B&S Diamond I/C 15.5HP Repair / rebuild

G

geertv

Forum Newbie
Hi, newbie in engine mechanics trying to repair the engine of an old garden tractor (15 year old).
The engine was starting fine, but after 20-30 seconds starts smoking extremely (burning oil). It keeps running pretty well, but you hear RPM isn't very stable, oil is not over-filled.
After inspection we see oil in air filter box and carburator, and it also leaked oil under the cylinder head.
I've read it can be a blown gasket with this engines so we opened it up a little more... Removed the oil, spark plug (pretty black) and the OHV cap (don't know what's the right name), there was still a bit oil in it but suppose thats normal. Then we opened the cylinder head, first remark the piston was very black and we do see some cracks in the gasket.
Here some images:
aa6a7c9d-7671-4194-b4b0-dddb46c33f1e.jpg
2d78fcae-fd75-4818-b1b5-80fbbb2e2579.jpg

What do you guys think, would that have been the issue ? Now it's open so far, anything else that should be replaced ?
Also removed breather tube which is open but als contains oil, can the cap also be removed as it seems to be stuck:
1fe298d7-67d8-451e-8366-610066882eb5.jpg

Don't know this can just be removed ?

So, here we are now... any tips welcome :)
 
yes the head gasket is blown
Replace it
Buy two because they blow all of the time
Also look into installing a tap in the fuel line as hydrolocking is a big cause of these gaskets going bad
Give the head a light lap on a sheet of glass with some wet & dry soaked in Kerro
It is not flat , B & S deliberately mill it rough so it holds the gasket in place
The Stens aluminium gaskets are a lot better than the B & S gaskets
The tube is the crankcase breathee reconnect it as the engine will not run properly if it is not connected and the side of the engine will get very oily & thus dirty which affects the cooling
 
bertsmobile1 said:
yes the head gasket is blown
Replace it
Buy two because they blow all of the time
Also look into installing a tap in the fuel line as hydrolocking is a big cause of these gaskets going bad
Give the head a light lap on a sheet of glass with some wet & dry soaked in Kerro
It is not flat , B & S deliberately mill it rough so it holds the gasket in place
The Stens aluminium gaskets are a lot better than the B & S gaskets
The tube is the crankcase breathee reconnect it as the engine will not run properly if it is not connected and the side of the engine will get very oily & thus dirty which affects the cooling
Click to expand...
Many thanks for your help and confirming blown gasket.
There is a tap in the fuel line, it has always been there, and we close it after usage.
I suppose the tube leaks oil to the airfilter due to the crack which causes pressure, right (try to understand exaclty what goes wrong) ? But why does it somke like hell, is it also sucking oil in the cylinder ?
Would you replace anything else except the gaskets now it's open ?
If I am correct, only issue might be I need special tool to measure distance between the valve caps and the rocker ? Or is there another way to mount it again ?
 
The tube vents the crankcase into the carburettor
Excessive pressure in the crankcase caused by the leaking gasket forces oil fume into the carburettor causing the smoke
The engine will normally knock a little when you first engage the deck, and when you turn the engine off
Some time it will blow a little black smoke as well when the PTO is engaged
You will need a tension wrench to tighten the head bolts and note the torque is in INCH pounds not foot pounds like a car head so divide by 12 as very few tension wrenches are marked in inch pounds
I use a 6" long one to avoid over tightening .You will also need a set of feeler gauges, a torx spanner & a metric ( 10mm ) spannar to set the valve lash
Note that the grub screw ( torx head ) is the locking device & the outer nut is the adjustment , people often get it backwards because that is how cars work.
Pull the pushrods out, check they are strait & reverse them when you put them back in as they rub on the guide blocks and wear thin
If they look really worn replace then they are only about $ 10 a piece .
I fit 2 steel ones for this reason as the aluminium ones will wear thin & bend .

There are hundreds of you tube videos about doing this but watch a few to get a feel for it.
Donny Boy , Taryl, steves Saloon are all recommended viewing .
Clean the worst of the carbon off the piston top & head but do not touch the ring of carbon around the bore .
Does not have to be spotless you are not doing open heart opperations in there
The B & S manual is on line & if you are lucky some one will post a link to a copy
If you are unsure about anything then ask .
 
Last edited:
Indeed, I think a lot of oil go's via the breather into the carb...
So, nothing else to change ? That's great news... learned a lot !
Someone knows where i can find the valve clearances for this engine ? And the torque for the cylinder head ?
Then I can close it and test :)
 
IN .003-.005
EX .005-.007

Original head torque was 220 in-lbs. Briggs increased it to 250 in lbs (20.8 ft-lbs (I use 21 ft-lbs)) using three stage step torque. (7, 14, and finally 21)

Torque head bolts per service manual shown sequence.
 
bertsmobile1 said:
The tube vents the crankcase into the carburettor
Excessive pressure in the crankcase caused by the leaking gasket forces oil fume into the carburettor causing the smoke
The engine will normally knock a little when you first engage the deck, and when you turn the engine off
Some time it will blow a little black smoke as well when the PTO is engaged
You will need a tension wrench to tighten the head bolts and note the torque is in INCH pounds not foot pounds like a car head so divide by 12 as very few tension wrenches are marked in inch pounds
I use a 6" long one to avoid over tightening .You will also need a set of feeler gauges, a torx spanner & a metric ( 10mm ) spannar to set the valve lash
Note that the grub screw ( torx head ) is the locking device & the outer nut is the adjustment , people often get it backwards because that is how cars work.
Pull the pushrods out, check they are strait & reverse them when you put them back in as they rub on the guide blocks and wear thin
If they look really worn replace then they are only about $ 10 a piece .
I fit 2 steel ones for this reason as the aluminium ones will wear thin & bend .

There are hundreds of you tube videos about doing this but watch a few to get a feel for it.
Donny Boy , Taryl, steves Saloon are all recommended viewing .
Clean the worst of the carbon off the piston top & head but do not touch the ring of carbon around the bore .
Does not have to be spotless you are not doing open heart opperations in there
The B & S manual is on line & if you are lucky some one will post a link to a copy
If you are unsure about anything then ask .
Click to expand...
Hello,

Why should the carbon ring around the bore be left? Is it to prevent any lip from damaging the rings at TDC?

Thanks.
 
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