I have just pulled down a 390 Cadillac engine out of a '59 for a rebuild. I am going to install an Isky 262 cam, give the heads a little bit of slap and tickle and drop on a tri power setup, however nothing crazy. The odometer says 56,650 miles, and based on the condition of the motor, I wonder if this may be correct as it is a tidy car. The engine was running on 7, however I am confident this was an ignition issue. I used an inside mic to measure the ridge on the bore, and it is around 0.001". All of the bearings have worn evenly, as have the pistons. The new factory piston to bore clearance is 0.0013" to 0.0017". The clearances on this engine range between 0.0035" & 0.007". The new factory compression ring to groove clearance is 0.0022" to 0.0035". The lower rings measure 0.0003", however the top rings have 0.008" clearance. My question is could I re-use the pistons with a new set of rings and a hand hone? They are Cadillac pistons, and I would rather use original quality pistons rather than take my chances with new stuff....so much of it seems to some from China now. Really would appreciate some feedback from those with more experience than me
Rering and put in new bearings and it will give you a good life. Give it a good old fashion valve grind, clean out all the oil gallery's - check the oil pump for any wear and you will be good to go
Rering jobs use to be commom. So, I agree with 35desoto, freshen up the bottom with rings and bearings. Valve job on the head with fresh gaskets and seals all around. Will is last as long as a full rebuild? Probably not. Just don't drive like a teenager and keep the oil clean. Good luck which ever way you go. -Dave
One more thought. If it were me, I'd want to check the big end of the rods and have the crank polished. That is if the crank checks out and doesn't to go undersize. Peace of mind is better than pieces of an engine. -Dave
As far as pistons go, there are cheap ones from China and there are NASA quality ones available. It all depends on how much you want to spend. A quick hone and rering job with bearings and gaskets will probably be around $500 and a good rebuild will be around $2500.
Been thinking, but not real fast. If the pistons you pull out are in good shape. Then, whether you need to replace them or not comes down to the condition of the cylinders. Once it's all apart, measure the cylinders to make sure they aren't barrel shaped or other wise worn. With this information you should be able to determine if you need to go oversize and buy new pistons. Still think a good rering would put you on the road. -Dave
Taper is what kills rings less than .005 will last a while. Question are you going to drive it another 100,000 miles
The pistons and new rings can go back in honed cylinders. Your "give the heads a little bit of slap and tickle" includes hard exhaust seats? jack vines
Just be sure to make use of a ridge reamer to clean up the bore. Then you will be good to go with a hone and rering it and enjoy.
Clean and inspect the pistons very carefully. Look for hairline cracks on each side inside near the pins. Ream the ridge off, then hone. This was SOP fifty years ago. In a rod, it may last the rest of your life.
How wide/thick are the old rings? If new unworn rings are wider and got the ring-to-piston side clearance down to 0.005" or so full depth, and other dimensions are good, and the pistons are crack free, then I'd consider reusing them. Deves says over 0.006" is too much for re-ringing re-use. http://www.deves.com/pistonrings_inst.php http://www.imperialclub.com/Repair/Lit/Master/002/page17.htm http://www.imperialclub.com/Repair/Lit/Master/002/page23.htm
Wow, thanks for the replies and wisdom. That's why I like this forum so much! The consensus is clearly to re-ring and put it back together. I measured the bores for roundness using an inside mic (not as precise as a bore gauge): 1: 4.0025 x 4.0035 2: 4.004 x 4.0035 3: 4.004 x 4.003 4: 4.004 x 4.0032 5: 4.0038 x 4.004 6: 4.0038 x 4.003 7: 4.0035 x 4.0035 8: 4.004 x 4.004 So the biggest roundness variation is 0.001" With regards to the top ring gap, has anyone ever heard of thicker compression rings? It would be handy if there was a +0.004" or +0.006" ring available. I plan to replace the big end and main bearings at the same time. The crank surfaces are in great condition with no grooves or scoring. I also have an oil pump rebuild kit which I will install. Your "give the heads a little bit of slap and tickle" includes hard exhaust seats? With regards to the heads, I have read here and also on another forum that Cadillac heads have a higher nickel content, and as a result are harder. I am in two minds whether I install hard exhaust seats, or just keep using 'valve saver' upper cylinder lubricant.
If I understand your question on the rings, you would like to them to make up for a slightly larger than standard bore? What you do is buy oversize rings and grind the end to set the ring gap to specs. A lot of people have run standard valve seats with unleaded gas and no additive and have no problems.
You can knurl the pistons if they are loose. Sounds like the cylinders are good. I would use the old pistons.
Glad some of us still remember the "old ways" our knowledge base needs to be passed on to the newer guys.
