using oil, using more gas to run, harder to start, fouling spark plugs and oil smoke (which is blue in color not white).

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Could I re-hone the cylinder and fit slightly oversize piston rings that can be bought?

tom3 said:

If the cylinder has any serious wear and scoring, you'll see it with the piston down in the hole.

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My cylinder does not have any serious wear or scoring?

Scrubcadet10 said:

I cant imagine oil getting to the cylinder with it just sitting. and the head off ...How much oil are you putting in it?
The engine holds approximately 5/8 qt (20 ozs; 0.6 liter).

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The oil was drained from the engine before I took the head off. Oil has seeped past the cylinder as it sat.

Oil seeping past bad rings is not uncommon. You have a couple choices. Get an oversive poston an rings and have a machine shop bore and fit new piston. Will cost more than the mower is worth probably. Or you get a set of standard rings and get a Brush Research ball hone appropriate to your cylinder material and deglaze the cylinder put new rings on original piston and put it together .

Hammermechanicman said:

Oil seeping past bad rings is not uncommon. You have a couple choices. Get an oversive poston an rings and have a machine shop bore and fit new piston. Will cost more than the mower is worth probably. Or you get a set of standard rings and get a Brush Research ball hone appropriate to your cylinder material and deglaze the cylinder put new rings on original piston and put it together .

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I think I will use a honing kit and then fit new piston rings

You will need a hone like this and not a spring hone.

Shorter Grass said:

Could I re-hone the cylinder and fit slightly oversize piston rings that can be bought?

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Hone the cylinder again and get standard piston rings.
A ball hone will do just fine.
You don't need a rigid hone that was shown above.
This isn't a 3 second top fuel engine.

If he is going to just deglaze and install new rings all he needs is a ball hone. If he is going to fit an oversize piston and rings he will need a rigid type hone

Hammermechanicman said:

If he is going to just deglaze and install new rings all he needs is a ball hone. If he is going to fit an oversize piston and rings he will need a rigid type hone

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Man if you going to fit an oversized piston/ring with a rigid hone you better have a machine to do it.
That means the cylinder wall has taper in it and you have to get it out for it to be true again.
Sunnen makes a machine to do it. It's called wet honing.
Again, it's a lawnmower engine. Not NASA.

Hammermechanicman said:

15000 Engine Cylinder Hone

Best by Far - Yet Competitively Priced.

www.lislecorp.com

You will need a hone like this and not a spring hone.

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I will get a hone like this and not the spring type.

Richie F said:

Man if you going to fit an oversized piston/ring with a rigid hone you better have a machine to do it.
That means the cylinder wall has taper in it and you have to get it out for it to be true again.
Sunnen makes a machine to do it. It's called wet honing.
Again, it's a lawnmower engine. Not NASA.

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I take you point - no oversize rings. I will do a deglaze and fit new standard rings

Hammermechanicman said:

If he is going to just deglaze and install new rings all he needs is a ball hone. If he is going to fit an oversize piston and rings he will need a rigid type hone

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I am going to do a deglaze and fit new standard piston rings

Shorter Grass said:

The oil was drained from the engine before I took the head off. Oil has seeped past the cylinder as it sat.

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I may have not followed this correctly. You drained the old oil, installed new oil and then ran the machine? Then it sat overnight? Or did you just install new oil and then let it sit overnight? If the latter, wouldn't that mean too much was put in or the engine was tilted?

Has a leak down test been done? I'm new and not as adept at diagnosing off symptoms alone. Usually, I start with that to try to narrow things down.

lefty2cox said:

I may have not followed this correctly. You drained the old oil, installed new oil and then ran the machine? Then it sat overnight? Or did you just install new oil and then let it sit overnight? If the latter, wouldn't that mean too much was put in or the engine was tilted?

Has a leak down test been done? I'm new and not as adept at diagnosing off symptoms alone. Usually, I start with that to try to narrow things down.

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I drained the oil and did not add new oil. The engine sat with the head off and the cylinder has a puddle of oil in it. Ovbiously it has seeped from the cylinder to the front of the piston.

I have not done a leak down test but can do this though I am fairly convinced that my piston/clinder/rings are the issue.

I have a Wisconsin Robin with stuck/dead rings. It too will pass a small trail of oil in front of the piston into the combustion chamber, head off.

I would throw a new Chinese engine on it and call it good. Home mechanic messing with the aluminum bore is only a band-aid in my opinion. Some of you other guys could probably make it work just fine.

slomo

slomo said:

I have a Wisconsin Robin with stuck/dead rings. It too will pass a small trail of oil in front of the piston into the combustion chamber, head off.

