I have a 1972 combat engine with 10,000 miles, do you think it should be bored or can i just change the rings. Cylinder looks good with no ridge, it has .007 between piston and cylinder. Ring gap .020 Cylinder measurement is 2.8755. Piston measures 2.8682
750 piston clearence
- Thread starter stevesbsa
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stevesbsa said:
For the price it is hard to beat the JCC replacements. The replacement Hepolites are excellent also but 3 times the money.
A little elongated hole-two per side is not a problem. If these two holes are joined by a slot that is seen after the oil ring is removed or from the inside of the piston then you need to replace them. Jim
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A new set of pistons would probably bring the clearance back about 0.002'', for a little while any way, as COMNOZ said the JCC pistons are plenty good enough, probably even better than the originals.My own opion is that they could do with a good quality top ring
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Trixie Combat had the Combat inferior pistons with the wide spaced coma oil holes plus an Al gasket that rubbed on pistons till one came apart in very mild new to me 3 wks bike. Replacements are JCC Tiawain supplied by Baxter cycle.
Expected clearance is more like .005" or be wary of seizure I hear.
Expected clearance is more like .005" or be wary of seizure I hear.
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This subject is about where I am at on a project engine I am building so when I saw the title I stopped to read and then that spurred more research.
One of my references is Modern Motorcycle Mechanics by J.B. Nicholson.
The clearance stated there for Combat Commando 750's with thermal slot is .0065" (top of skirt) and .0035" (bottom of skirt).
What is the reference to (bottom of skirt) about and this would seem to put a taper in the bore.
Is my understanding correct?
This is the way I would describe making a clearance measurement when using a feeler gauge method of measurement:
the area being measured is about a 1/2" into the cylinder from the top, generally with the piston reversed so that it is easier to handle. The feeler gauge is placed between the piston and the cylinder wall 90 deg to the wrist pin. The feeler gauge takes some pull to move it (spring gauge 5lbs)
IF that is correct, how is a bottom of skirt measurement accomplished and where?
One of my references is Modern Motorcycle Mechanics by J.B. Nicholson.
The clearance stated there for Combat Commando 750's with thermal slot is .0065" (top of skirt) and .0035" (bottom of skirt).
What is the reference to (bottom of skirt) about and this would seem to put a taper in the bore.
Is my understanding correct?
This is the way I would describe making a clearance measurement when using a feeler gauge method of measurement:
the area being measured is about a 1/2" into the cylinder from the top, generally with the piston reversed so that it is easier to handle. The feeler gauge is placed between the piston and the cylinder wall 90 deg to the wrist pin. The feeler gauge takes some pull to move it (spring gauge 5lbs)
IF that is correct, how is a bottom of skirt measurement accomplished and where?
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Flat wide feeler gauge will not give true round bore gap. A round one would be better, then divide by 2 to get the gap value. But best is mic piston and a bore gauge as each piston has specific levels down bore to take a measure.
Ms Peel had too big a bore on too small pistons and my wallet too thin for new pistons and re-bore, so had them knurled and worked a treat but did show smoothing over after 7000 very very tough miles. No slap sounds and for sure no siezures.
Ms Peel had too big a bore on too small pistons and my wallet too thin for new pistons and re-bore, so had them knurled and worked a treat but did show smoothing over after 7000 very very tough miles. No slap sounds and for sure no siezures.
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Duh, if ya stick something down the gap til it binds, you get the total of both sides of the gap, you ain't measuring a centered piston but one shoved completely to one side, so when piston is centered by rings it will take up one half of the width of the measuring stick. A thin enough flat feeler gauge may conform to the bore arc enough not to matter but tends not to fully conform so harder to get good measure that way. Stick small drill bits or what ever in the gap then measure that and divide by two. But here we run into the depth down bore vs piston and bore size so may be closest enough for a shade tree like me, but worth being aware of.
