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Briggs 31Q777-0305-E1

Head gasket blew in 1 day after I replaced it. In the usual place where no bolt can tighten it down.

Engine is on a Cub Cadet that I bought used. Guy said the head gasket keeps blowing and he is right.

After I replaced the head gasket I noticed the push rod adjuster was hitting the valve cover as you could hear it and feel it. Turns out someone put a longer Intake push rod in. I don't know if this caused any damage other then the dent on the valve cover.

After replacing the push rod with the correct one the engine ran fine. Then when I engaged the blades I could hear a fast knocking sound for 1 second and then it goes away and was mowing fine. I mowed maybe for 5 minutes then I re-engaged the blades smoke poured out the exhaust. Head gasket blew again.

I'm wondering if the PTO is making the knocking sound and I'm not sure how to stop the head gasket from blowing.


Any information or help is gratefully appreciated.

Repeat head gasket failure would indicate either the head is warped or the block at the head mating surface. I have seen both. Including one where the block had machining marks still in the surface.

The head was replaced with a new one, so I'm guessing its the block. Is there a way I could flatten it out evenly?

It is a right royal PIA but you can file, scrape or linish it flat with the wet & dry glued to a sheet of glass.
All depends upon you compentcy with hand tools.
I have resurrected "unrepairable" inteks by fitting a thick copper head gasket.
That does two things.
Firstly it takes up space on the bad design and secondly it adds 1/16" ( well 3/32 " to be truthful ) to the bore length so drops the CR just a touch,
A good polish on the head & piston top also knocks it down a tad.

If you want to go that way give Lanni a buzz at CGU Home Page
I think he actually keeps Intek gaskets as a stock line if not he will cut you one in any thickness you like and he sends them full soft ready to install.

I have been using him for years great bloke to deal with, very reasonable prices , fast turnaround and always ready to give you some good advice.

Going to try and flatten it down. I did notice some scratches where the gasket keeps blowing, not sure if that is aiding in the failure of the gasket.

Any thoughts on the fast knocking sound when engaging the blades? The mower has a electronic PTO. I held a broom handle against the back of the block and to my ear to see if I could hear or feel the noise but it did not seem like it was coming from the the engine, though it would puff out a little smoke when turning on the blades. Maybe the gasket was failing?

I also have Craftsman LT1000 with a 17.5 Briggs OVH. It has actually been on fire due to leaves clogging the muffler. The fire melted the engine shroud, gas tank, parts of the carb and partially welded a valve cover screw to the engine. The mower still runs great 4 years later.

*Edit*

Actually just going to get a copper gasket. Seems like the more logical choice. I'm not sure what thickness to get .016, .021, .032, .042, .050, .063, .093, .125

https://www.briggsandstratton.com/us/en/support/manuals/results?NTT=31Q777-0305* engine operators manual & parts list for your engine

As noted you could try the "manual mill" method. Get a good 12" + double cut file from Sears . It will cost you ?$8-$10, but should last many years. Ideally you need a mechanics flatness gauge, but they are expensive. Borrow the rule of a 90 degree (right angel) . Slide it out. Go to your table saw or a piece of glass and try to clean up the "flatness" on one side of the steel rule. Use emery paper. Keep checking the flatness against the clean table of the saw until no "light band" is visible and no feeler gauge will fit under. Then take 3 criss cross readings with it of the cylinder top. Dead flat to .001" is the goal. Rotate the piston down and protect is from filings and any other openings.Use a blue marker and alcohol. Keep file dead flat and remove material . Clean file. Smear cyl top with blue. More forward strokes. Clean top with alcohol and rag. Vacuum any filings. Cleanup should only be ? .005-.010"? There is risk if too much material is removed, Piston slap. IMO

darrinster said:

Going to try and flatten it down. I did notice some scratches where the gasket keeps blowing, not sure if that is aiding in the failure of the gasket.

Any thoughts on the fast knocking sound when engaging the blades? The mower has a electronic PTO. I held a broom handle against the back of the block and to my ear to see if I could hear or feel the noise but it did not seem like it was coming from the the engine, though it would puff out a little smoke when turning on the blades. Maybe the gasket was failing?

I also have Craftsman LT1000 with a 17.5 Briggs OVH. It has actually been on fire due to leaves clogging the muffler. The fire melted the engine shroud, gas tank, parts of the carb and partially welded a valve cover screw to the engine. The mower still runs great 4 years later.

*Edit*

Actually just going to get a copper gasket. Seems like the more logical choice. I'm not sure what thickness to get .016, .021, .032, .042, .050, .063, .093, .125

Click to expand...

As for gasket size, go somewhere between double & triple the std Briggs gasket thickness.
The Intek head gasket failure seems a random occurrence , some engines are plagued by it others never give a minutes troubles.
One of the local shops here does so many they made a jig to hold the head in the mill so I drop any I get over there for a quick clean up.
Before finding them I would do some engines every season.

While the head is off check the rod by pressing lightly on the piston as you rotate it over TDC,
If the rod is loose or excessively worn you will feel it "click" as the play goes from the top to the bottom .
Before you spend too much get under the engine and check for both end play and wear in the bearing.
Electric PTO do thump when they take up movement and it might be shudder in the clutch, a bad bearing in any one of the deck pulleys or a worn spindle bush.

bertsmobile1 said:

As for gasket size, go somewhere between double & triple the std Briggs gasket thickness.
The Intek head gasket failure seems a random occurrence , some engines are plagued by it others never give a minutes troubles.
One of the local shops here does so many they made a jig to hold the head in the mill so I drop any I get over there for a quick clean up.
Before finding them I would do some engines every season.

