Engine bore measurements

I'd likely put it back together and see how it runs with the existing rings, but better to lightly deglaze the cyl and replace the rings as well as touching up the valves.

If you are going to do anything else the complete cyl and piston set would be the best - basically a full rebuild to original.

Also check all the bearings - likely replace the con-rod along with the piston if you are going that far.

Just looking at the picture I would say it is an oil burner. All that black crud on the top of the block confirms it. Also .003"wear seems a bit excessive, does the manufacturer give a maximum cylinder wear spec? All said, it will probably run fine as is, but consume some oil. If money was no object a total rebuild would be in order if you can still get parts. Greg O

=A0 =A0 =A0 =A0Parallel

=A0 =A0 =A0 =A0 =A0 +0.0030"

=A0 =A0 =A0 =A0 =A0 =A0 =A0 +0.0025"

=A0 =A0 =A0 =A0 =A0 =A0 =A0 -0.0005"

=A0 =A0 =A0 =A0Parallel

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 3.131"

=A0 =A0 =A0 =A0 =A0 3.135"

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 3.136"

The rings are really what seals the works, the piston is just the carrier. You need to check the ring grooves for wear, too. Most any book on engine rebuilding will have suggested limits which should applicable for similar sized bores and pistons. I know B&S has limits on the ring gaps when inserted in the bore. Slop up and down in the piston ring grooves is checked with feeler gauges. Too much and you'll need a new piston. Don't forget to check the rod for twist, too. Crankpin needs to be checked for wear, too. No sense putting a lot of effort into redoing the cylinder bore if the bearings are shot. Hope you can get replacement parts.

Stan

You didn't mention whether you had run the engine before taking the compression test, but the rings aren't supposed to be dry for the test (long period of storage or non-use).

The piston and bore are probably fine, as-is. You mentioned that there was very little ridge at the top of the cylinder. The ridge is an excellent indicator of wear, generally, if no other problems are present (piston cracks, scored cylinder, signs of being run with too little oil, etc).

The piston skirt, for the most part, is what guides the piston in the cylinder. There were high performance modifications done to racing pistons where several "buttons" of synthetic material were set into counnterbored holes in the skirts. The buttons then guided the piston, reducing the area of the skirt contact with the cylinder to just a couple of square inches, thereby reducing friction and heat.

At the point where you are now, the valves should be re-done, and the valve guides checked and/or replaced. Replacement would be best if they're available. The length of the valve stems will most likely need to be shortened after grinding the valve faces and seats. There will be a spec for the gap to the cam surface wih the valves closed. The gap insures that the valves will fully seat after the parts have expanded from normal engine heat.

After checking the piston grooves with a new set of rings, if the grooves are OK, then removing the ridge and honing may be all the cylinder work you need to do. Slightly oversize rings, as recommended earlier, would be best, and there is a spec for the gap at the ends of the rings (while each ring is positioned squarely in the cylinder (pushed down in the cylinder a certain distance with the bare piston).

After checking rod bearing and the main bearing surfaces and dimensions, you should be ready to reassemble the engine.

The ring replacement procedure, for now, will very likely give you years of reliable performance. In another 50 years, or for the engine's 100th anniversary, you can go for the .010" oversize piston and cylinder boring job (better get the parts now, though).

Thanks to everyone for the suggestions!

The manual suggests decarbonising the engine every 400 hours, so the build-up of soot may be normal.

Best wishes,

Chris

Thanks for the advice. I just replied to your Kohler e-mail with some thoughts.

Best wishes,

Chris

I hadn't run the engine first, no. So the bore was likely drier than it would be in normal use.

No marks on the bore. It's smooth and shiny. Very few marks on the piston either. Parallel to the crankshaft, I can still see some machining marks on the piston.

The valves look good, at least visually. The seats are smooth and without erosion, and the tappet clearances are close to the specified values. Adjustment of the clearances is using circular shims, a system I haven't seen before. I'm somewhat reluctant to grind the valves because of having to clean up the paste afterwards, but I might be persuaded.

I'm doubtful I can get slightly oversize rings, but I'll check.

Now that's a nice thought!