I would throw a new Chinese engine on it and call it good. Home mechanic messing with the aluminum bore is only a band-aid in my opinion. Some of you other guys could probably make it work just fine.

slomo

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I have rebuilt engines before and am an engineer so will get it sorted, in fact I'm even more determined after reading what you have written lol.

Shorter Grass said:

I will get a hone like this and not the spring type.

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That hone is worth more than that engine.
This will do ya, http://www.brushresearch.com/

Scrubcadet10 said:

That hone is worth more than that engine.
This will do ya, http://www.brushresearch.com/

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Other hone was too expensive for the application

The 2 hones are for very different uses. The ball hone is for what is called "glaze breaking" which just reconditions a smooth bore to prepare for new rings. It won't correct a tapered bore. The Lisle rigid hone is for honing a cylinder for an oversize piston. It will correct a tapered bore. What hone is used is dependent on whether just reconditioning the bore for new rings or fitting an oversize piston.

Hammermechanicman said:

The 2 hones are for very different uses. The ball hone is for what is called "glaze breaking" which just reconditions a smooth bore to prepare for new rings. It won't correct a tapered bore. The Lisle rigid hone is for honing a cylinder for an oversize piston. It will correct a tapered bore. What hone is used is dependent on whether just reconditioning the bore for new rings or fitting an oversize piston.

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Deglaze and standard rings being fitted

Shorter Grass said:

Deglaze and standard rings being fitted

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You will want an aluminum oxide 320 or 400 grit hone for your size cylinder

Hammermechanicman said:

You will want an aluminum oxide 320 or 400 grit hone for your size cylinder

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I am surprised as the grit is coarser than I would have imagined?

If it was an sleeved cylinder it would use silicon caebide at 600 grit. Thoae numbers are from the Brush Research catalog

Unless you are using teflon coated or chrome plated rings the bore has to be ROUGH
The action of the rings running over the peaks weas the tops off but leaves to rootsThe roots hold the oil that actually makes the seal between the rings & the bore

bertsmobile1 said:

Unless you are using teflon coated or chrome plated rings the bore has to be ROUGH
The action of the rings running over the peaks weas the tops off but leaves to rootsThe roots hold the oil that actually makes the seal between the rings & the bore

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The cylinder looks remarkably smooth to the eye and touch but I will remove the piston later and inspect the cylinder and piston rings then

Shorter Grass said:

The cylinder looks remarkably smooth to the eye and touch but I will remove the piston later and inspect the cylinder and piston rings then

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Bert is correct. New rings put into a smooth cylinder will never seat in properly and will usually be an oil burner. The use or a hone to "break the glaze" or condition the cylinder for new rings is needed when you install new rings. Very important on aluminum bore engines is to change oil after a couple hours of break in running. Aluminum bores cast off more metal seating rings than an iron bore does.

With the piston out, ball hone.
When ball honing keep the cylinder wet with mineral spirits.
When done wash the cylinder with warm soapy water and rinse then dry.
Then with clean paper towels use automatic trans oil to wipe the cylinder down.
You will see a black film when doing this. That is the residue from the ball hone and cylinder wall.
Good luck.

Richie F said:

With the piston out, ball hone.
When ball honing keep the cylinder wet with mineral spirits.
When done wash the cylinder with warm soapy water and rinse then dry.
Then with clean paper towels use automatic trans oil to wipe the cylinder down.
You will see a black film when doing this. That is the residue from the ball hone and cylinder wall.
Good luck.

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Would this home still be ok as this was the one I had ordered before I saw your message.

Not in the hands of an ameteur .
You have to shove it in and pull it out very quickly without allowing it to go too far in or too far out .
Getting a good hatch without taking too much material off requires quite a bit experience.
Bad enough with iron bores but a disaster with aluminium bores

Shorter Grass said:

Engine Small Brake Cylinder Hone 32-89mm Bore Honing Tool Flexi Shaft S-2ECH3 on OnBuy

SATRA S-2ECH3 is engine cylinder honing tool with adjustable tension. Perfect for honing and de-glazing car brakes and clutch cylinders. Size...

www.onbuy.com

Would this home still be ok as this was the one I had ordered before I saw your message.

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Yes.
Go slow to get the feel of it.
Only let half of the stone come out of the bore each way.

bertsmobile1 said:

Not in the hands of an ameteur .
You have to shove it in and pull it out very quickly without allowing it to go too far in or too far out .
Getting a good hatch without taking too much material off requires quite a bit experience.
Bad enough with iron bores but a disaster with aluminium bores

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If i pull it through once at a steady speed, start to finish in about 10 seconds?

Shorter Grass said:

If i pull it through once at a steady speed, start to finish in about 10 seconds?

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At quick pace to 10 seconds is about right.
Look up a youtube on honing basics of cylinder bore.
It's really not hard at all.