While the head is off check the rod by pressing lightly on the piston as you rotate it over TDC,
If the rod is loose or excessively worn you will feel it "click" as the play goes from the top to the bottom .
Before you spend too much get under the engine and check for both end play and wear in the bearing.
Electric PTO do thump when they take up movement and it might be shudder in the clutch, a bad bearing in any one of the deck pulleys or a worn spindle bush.

Click to expand...

I did as you instructed and found no play or click when pressing on the piston while turning the flywheel at TDC. It actually felt really solid. The only noise was coming from below. Actually tested my other mower that has a briggs OHV 17.5HP and it is a little noisy when moving the flywheel.

Just to add some more detail, when I pulled the PTO switch, the engine would do a vibration/shudder, clack sound and puff out some smoke. I checked the engine mounting bolts and they were tight.

I will test the deck hardware tomorrow. Is there a way of testing the PTO if it's bad?

Electric PTO's are sudden death.
It is either off or on, no slip so it puts an instant load on the engine.
The load on the engine makes it slow down so the governor opens up the throttle and pumps in the gas.
The engine slowing under load is the same as you backing off and developes a high vacum load which will suck oil past your rings ( or gasket ) if there is any gaps.
Then the engine picks back up so you get a little puff of whits smoke from the ooil then black smoke from the extra gas.
You notice this more an small engines and in particular from small single cylinder engines.
With the head gasket sorted you should not get the white smoke but the black smoke may persist.

bertsmobile1 said:

Electric PTO's are sudden death.
It is either off or on, no slip so it puts an instant load on the engine.
The load on the engine makes it slow down so the governor opens up the throttle and pumps in the gas.
The engine slowing under load is the same as you backing off and developes a high vacum load which will suck oil past your rings ( or gasket ) if there is any gaps.
Then the engine picks back up so you get a little puff of whits smoke from the ooil then black smoke from the extra gas.
You notice this more an small engines and in particular from small single cylinder engines.
With the head gasket sorted you should not get the white smoke but the black smoke may persist.

Click to expand...

When I get it all back together I will record it starting and engaging the blades.

bertsmobile1 said:

Electric PTO's are sudden death.
It is either off or on, no slip so it puts an instant load on the engine.
The load on the engine makes it slow down so the governor opens up the throttle and pumps in the gas.
The engine slowing under load is the same as you backing off and developes a high vacum load which will suck oil past your rings ( or gasket ) if there is any gaps.
Then the engine picks back up so you get a little puff of whits smoke from the ooil then black smoke from the extra gas.
You notice this more an small engines and in particular from small single cylinder engines.
With the head gasket sorted you should not get the white smoke but the black smoke may persist.

Click to expand...

Those two sentences in bold conflict with each other. Opening the throttle increases manifold pressure, not the other way around.

The on/off (i.e., no gradual application) nature of the electric clutch is why I engage the mower at a barely-above-idle engine speed and then bring the engine up to operating rpm. I also eliminated the nanny-state mower shutdown switches so that I don't put a hundred unnecessary disengage/engage cycles on the clutch every time I mow.

The clunking...a basic issue. Was the engine properly torqued to the chassis and the PTO to the engine? When I had my Intek out I could not find torque values for the hold down bolts so I used the bolt head markings and a chart. The clunking reminds me of shot engine or trans mounts on a car everything moves around with power application. Just a thought. Also the white puffs on closing and then opening throttle may be partially due to valve guide clearance (wear) or bad seal (s).

I put another OEM Briggs gasket on again and did a super clean of the gasket surface.

If you guys can watch the video I recorded of the engine running and can see if you hear any issues. I don't really want to throw much more money into it. If I throttle the engine up quickly it's puffing out white smoke.

Here is the video https://www.youtube.com/watch?v=7qTu1uRHamA Another video showing the blades being engaged. Is the sound normal? https://www.youtube.com/watch?v=AU0hIY41l2E

*EDIT* Added better quality video. Some of the rattle is the muffler and the heat shield. The shroud is not bolted down.

*Update*

I drained the 10w30 oil which smelled a little like gas. I put in SAE 30. Instead of white smoke puffing out when engaging the blades, it's black. Which is a great improvement to me as my other riding mower does this and it has never gave me any problems.

With a combination of:

1: The wrong Intake Push Rod being too long and hitting the valve cover and I think it was not allowing the oil to re-enter the crankcase, as there was a huge amount of oil in the valve cover after running. ( I installed new Intake and Exhaust Push Rods)

2: 4 Different size Head bolts. I replaced all 8, with new correct bolts. I torqued them down in small amounts in inch pounds. 50, 100, 150, 200, 220, Finally 230 for just a little extra torque just to make sure it was not under torqued. Each torque amount I checked it 3 times. Worth the extra time so you know it's correctly torqued.

Did some mowing and everything seems to be good now. The engine does not shake like it use to when engaging the blades.

Thanks to everyone for their help. Hopefully this holds up.