Best wishes,

Chris

Pistons are not round they are tapered in length when new and oval shaped. Pistons should only be measured on the skirt near the bottom. That will tell you how bad they are worn. Also Pistons are usually about .025 undersize where the rings are. So checking with a feeler gauge tells you nothing. On cars engines it recommended to rebore when the taper in the bore is .005" or greater. If you use cast iron rings I have seen as much as .012" taper and not burn oil. Only you may get 60 to 70K miles before it becomes an oil burner. I have seen this with SBC engines.

The reasons for the undersize around the rings is, 1 the difference in expansion in this area, 2 the compressed gas/air mixture goes around the groove to force the rings against the cylinder wall. That is why there is a

From what you are telling us is you could ring the engine and install new bearings if they are worn out of spec. Most likely on such an old engine the valve seats are cast iron and worn and or corroded. A valve grind is good idea. It may be that all you need is a valve grind to bring compression back up. A lawn mower shop can grind the valves with out tearing the engine down and install new bronze guides at the same time if necessary. Most likely the carbon is from worn valve guides.

If this were my engine I would grind the valves and put it back together. But then it's not mine. Also parts may be hard to find.

Richard W.

Carbon is one thing, oily soot is another. I ran a small engine shop for a few years. I can not tell you how many of these small engines I have rebuilt over the years! Do what you want, but with .003" wear in the cylinder even with new piston rings, it will continue to burn oil! Someone mentioned Kohler engines, pretty sure maximum wear for a Kohler was .003", right where you are at. I could run out to the shop and look it up, but I am too lazy! If you want a great running engine, it needs a rebuild. It will run good like it is, but I guaranty it will burn oil! Greg

And a J.A.P. engine didn't from new??? They ALL burned some. Just depends on what you consider to be excessive.

These small engines don't have valve guide seals, so they all burn some oil. The more worn the valve guides the more oil is consumed.

Richard W.

The valves, seats and guides all look in good order. I'm sure these engines burnt a little oil even when new. Surely some oil would sneak past the rings, even in a new bore? The manual for this engine suggests adding one tablespoon of engine oil to each gallon of fuel, when "commercial petrol" is used. Whether this is to lubricate the carburettor or to make up for burnt engine oil, I don't know. But as long as I don't have to top up the oil too often, I don't mind. Hopefully it won't burn enough oil to get from the "High" to "Low" mark in a season, but I'll still check the level every time I start the engine.

Best wishes,

Chris

I think they're cast iron rings in a cast iron bore, with an aluminium piston.

Chris

Is ridge removal essential? How do you remove the ridge without reboring the cylinder? I remember being told once that if the piston rings hit the ridge when the engine was run fast, it could damage the rings, but I don't know if this is true.

Chris

Yes it's true. They use a ridge reamer for this. It's used with a wrench to spin it.

Some are CI rings and others are chrome rings. I don't think I have ever heard of a chrome-molly option for small engines. Cast Iron rings are fine unless you have the aluminum cylinder.

Also I forgot to say in the earlier post, pistons are not only oval, but taper in length. That is why you measure at the bottom of the piston.

From what I see I would put it back together and see how it runs. I have seen Briggs engines run with as little as 5 pounds of compression.

Richard W.

My 24 yr old tech snow blower engine goes down about 1/4 of the range in a season. I had one B&S 3.5 that would go down 1/2 per season of grass - maybe 10 hours as opposed to 5 for the snow blower. We're not talking high time seasons here now that I'm only doing my own property with no offspring trying to earn comic book money. Gerry :-)} London, Canada

A tool that's commonly referred to (here in he USA) as a ridge reamer, will remove the cylinder ridge with a HSS or possibly carbide cutter that's rotated with a wrench.

Many RRs have a stop, which will prevent the cutter from cutting oversize. If the stop isn't adjusted properly, the ridge won't be completely removed, or the possibility of removing too much cylinder wall exists. The user should be careful as the RR reaches the size of the cylinder bore.

Not removing the cylinder ridge risks damage to the new top piston ring and the piston ring land that it sits on, and the second ring.

Small engine repair generally involves having a few specialized tools.. ridge reamer, piston ring compressor, piston ring groove cleaning tool, ring expander, valve spring compressor and a few others.

It IS true, and there is a little thing called a ridge reamer that just takes the ridge out. You do this BEFORE removing the piston unless the piston is removed from below.