Richie F said:

At quick pace to 10 seconds is about right.
Look up a youtube on honing basics of cylinder bore.
It's really not hard at all.

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Would a spray of WD40 help?

Shorter Grass said:

Would a spray of WD40 help?

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WD-40 is not really a lubricant. And it's not clear to me it would help at all. Really, what you want is a liquid that helps remove the grinding/honing media from the cylinder. "Lubrication" of the honing operation is really secondary.

For that reason, I hone in a utility sink that is filled with enough hot, soapy water to cover half the cylinder. Obviously, safety requires a cordless drill motor, but the variable speed comes in handy. IMHO, a true detergent is the only thing that will remove the honing media, some of which is microscopic. At one time I worked in a shop where finished cylinders were even run through a dish washer. Once you understand that the media of the honing stones is harder than diamond and continues to cut metal if left behind, then getting rid of the media left by the honing process becomes the primary key to long engine life.

Just my 2 cents.

Born2Mow said:

WD-40 is not really a lubricant. And it's not clear to me it would help at all. Really, what you want is a liquid that helps remove the grinding/honing media from the cylinder. "Lubrication" of the honing operation is really secondary.

For that reason, I hone in a utility sink that is filled with enough hot, soapy water to cover half the cylinder. Obviously, safety requires a cordless drill motor, but the variable speed comes in handy. IMHO, a true detergent is the only thing that will remove the honing media, some of which is microscopic. At one time I worked in a shop where finished cylinders were even run through a dish washer. Once you understand that the media of the honing stones is harder than diamond and continues to cut metal if left behind, then getting rid of the media left by the honing process becomes the primary key to long engine life.

Just my 2 cents.

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Thank you and it is probably what I will do also

It occurred to me that you could find a soup can (or other steel can) about the size of your cylinder and use that to practice your honing technique. You'll want to end up with hone marks between 30 and 45° off the head gasket surface. As was said, it's really very easy... once you've done it 20 times !!

Shorter Grass said:

Engine Small Brake Cylinder Hone 32-89mm Bore Honing Tool Flexi Shaft S-2ECH3 on OnBuy

SATRA S-2ECH3 is engine cylinder honing tool with adjustable tension. Perfect for honing and de-glazing car brakes and clutch cylinders. Size...

www.onbuy.com

Would this home still be ok as this was the one I had ordered before I saw your message.

Click to expand...

The biggest problem with that type of hone is that the face of the stones are flat. When you put them in a round cylinder the sbarp edges of the staoes are what makes contact. No matter what grit you get the stone's sharp edges will gouge the aluminum. 30+ years ago you could get stone's with radiused faces that worked on aluminum. Also if not experienced with the spring hone it is very easy to put vertical grooves in the bore inserting and removing it. Another problem with the spring hone stones is that the material will load up an cause scratches in the finish. Ball hone much user friendly. I put the cylinder in a bucket of kerosene. Remember! You only get one shot at it. If you must use a spring hone get 600 grit stones. The cheap Chinese stones are always more coarse on aluminum than what you think and the one you linked to lists 220 grit stones that will destroy an aluminum cylinder. I ruined my share of 5HP Briggs engines learning these things.

Shorter Grass said:

Would a spray of WD40 help?

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No!
Use mineral spirits or kerosine.
The hone you are getting is just fine.
The square corners will round a little as it breaks in.
Stop thinking about it and do the job.
I've been rebuilding cars/ trucks/snowmobiles/engines/trannys/rears for 50 years.
Don't overthink the wheel.

And getting back to your previous post.
The hone goes into the cylinder stationary it is rotated while moving up & down then turned off squeezed in and pulled out stationary.
THis is very important in an alloy bore or as the rotating stone is removed it will chamfer the top of the bore and you really do not want that to happen .
The ball hones were invented and used because of the short comings of flat hones for deglazing
Also getting the pressure just right is another skill that come after you have buggered a few cylinders.
The ball hones have a preset pressure so no chance for errors unless you use the wrong size.

Bert
He's just deglazing a cylinder bore.
Doesn't care about the physics of it.
The more he knows, the more confused he will become.
"NASA, we have a lawnmower engine to fix".

What could go wrong? The linked to spring hone lists 2 inch 220 grit stones of an unknown material. Aluminum oxide? Silicon carbide? User has never deglazed a cylinder before so getting the proper 45 degree crosshatch pattern on the first try shouldn't be hard. With 2 inch stones no worries about popping the stones out of the top or bottom of the bore. It's not the space shuttle go for it. Can't be that hard.

I think enough about talking about this.
Good luck with the repair, one way or another.
Over and out.

just be sure to clean everything inside and out in the engine, and keep cleaning the bore afterwards until a white paper towel comes out white after wiping it