My concern is more around the larger ring to groove clearance (i.e. 0.008" vs 0.002" when new). I have sent an email to Hastings to see if they can tell me if any of their 2mm ring sets could be used with the Cadillac pistons. 2mm is 0.006"larger than 5/64 - this would bring the ring gap back to 0.002"...perfect!!
Look for shops that have been around a long time. I got a old Hastings knurling machine that came with the manual most shops knurl both sides but this manual said only knurl the non thrust side. I don't know if they are available any more but the used to offer shims for ring grooves.
Every auto machine shop used to have a machine to do this. It is not very difficult or time consuming. The local shop used to have one that printed "Perfect Circle" no doubt furnished by the Perfect Circle piston ring people. I expect sunbeam's Hastings model prints Hastings? The machine sort of clamps the piston and presses the skirt against a cylinder with a pattern on it. The operator turns a handle and presses a pattern into the piston. This expands the skirt slightly and leaves raised ridges and bumps. Around 1960 I read in Hot Rod magazine about a hot rod 283 Chev engine built for the street, but using forged solid skirt pistons. These were supposed to be heavy duty racing only parts because they had to be installed with .012 clearance and they slap like mad until they warm up. The mechanic fitted them to the correct clearance then knurled them to standard size. They never slapped or made noise and when the article was written the motor had 30,000 miles with no problems. This is the most creative example of knurled pistons I know of.
The hasting unit just uses a standard knurl like a lathe tool It rotates the piston back and forth about an 1/8 of a turn. The piston is in upside down and you feed the tool up by hand. You don't even have to remove the rods.
Was the 0.008 " side clearance with a used/worn ring? Would the 0.008" feeler gage go full ring depth, or half way? The ring groove is likely worn tapered and only as deep at the original ring as it lost some face material. The back side might have had a chamfer, and the wear will follow that feature too. http://www.tpub.com/engine2/en2_files/image104.jpg\ I'd only put stock inch sized (5/64") inch rings in the groove, and only if the side clearance with a new inch width ring was less than 0.006"..
Hi Dan, The feeler gauges go full depth with the used rings. I had the ring pushed all the way into the groove when I measured it. I will go and have another look tonight and see if I can see any steps in the groove.
They used to sell a spacer ring to correct that problem... you had a hand tool that opened up the ring groove a little and you put in a flat ring.. kinda like a 3 piece oil ring rail under the ring. They were sold under the name GI60... I put in a lot of them in the 70's
All your clearances say to me that you should reuse the existing pistons with maybe what we called land rings in the top ring to take the "float" out of that one. When the rings have too much side clearance they end up with a "flutter" which helps break the ring and cause loss of compression, ring groove damage and ultimately piston collapse. However all the piston and bore measurements you have quoted suggest that this engine will give you good service with a hone , new rings, new big and main bearings and the usual clean out of all oil passages and water ways. With new rings it is a good idea to freshen up the valve seats and valve stem seals.
I have pretty well arrived at the same answer, although if Hastings have a 2mm ring with the same axial width I will have a go at that for the top ring. This would be better than a spacer IMO. I have a local guy who does heads, so I am going to drop them off with him tomorrow. He will give them a clean and then crack test them. If they are OK I will get him to clean up the surface, install k-lines, cut a 3 angle valve job and also relieve the back of the valves. I can then gasket match the ports, polish the chambers and do a little bit of port work (mainly in the exhaust). Not looking for anything crazy, just trying to increase efficiency.
Find a good old machine shop to knurl the piston skirts as others have said. Pistons can then be fit to each cylinder by knocking down the knurling slightly with crocus cloth after trial fit of each piston. Did many, many engines both street and oval track in the 60's this way. Bonus effect is that oil stays in knurling "grooves" for better lubrication and cold start lubrication.
OK rather then read the entire thread here are the numbers you gave us: .0013 to .0017 and the numbers you gave us are .0035 and .007 so unless you measured it wrong or wrote your numbers down wrong you range from .0018 to .0053 beyond maximum tolerance. So you tell me do you need new pistons? measure your bores from the top to the bottom and see how much taper you have also measure them cross wise and make sure that they are round. of the cylinders are out of round or you have very much taper you will need to bore it, or if a new set of stock pistons doesn't get you within tolerance you will need to bore it. I can tell you this even if you knurl the existing pistons you are not going to pick up .0035 and that is your tightest piston.
Maybe I missed it, but assuming it has the original pistons do we know if they were cast or forged? I could see Cadillac using cast pistons to keep things quiet on a cold start but I could also imagine them using a forged piston for more durability. And that would make a difference in clearance as well.
The Jalopy Journal