I'm going to have to keep an eye on Peels piston .0065 gap as just on boarder line of acceptable in Hi Compression boosted heat flow use. If I find signs of scuffling may have to have pistons narrowed at tad. On my plain Jane Trixie I just had local shop hone the bore and stuck in out the box .040" pistons w/o checking the gap. I'll never press heat or rpm loads on Trixie just want cheapest decent repairs that last w/o seeking perfection as in Peel.
I'm going to have to keep an eye on Peels piston .0065 gap as just on boarder line of acceptable in Hi Compression boosted heat flow use. If I find signs of scuffling may have to have pistons narrowed at tad. On my plain Jane Trixie I just had local shop hone the bore and stuck in out the box .040" pistons w/o checking the gap. I'll never press heat or rpm loads on Trixie just want cheapest decent repairs that last w/o seeking perfection as in Peel.
If the pistons are not the failure prone variety with the slot under the oil ring then I would ring it and run it. Jim
unfortunately,the pistons do have the cut out that goes from one slot to the other in the oil ring grove. Also mic out the cylinder to 2.8758 and piston to 2.8674 with a piston clearance of .0084 pretty close to my feeler gauge reading of .007. Looks like I'll be boring to fit new pistons. I guess you can get .010 over pistons. Most local auto machine shops shouldn't have a problem boring this size cylinder, should they.
unfortunately,the pistons do have the cut out that goes from one slot to the other in the oil ring grove. Also mic out the cylinder to 2.8758 and piston to 2.8674 with a piston clearance of .0084 pretty close to my feeler gauge reading of .007. Looks like I'll be boring to fit new pistons. I guess you can get .010 over pistons. Most local auto machine shops shouldn't have a problem boring this size cylinder, should they.
Steve, your bore is aprox 0.001'' oversize and you want to rebore it ??., no you can't get 0.010'' pistons unless you can find old stock,new std pistons should be about 2.870'' bringing your clearance back to about 0.0055'', near spot on.
Thanks, Yeah, Your right The spec on the cylinder shows 2.8750/2.8754. I guess it doesn't make sense to bore. The JCC piston sounds like it will be ok from baxter cycle. How about the rings? Any brand better than the other or do they come with the pistons?
Thanks, Yeah, Your right The spec on the cylinder shows 2.8750/2.8754. I guess it doesn't make sense to bore. The JCC piston sounds like it will be ok from baxter cycle. How about the rings? Any brand better than the other or do they come with the pistons?
Thanks, Yeah, Your right The spec on the cylinder shows 2.8750/2.8754. I guess it doesn't make sense to bore. The JCC piston sounds like it will be ok from baxter cycle. How about the rings? Any brand better than the other or do they come with the pistons?[/quote]
They do come with the pistons but they are only a plain cast iron ring, there are better rings made....
They do come with the pistons but they are only a plain cast iron ring, there are better rings made....
whats the word on the emgo pistons? Finally found standard size pistons, the only ones that were available were JP,emgo and the expensive light weight pistons or some italian type. Im gonna use hasting rings, i guess their better than stock emgo. No jcc pistons from baxter cycle. They only had the emgo pistons. The question about the combat engine piston, they are the same as stock. The parts manual shows a different number for the 9 to 1 and the combat. Everyone's opinion is that they are the same, whats the deal with the P/N ? Thanks for everyone's patience, I'm sure all these questions have been ask before. There is so much information on this site, if a guy was to read it all, nothing would get done in the garage.
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When I first rebuild Combat Ms Peel, the bore was found to have been done .020"
over plus tapered wear. Hone job fixed the taper but left pistons a bit sloppy, like .007-9" clearance, so they said just knurl the pistons and that worked a treat till over rev event bent stuff. Knurls showed they'd worn mostly down by 7000 miles but undetectable noise or function issues.
over plus tapered wear. Hone job fixed the taper but left pistons a bit sloppy, like .007-9" clearance, so they said just knurl the pistons and that worked a treat till over rev event bent stuff. Knurls showed they'd worn mostly down by 7000 miles but undetectable noise or function issues.
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