Briggs & Stratton OHV Plastic Engine No Compression

[RevB]

I don’t even know of a machinist in our area that will grind valves. When you factor in your time back and forth, and paying labor cost, it would be on the expensive side. Even new valves need to be lapped in to the old seats.

Automotive repair or engine rebuilder....most straight machinists don't.

 

[GearHead36]

Lapping does not increase compression.

I disagree. I used to race karts with 5 HP Briggs flathead engines. I ran open class, and the engines peaked out around 6000 RPM. At the end of a race, compression would be noticeably down from the start. I would have to basically do a valve job after every race. I'd remove the head and valves. I'd decarbon the valves by spinning them in a drill press, and cleaning up with steel wool. Then lap the valves. I'd use 3 compounds. Course, fine, and polish. Re-set the valve clearances, then reassemble. Compression went way up after doing this.

 

[RevB]

I disagree. I used to race karts with 5 HP Briggs flathead engines. I ran open class, and the engines peaked out around 6000 RPM. At the end of a race, compression would be noticeably down from the start. I would have to basically do a valve job after every race. I'd remove the head and valves. I'd decarbon the valves by spinning them in a drill press, and cleaning up with steel wool. Then lap the valves. I'd use 3 compounds. Course, fine, and polish. Re-set the valve clearances, then reassemble. Compression went way up after doing t

I disagree. I used to race karts with 5 HP Briggs flathead engines. I ran open class, and the engines peaked out around 6000 RPM. At the end of a race, compression would be noticeably down from the start. I would have to basically do a valve job after every race. I'd remove the head and valves. I'd decarbon the valves by spinning them in a drill press, and cleaning up with steel wool. Then lap the valves. I'd use 3 compounds. Course, fine, and polish. Re-set the valve clearances, then reassemble. Compression went way up after doing this.

After every race. A complete valve job. Decarbon. Ohhh Kay.

 

[GearHead36]

After every race. A complete valve job. Decarbon. Ohhh Kay.

It wasn't complete valve jobs. I didn't replace valve guides, and I didn't cut valve seats or valves. I was a teenager, so this kept me out of trouble.

 

[slomo]

After every race. A complete valve job. Decarbon. Ohhh Kay.

Not like Briggs engines, well, guess they do make racing engines. Going to say they are not high rpm engines with any form of durable valvetrains. More Diesel like compared to gas motors. None are made to last 100,000 miles either like cars do.

 

[GearHead36]

Not like Briggs engines, well, guess they do make racing engines. Going to say they are not high rpm engines with any form of durable valvetrains. More Diesel like compared to gas motors. None are made to last 100,000 miles either like cars do.

Like I said, not a complete valve job. And these were all-out racing engines. The only OEM parts were: the block, head, lifters and spark plug. The crank, cam, & piston were all high end racing parts. Racing carb & intake, too. Straight pipe exhaust. The engines turned WAY more RPMs than stock and produced WAY more power. The fuel was 100% methanol. Blown engines were not uncommon. The valves took a beating. A single race probably put more stress on the valves than a typical engine would see in its entire life. My point was that lapping the valves DID restore compression.

 

[sgkent]

Here is what I learned working in a high end machine shop back when I was still building racing engines. Valves seal. Let's say they start at 100% sealing. As the valve guide wears the valve rocks in the worn guide at which point it loses some of its sealing. Carbon can build up under the valve if the seat contact area is too wide. The optimal seat contact area cuts thru any carbon that builds up. Now comes DIY home guy or small shop repair. They clean the valves and hand lap them. Maybe the sealing was down to 85% so they now have it back to 95%. The engine runs better. Did this solve the worn guide issue - no. Did this solve the seat contact area issue - no. But they charged for their services and the problems will be back but sooner this time. If they decide to replace the valve guides, the valves are no longer 100% concentric with the seats so this does not get them back to 100%. The only way to do that is to replace the guides, use a cutter or stone and a pilot in the guide to keep it concentric with the seat, then shoot the seat for 3 angles and surface the valve a little, then tip it for the same height. Nothing else will bring back 100% including hand lapping. But hand lapping is better for a while than leaving the valve and seats full of carbon. I plan to rebuild a B&S mower next spring when I heal from a heart bypass surgery. I already have the machine shop lined up who will replace the guides and do the seats and valves. They build trophy cart racing motors and are set up to do it. Hand lapping is ok as long as one understands the limitations of it and doesn't expect the engine to be 100% what it was when the guides and valves were new. If I set valves on a L-head engine, I clean the valves and seats, then hand lap. 95% is better than 85%. If I plan to rebuild an engine, then we go a different route. Most small shops are not set up to replace guides and machine shop services are mostly gone due to the throwaway society we live in.

 

[RevB]

Here is what I learned working in a high end machine shop back when I was still building racing engines. Valves seal. Let's say they start at 100% sealing. As the valve guide wears the valve rocks in the worn guide at which point it loses some of its sealing. Carbon can build up under the valve if the seat contact area is too wide. The optimal seat contact area cuts thru any carbon that builds up. Now comes DIY home guy or small shop repair. They clean the valves and hand lap them. Maybe the sealing was down to 85% so they now have it back to 95%. The engine runs better. Did this solve the worn guide issue - no. Did this solve the seat contact area issue - no. But they charged for their services and the problems will be back but sooner this time. If they decide to replace the valve guides, the valves are no longer 100% concentric with the seats so this does not get them back to 100%. The only way to do that is to replace the guides, use a cutter or stone and a pilot in the guide to keep it concentric with the seat, then shoot the seat for 3 angles and surface the valve a little, then tip it for the same height. Nothing else will bring back 100% including hand lapping. But hand lapping is better for a while than leaving the valve and seats full of carbon. I plan to rebuild a B&S mower next spring when I heal from a heart bypass surgery. I already have the machine shop lined up who will replace the guides and do the seats and valves. They build trophy cart racing motors and are set up to do it. Hand lapping is ok as long as one understands the limitations of it and doesn't expect the engine to be 100% what it was when the guides and valves were new. If I set valves on a L-head engine, I clean the valves and seats, then hand lap. 95% is better than 85%. If I plan to rebuild an engine, then we go a different route. Most small shops are not set up to replace guides and machine shop services are mostly gone due to the throwaway society we live

Like I said, not a complete valve job. And these were all-out racing engines. The only OEM parts were: the block, head, lifters and spark plug. The crank, cam, & piston were all high end racing parts. Racing carb & intake, too. Straight pipe exhaust. The engines turned WAY more RPMs than stock and produced WAY more power. The fuel was 100% methanol. Blown engines were not uncommon. The valves took a beating. A single race probably put more stress on the valves than a typical engine would see in its entire life. My point was that lapping the valves DID restore compression.

Methanol burns about two times slower and only has 9500btu/lb compared to 18,400btu per pound for gasoline. My guess is these were not blown or turbocharged so there is no way you could get the power you claim unless you burned 3 times the amount of methanol compared to gasoline. And...the engine's EGT would be in the toilet with methanol due to the latent heat of evaporation being about 3.5 times higher than gasoline which means everything runs far, far cooler, a blessing for valves but hell on cylinders/rings/pistons as any residual lubricating oil would immediately be washed away by the methanol. The claim of "way more power" requires at least 3+ times more methanol per intake charge than gasoline to produce the same amount of energy per unit volume. Even reducing combustion chamber volume to 13:1 won't get you there normally aspirated.