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By slumlord444 - 13 Years Ago
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I rebulit my 312 originaly in 1969 and bored it .040 over with pistons from my local Ford dealer, or at least thats what I remember. Rebuit it again in 1980 and had to replace one piston. Got it from Ford. Pulled the heads today to go through it again and discovered that the pistons are stamped .0425. Memory fails me on this one. All I can remember is it was .040 over. Depending on the condition of the pistons when I finish disasembling the engine I may want to go with new forged pistons. Am I going to be able to use .040 over forged pistons in the existing bore, or am I going to have to re-bore to .060 over? Checked my Ford parts book and they show the 312 pistons as .020 plus, .030 plus, and .040 plus. 292 pistons are shown as .020, .030, and .040. No plus. Anybody able to shed any light on this mess?
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By slumlord444 - 13 Years Ago
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Surely somone on a Y-Block website knows something about this??
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By aussiebill - 13 Years Ago
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slumlord444 (2/8/2012)
I rebulit my 312 originaly in 1969 and bored it .040 over with pistons from my local Ford dealer, or at least thats what I remember. Rebuit it again in 1980 and had to replace one piston. Got it from Ford. Pulled the heads today to go through it again and discovered that the pistons are stamped .0425. Memory fails me on this one. All I can remember is it was .040 over. Depending on the condition of the pistons when I finish disasembling the engine I may want to go with new forged pistons. Am I going to be able to use .040 over forged pistons in the existing bore, or am I going to have to re-bore to .060 over? Checked my Ford parts book and they show the 312 pistons as .020 plus, .030 plus, and .040 plus. 292 pistons are shown as .020, .030, and .040. No plus. Anybody able to shed any light on this mess?OK, leave the maybes out and deal with what your actual bore sizes mike up at? then you will know if you may only need to hone them for the forged pistons, they run more clearance than cast, but until you measure everything you,re guessing, also check what forged slugs are available, i,m sure something will work out. regards bill
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By Ted - 13 Years Ago
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As Bill suggests, mic the cylinders and the pistons and verify what you have. Nothing says there were not some +002½”over 0.040 over pistons for special cases and you got a set. If the pistons are indeed 0.0025" over a normal bore size, then in the back of my mind the 0.0425” pistons are made with either cleaning up an existing 0.040” bore in mind or being a fix for 0.040” bores that were overshot during the initial machine work. Sort of like the red and blue pistons that were available from Ford in the standard sizes to compensate for variability in the standard bores. With myself doing a large number of custom pistons, I do odd ball bores pretty regularly simply to minimize taking any more out of a cylinder than is absolutely necessary. The key here is insuring that a ring set for whatever bore size I target for is actually available before ordering the pistons.
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By 314 - 13 Years Ago
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there were a lot of those .0425 pistons made fot the 312.it was the only size made like that i ever heard of.no idea why.
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By slumlord444 - 13 Years Ago
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Did an approximate measure of the bore with an inside snap guage and an outside mike. Came up with 3.850 or 3.851. If the pistons are 3.8425 that would give me .0085 piston clearance. Does that sound about right? Will pull the pistons tomorrow and mike the pistons. Tying to locate an inside mike to get an accurate measurement on the bore. I am thinking new .040 over pistons will measure 3.840 and end up .011 piston cleareance which I think is way too much. Need to do some research and find an inside mike.
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By charliemccraney - 13 Years Ago
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First I'd get it measured exactly. Check for taper and eccentricity (out of round). But if it is that big, I'd say it's time to go to .050 or .060 over.
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By slumlord444 - 13 Years Ago
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Actualy the .0085 is about right for piston clearance. Think I was one decimal place off. Big difference. My old Hot Rod Magazine engine manual recomends .003 to .007 for street and up to .012 for racing so the .0085 would be good. Found an inside mike to borrow next week. Pulled the pistons today and miked them. They are definately 3.8425. Pistons checked out fine so far so my plan it to reuse them and re-ring it and install new rod and main bearingw while it is apart. ARP rod bolts, head bolts, and main bolts also. Problem now is rings. Checked bill from the 1979 and used .040 over rings then. I need to check the end gap on the old rings. If there is a problem, not sure if I will be able to get the .0425 rings. Could go with .060 rings and file them to fit but that would be a pain. Will measure and check availability. Good news is that the cam I thought was flat looks fine and I plan to re use it. Lack of performance was due to low compression.
