Engine bore measurements

Hi folks,
I have a question for the engine guys. I'm working on a JAP Model 5 Type 53 single cylinder petrol engine mounted on a Ransomes "Mastiff"
cylinder mower. The engine dates from 1962. Cubic capacity is 412 cc with a power output of 4.5 hp at 2,200 rpm. I particularly like this engine because it's solid yet very precisely built.
I want the engine to be reliable so I can use the mower frequently. When I first turned the starting handle to test the compression, it seemed poor. I then removed the cylinder head and found that the piston seemed loose in the bore. I could see a gap around the piston and thought that I could even see the top compression ring through the gap (but it was hard to be sure because of the reflections). With the piston at the top of the bore I estimated the gap to be 0.025" on the diameter with a feeler gauge. The piston wobbled in both directions, but slightly more perpendicular to the crankshaft. The only other engine I've seen with a piston this wobbly would never run reliably (I just realised that the unreliable engine used a cast iron piston in an aluminium bore, whereas this latest engine uses an aluminium piston in a cast iron bore - perhaps a wobbly piston is a much bigger deal in the first case because of the differing coefficients of thermal expansion?).
But the bore looked good. I wondered if perhaps the wear was much worse on the piston than the bore, because the aluminium piston is softer than the cast iron bore, and obviously the piston is in contact with the cylinder wall all the time, whereas parts of the wall are only intermittently in contact with the piston. So I bought a bore gauge to measure the wear. I now feel a bit dumb, because I later discovered two other problems that were causing the compression to seem poor: a slightly loose spark plug and a deliberate blockage in the inlet manifold left by the previous owner to stop dirt getting inside. I have now checked the poppet valves too. The seats look good and the valves are closing properly. The compression is now better, but not as good as in some other engines I own.
I went ahead and measured the bore. Here are three pictures. The first shows the bore (that ridge looks far worse in the picture than in reality; it's actually only visible on one side of the bore, and can barely be felt). The second shows my Etalon vernier calipers, which I used to set the zero on the bore gauge. The third shows my Mercer bore gauge.
http://www.mythic-beasts.com/~cdt22/jap_engine_bore.jpg
http://www.mythic-beasts.com/~cdt22/etalon_vernier_calipers.jpg
http://www.mythic-beasts.com/~cdt22/mercer_bore_gauge.jpg
The bore is nominally 80 mm or 3.150".
I got the following measurements with the bore gauge:
Perpendicular (to crankshaft) Parallel Top (of bore) +0.0035" +0.0030" Middle +0.0030" +0.0025" Bottom -0.0000" -0.0005"
I considered setting the vernier calipers using the top land of the piston for a relative measurement, instead of using the nominal 80 mm, but didn't try it because I was interested in the distribution of the wear. I was careful to avoid ridges and chamfers. I repeated the measurements, including setting the 80 mm using the vernier calipers (oddly, I found the vernier scale easier to read with the lines horizontal - I wonder if it's related to this? http://en.wikipedia.org/wiki/Vertical%E2%80%93horizontal_illusion ), and reversed the sense of the bore gauge. I got exactly the same results, so the repeatability is good. The bore gauge is graduated down to 0.0005" and the vernier calipers to 0.001".
I also measured the piston diameter using the calipers, with the following results:
Perpendicular (to crankshaft) Parallel Top land 3.126" 3.131" Middle (or close) 3.138" 3.135" Skirt 3.140" 3.136"
That looks like a fairly consistent 0.003" wear on the bore in the area swept by the rings. I don't have an official diameter for the piston, but Don Nichols suggested finding the recommended clearance between the piston and bore. The engine's manual states 'RECOMMENDED CLEARANCES ON RECONDITIONING Piston - On diameter of piston: 0.009" skirt, 0.018" top land'. Presumably that means the piston is slightly conical?
The gap around the piston really bugged me at first, but given those figures, maybe the wear isn't so bad. The clearances sound big, but I wonder if I'm making the error of expecting an aluminium piston to fit a cast iron bore as closely as a cast iron piston at room temperature? Looking at the coefficients of expansion, a temperature rise of a few hundred centigrade is going to close that gap considerably.
Almost all parts are available for this engine, so I have a lot of options, ranging from free to expensive. But I want to get this rebuild right first time.
I could:
- Do nothing. - Replace the piston rings only. - Replace the piston with a new standard piston and new rings. - Get the cylinder bored oversize, fit an oversize piston and rings. - Fit a brand new cylinder, new piston and rings.
My priorities are (in order):
- To have a reliable engine that starts easily and develops rated power. - To maximise the life of the engine (i.e., not rebore if unnecessary). - To save money.
Based on the evidence, my gut reaction is to go for a new set of piston rings only. But I'm unsure how to inspect rings for wear. Can anyone advise me here?
Sorry for the length of this message! I'd be very interested to hear what people think.
Best wishes,
Chris
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On Mon, 01 Mar 2010 04:16:07 +0000, Christopher Tidy

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.
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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
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Greg O wrote:
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
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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
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wrote:

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

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
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On Thu, 04 Mar 2010 22:15:15 +0000, Christopher Tidy

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
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On Thu, 04 Mar 2010 19:25:50 -0500, the infamous Gerald Miller

Other than annual oil changes (when I remembered) I can remember buying only a few quarts of oil for my mowers which was used to top off the crankcase. Most didn't use oil.
-- An author spends months writing a book, and maybe puts his heart's blood into it, and then it lies about unread till the reader has nothing else in the world to do. -- W. Somerset Maugham, The Razor's Edge, 1943
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wrote:

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
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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).
--
WB
.........


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Wild_Bill wrote:

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
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Wild_Bill wrote:

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
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Yes it's true. They use a ridge reamer for this. It's used with a wrench to spin it.
(Amazon.com product link shortened)
Richard W.
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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.
http://buy1.snapon.com/catalog/item.asp?P65=&tool=all&item_ID 762&group_ID87&store=snapon-store&dirtalog
http://www.jcwhitney.com/jcwhitney/product/images/large/G_14236G_CL_1.jpg
http://buy1.snapon.com/catalog/item.asp?P65=&tool=all&item_ID 821&group_ID96&store=snapon-store&dirtalog
--
WB
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Wild_Bill wrote:

Got a ring compressor, expander, deglazing hone and now a bore gauge. I don't have a ridge reamer or compression tester yet.
Chris
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On Thu, 04 Mar 2010 22:20:14 +0000, Christopher Tidy

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.
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snipped-for-privacy@snyder.on.ca wrote:

The piston came out from below, though given the size of the ridge, I suspect it would have come out the top too.
I just found my lost feeler gauge, so I'm going to check the ring grooves tomorrow.
Chris
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On Thu, 04 Mar 2010 22:20:14 +0000, the infamous Christopher Tidy

Ridge removal is imperative if you replace pistons, lest you break the new rings on the slightly lower ridge which was created by the old rings riding in the worn lands ofthe old pistons. Generally, ridge removal is required to remove the old pistons, so it's not a problem. If you can remove the old pistons without removing any small ridge, there likely isn't a problem, either.
-- An author spends months writing a book, and maybe puts his heart's blood into it, and then it lies about unread till the reader has nothing else in the world to do. -- W. Somerset Maugham, The Razor's Edge, 1943
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