N.O.S. Posted August 23, 2015 Share Posted August 23, 2015 (edited) We need to put some standard pistons in a standard never machined block, because 3 of the original pistons have badly worn ring grooves, causing the rings to break up. Bores are good so apart from a light honing new pistons / rings are all that is required. But we've run up against a piston sizing issue. It would appear that modern (good quality) replacement standard pistons these days seem to be 2 1/2 to 3 thou smaller diameter than original pistons. This means there is too much slop in the original bores of our engine. The new pistons are machined to 3.12325" measured at skirt. If bores are machined to std size of 3.125" plus 1 thou, the clearance would be 2 3/4 thou - about right for recommended clearance. The original pistons from this engine, and several other originals we have found, measure 3.125" at skirt and fit with the correct clearance in our bores, so we can only assume that the working clearance was obtained new by machining the original bores to 3.125 plus 2 1/2 thou clearance. I guess it is true to say that all standard pistons are now used in blocks that will have been sleeved back to standard size, and the new liners would be as a matter of course be honed to suit the new pistons - so this discrepancy between new and old piston diameters would never be an issue. We are fortunate enough to have access to a set of good used pistons which we can fit in order to achieve correct clearances and avoiding rattle which would surely result if new pistons were fitted giving 5 - 6 thou clearance. The only alternative is to bore out 10 thou or bore out and liner back to original size. Has anyone else encountered similar issues with fitting new pistons into original bores? Edited August 23, 2015 by N.O.S. Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted August 23, 2015 Share Posted August 23, 2015 Hi Tony, Have you measured the bores at the bottom of the bore? This will give you true size as there is little if any wear in that region. Normally on an original engine it would be bored to the correct size and a range of graded pistons would be available to cope with machining tolerances. I cannot see them machining extra clearance, that sort of thing is done on overhaul when you hone to fit pistons supplied. Never bore and hone a block without the pistons being available to measure. Differing clearance on pistons is due to different expansion rates, depending on the alloy or type of piston. regards, Richard Quote Link to comment Share on other sites More sharing options...
ruxy Posted August 23, 2015 Share Posted August 23, 2015 Are you measuring correctly ? modern pistons are cam ground oval , when at operating temperature - they will be true. With modern machine tools , pistons and bores nominal dia. are tighter held - they are more like peas out of a pod. IIRC letter graded are to .0005" , for selective assembly to bore , doubt if you will find now. Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted August 23, 2015 Author Share Posted August 23, 2015 (edited) No Richard, we haven't measured the bores top or bottom. This is strictly a 'roadside' rebuild :blush: The micrometer is a borrowed WW2 U.S. metric unit (no doubt from a USAAF base!) but good enough to measure the difference between new and old pistons. We have measured the old and new pistons, and running both pistons down the bore we get adequate clearance with the old but too much clearance at 90 deg. to the crankshaft with the new pistons. The engine runs well with the original pistons - we think it will rattle like heck with the smaller new pistons. The fact that the new pistons are smaller to start with means there is no way to get the clearance down to reasonable limits without closing up the bore. I suppose the simple answer is that nobody needs to put new pistons in an original engine anymore without having bores sleeved and machined accordingly. Edited August 23, 2015 by N.O.S. Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted August 23, 2015 Author Share Posted August 23, 2015 Ruxy - we're measuring the skirt low down at 90 deg. to gudgeon pin, I'm sure the originals are oval too but we measured both at the same position. This is more of a comparative measuring exercise as it is obvious the new pistons are smaller than the original pistons and too small for the bore. We also tried one of a set of French pistons supplied with a new Hotchkiss block - these are about the same as the original pistons i.e. larger than new manufacture. Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted August 23, 2015 Share Posted August 23, 2015 Hi Tony, just as a point, oval ground pistons are not a modern design, they were common way back. In the situation you have, new pistons in a used bore, probably lost a couple of thou already, there used to be PEP pistons, designed to use in a slightly worn bore as an alternative to reboring. Quote Link to comment Share on other sites More sharing options...
Edd80inch Posted August 25, 2015 Share Posted August 25, 2015 When I rebuilt my Jeep engine it was found to be sleeved during a previous rebuild by the BAOR. Steve Rivers at Dallas autos supplied a new set of liners and pistons. It may be worth getting a professional engine machine shop to measure up your block and pistons to see if they are serviceable. I've yet to see an engine with worn pistons but no wear at all in the bores. Edd. Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted August 25, 2015 Author Share Posted August 25, 2015 One piston is good and could be used again, three pistons are good on diameter and skirt but the ring grooves are badly worn top and bottom, causing up and down play on rings, result is broken rings. Bores are not worn, some slight scoring presumably caused by broken rings came out with a gentle hone. The jeep was used on an airfield with a PTO mounted generator so maybe this was caused by running on light / no load for extended periods? It seems pointless to strip out engine to either bore out to + 10 or bore out and sleeve back to original just so that new pistons can be used, hence our decision to use a used but serviceable set of pistons. It is interesting to note that somewhere along the line the size of a new standard piston has shrunk, to the extent that they cannot be used in original bores. Not that the requirement to do so is very likely, our situation must be quite unusual. Quote Link to comment Share on other sites More sharing options...
Grasshopper Posted August 25, 2015 Share Posted August 25, 2015 It seems pointless to strip out engine to either bore out to + 10 or bore out and sleeve back And there was me thinking that this thread was going to bet boring... Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted August 30, 2015 Author Share Posted August 30, 2015 (edited) Well the "pre-owned" pistons (job done by someone more experienced than me) worked a treat - problem solved and a nice quiet engine is the result. The most likely explanation comes from a very experienced Jeep engine rebuilder, who reckons pistons from the Willys approved wartime (and late 40s) supplier were notorious for being out of size spec - as much as plus or minus 5 thou. Normal practice was, and is, for pistons to be grouped in matched sets and blocks machined to suit as normal practice. But modern manufacture results in more accurate new pistons, thus oversize sets of such difference within the standard size category would not be readily available. It may be that nobody else gets caught by this, but if you do you now know what to do Edited August 30, 2015 by N.O.S. Quote Link to comment Share on other sites More sharing options...
deadline Posted September 24, 2015 Share Posted September 24, 2015 I don't think your knowledgeable engine re builder is right... First, there would be a TM of fits and allowances for the Jeep used by 4th or 5th echelon shops for rebuild. I highly doubt that a piston would be accepted if +/- .005 do you have any proof of this?? Second TM9-803A shows how to fit a piston to a block... that basically ensures that even if piston specs are off, you must go through as many pistons as you need to get the proper clearance (.003 or 10lbs on a tension gauge). Could an engine simply be slapped together? Sure... it would also die a quick and horrible death. Old wives tales are just that.... Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted September 25, 2015 Author Share Posted September 25, 2015 (edited) No, I have no proof that 5 thou was the upper limit of piston size range. This is what I was told by someone who has a good standing in the Jeep engine business here in the UK. But on the basis that we have found 3 sets which are 2 1/2 to 3 thou adrift, it seems quite plausible to me that that there will have been some pistons made with a greater sizing error. Whether it is actually 3 1/3, 4 or 5 thou is really not important to me in the great scheme of things. I accept the idea that pistons were made with a wide tolerance - how else could you explain the situation we encountered? Believe it or not, not every situation we encounter in life is covered by a TM :trustme: Edited September 25, 2015 by N.O.S. Quote Link to comment Share on other sites More sharing options...
Blackpowder44 Posted September 25, 2015 Share Posted September 25, 2015 Technical info. taken from EMERS . John Quote Link to comment Share on other sites More sharing options...
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