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By NoShortcuts - 13 Years Ago
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In going through an unfired Ford authorized remanufacturer's 312 in the early '80s before selling it, I found that it had Ford 0.0425 cast pistons in it, as I recall.
Weird!?!
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By slumlord444 - 13 Years Ago
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Thats what I keep telling myself!
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By Ted - 13 Years Ago
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By the book, the wear limit is 0.0045” for the original cast pistons. When new, the oem piston to wall clearance was 0.0015-0.0025”.
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By slumlord444 - 13 Years Ago
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Ted, checked my Ford Shop Manual and you are dead on. I got my numbers from the 1962 Hot Rod Performance Manual that I originaly got most of my build info from back in 1968 when I originaly built this engine. Not sure where they got those numbers. Re buit it in '80 after spinning a rod bearing in it when I missed a shift several years before. Put a spare engine in it and left the race motor set for some time. Also had a cracked skirt on one piston at that time. Put it back together and been driving it since then with no major problems other than performance gradualy dropped off. No piston noise or excessive oil use. Checking my records I put standard .040 over rings in it at that time. No idea what rings I used in '68. I am looking for the proper rings now. Grant offers something that should work and I will check other sources. I will have a retired mechanic friend who is much better with micromoters and decimal points that I am check out everything. I think with new proper sized rings and Mummert heads I should be fine. I may have the block decked just to make sure head gasket problem does not come back. I think I will have machine shop do the cylinder hone job while it is there and drill and tap the mains for Mummerts ARP main bolts. I will use ARP rod and head bolts also. This will probably be the last time I go through this engine so I want it as right as I can make it without going bankrupt and get as much performance out of it as I can and keep it streeatable.
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By slumlord444 - 13 Years Ago
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Finaly got an inside micromoter and measured the bore and the pistions. Near as I can tell the bore is 3.847 and the pistons are 3.8425. This gives me .0045 skirt to cylinder clearance which is on the loose side but I can live with that. The Ford shop manual shows .0045 as the piston to bore clearance-bottom of skirt wear limit.The problem is coming up with proper rings. Checked my records and used .040 over rings when I rebuilt it in 1980. Got no idea what I used in 1969. Checked the ring gap with the current .040 over rings and came up with close to .050 which is way too much. Not having any luck finding rings close to 3.847. Found 3.850 rings but they are metric and the ring thickness don't match up. My choices seem to be using 3.840 rings which worked last time but I am concerned with the ring gap. Or I could use 3.860 rings and file them to get the proper end gap. My concern is that if I do that will the curvature of the ring fit the bore properly and seal since they are being squeezed down to fit the bore? This may or may not be a problem since the 3.840 rings in the 3.847 bore workd fairly well . Question is would I be better off using the 3.840 rings with a large end gap or use 3.860 rings and file them for the proper end gap? Unless someone knows where I can get 3.847 rings? I am thinking bad things for my Ford dealer selling me these pistons in 1969!
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By charliemccraney - 13 Years Ago
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Since "This will probably be the last time I go through this engine so I want it as right as I can make it without going bankrupt and get as much performance out of it as I can and keep it streeatable," I'd bite the bullet and get the next oversize. It'll solve the ring issue, too.
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By NoShortcuts - 13 Years Ago
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What follows is lifted from a Perfect Circle Doctor of Motors Service Manual, printed 9/1973, page 64... (you can tell it's old because it was printed in the U.S.A.!)
"It is important that all rings have at least the minimum gap which is necessary to provide for the difference in expansion which may occur between the piston ring and the cylinder."
Ring diameter 3 to 3-31/32 . . . . . .010
"However, maximum end clearance tolerance is not critical and may be several times the minimum without affecting engine performance. A conservative maximum end clearance is one which is .040" greater than the minimum."
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Engines are being machined and assembled with much closer tolerances than they were 40 years ago. The ring gap you presently have is not ideal from a performance standpoint today.
As I 'effort' to put my latest engine together, I continually have to remind myself how I'm really going to use it so that I don't slip into 'overkill' and so that I don't sink so much into building it that I can't enjoy driving it when I get done...
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By slumlord444 - 13 Years Ago
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.040 over rings with .040 t0 .050 end gap will run and work but I am not too thrilled about it. Anyone have thoughts on how .060 over rings filed down to the proper end gap will work? I am concerened with the curvature of the rings sealing to the cylinder walls properly.
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By aussiebill - 13 Years Ago
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slumlord444 (2/15/2012)
.040 over rings with .040 t0 .050 end gap will run and work but I am not too thrilled about it. Anyone have thoughts on how .060 over rings filed down to the proper end gap will work? I am concerened with the curvature of the rings sealing to the cylinder walls properly.I dont think your calculations re the large gap is right, that to me is far too big a gap.
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By slumlord444 - 13 Years Ago
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The gap was measured with a feeler gauge and the ring I just took out of the engine about 1" down the bore. Rings have probably 5,000 miles on them, more or less. Yes i know the gap is too large per specs but the engine ran prety well that way untill the valves or valve guides gave out. Leak down test showed a couple of exhaust valves leaking badly and also the head gaskes were leaking. Still trying to find out if .040 over rings with a large end gap or filing .060 over rings down to the proper gap is the lesser of the two evils unless someone can tell me where to find .050 over rings, which would be fairliy easy to file down to fit mhy 3.847 bore.
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By slumlord444 - 13 Years Ago
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Still trying to find rings that or close enough to file down. Local machine shop who will be decking the block and honeing it tried to find oversize file to fit rings from his suppliers for building stock car engines. No luck there. His suggestion was to use .060 over rings and file them to fit. Said he would like to try a .060 over ring in the bore and see how the curvature of the ring fit the bore. Anyone out there got a used .060 over 312 top ring laying around they could spare? Willing to pay postage and reasonable fee.
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By Hoosier Hurricane - 13 Years Ago
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Slumlord: I probably have a used .060 ring somewhere, if I can identify it. I'll look.
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By 314 - 13 Years Ago
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if your worried about cost why dont you look for a used set of .060 pistons.a bore job is not that much.thats if your block will clean up at .060.the way your going you will probly end with a fair engine at best.
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By charliemccraney - 13 Years Ago
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I agree. If you can't afford to do it right now, then you can't afford to do it again. It really is time for the next oversize if you want to do it right.
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By slumlord444 - 13 Years Ago
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Trust me, I do realize that boring to .060 may have been the solution. I never thought about used pistons. Good idea. I actualy found a set of rings today. With all the info and help that is available on line, we sometimes forget that low tech sometimes works better. I had e-mailed several ring manufacturers including Grant, and all said they could not help me. Just for the heck of it I decided to call Grant Rings today. Talked to a nice Tech guy who was actualy in the plant when I called. Had a bit of an accent but knew exactly what I was talking about and took all my info on bore size and ring dimensions. Came up with a set of .050 over rings in cast or chrome. $120 for chrome direct from them but a lot cheaper than a re- bore and new pistons. Bore is 3.847 so filing 3.850 rings to fit should be no problem. What I am prety sure he did was to look up invidual rings by dimension and basicly build a set from the different standard applications that they had. I was able to find the correct size rings on their web site but one set had the right top ring and the correct oil ring but the wrong second ring. He just pieced it together. No problem. Love this guy! Thanks to all for your suggestions. Maybe this info will help someone else in the future
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By Hoosier Hurricane - 13 Years Ago
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Slumlord: Found a .060 ring, but see you no longer need it. Also found a set of used .060 pistons that appear to be low mileage, but obviously you won't need them either. Glad you found the rings you need.
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By slumlord444 - 13 Years Ago
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Thanks for looking. Hopefully I won't need them. Should get rings next week. Will let you know how they work out.
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By slumlord444 - 13 Years Ago
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The rings came in from Grant today. Stuck a top ring into the bore to see what I had. About .009 end gap. According to the instructions that came with the rings I should have .003 to .004 per inch of cylinder diameter. That comes out to .0115 to .0153 as near as I can figure. My Ford Shop Manual says .012 to .029 with a standard bore. I am thinking somewhere in the middle of those numbers should be fine. Should be a lot better than the .050 I had with .040 over rings. It should be no problem to get the clearance right with a little careful fileing. What exact gap would the experts here recomend?
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By charliemccraney - 13 Years Ago
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I'd go with the ring manufacturers suggestion. They know their rings best.
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By slumlord444 - 13 Years Ago
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That makes sense. Any idea weither it is any better to go on the tight or wide end of their range?
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By charliemccraney - 13 Years Ago
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Wider if you'll abuse it now and then. Narrower for a tame cruiser.
Do they give only that one range? Some manufacturers ofter different ranges depending on the type or use of the engine.
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