mopowers
Well-Known Member
My dad's getting ready to rebuild a 79 360. What is the max allowable cylinder wall clearance? I'm not sure if it would be the same as for the late 60's service manuals I have. Thanks!
Max Piston-Cyl Wall Clearance
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twayne24365
Well-Known Member
really depends in the pistons you are going to use, mist importantly the material of the piston..
nm9stheham
Well-Known Member
As above... cast (including hypers) run tighter than forged.
I've run .010" plus with forged pistons before. Revved to the moon and ran like a bat out of hell. Sounded like a diesel till it warmed up.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I had a 69 340 that was at .007 when I took it apart. Yeah it used a bit of oil, but I think most 340s did.Not enough to see blue out the tailpipe, but a quart between changes was common.I took a few other hi-milers apart that were also at or near .007.
I think my hypers went in at .0015; and they now have over 100,000 miles on them.
But if you're hoping to save a bore-job, I think that's not the way to look at this. When the running clearance gets sloppy, the rings can't seal properly as the piston moves around in the bore. Lack of ring seal sends the pressure into the crankcase; where it causes all kinds of problems. Of course lack of ring seal also reduces the cylinder pressure, which reduces performance. The tighter the running clearance is without causing damage,the better the ring seal can be, and the greater the performance potential of the short block will be. It's been said that the power of the engine is mostly created by the heads, but if the rings leak away the pressure.........well, it's hard to blow up a tire-tube with a hole in it.
I think my hypers went in at .0015; and they now have over 100,000 miles on them.
But if you're hoping to save a bore-job, I think that's not the way to look at this. When the running clearance gets sloppy, the rings can't seal properly as the piston moves around in the bore. Lack of ring seal sends the pressure into the crankcase; where it causes all kinds of problems. Of course lack of ring seal also reduces the cylinder pressure, which reduces performance. The tighter the running clearance is without causing damage,the better the ring seal can be, and the greater the performance potential of the short block will be. It's been said that the power of the engine is mostly created by the heads, but if the rings leak away the pressure.........well, it's hard to blow up a tire-tube with a hole in it.
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mopowers
Well-Known Member
The piston are stock 79 360 pistons. Is there an acceptable max tolerance? I realize too much clearance and lack of ring seal kills cylinder pressure. I'm just curious at what point a bore job and new pistons are necessary.
When the piston to bore clearance is too great, when there is too much taper in the bore or if the cylinder is too far out of round. If one of these measurements are exceeded or any combination of these measurements are close to be exceeded it should be bored.
ValiantOne
Well-Known Member
According to 1980 fsm for stock pistons, 360 engine 2 bbl, .0005 -.0015, with 4 bbl .001 -.002 specified clearance. I can't find a maximum cleareace
Max cyl bore out of round .005, max taper .010
Best
CE
Max cyl bore out of round .005, max taper .010
Best
CE
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ValiantOne
Well-Known Member
Fsm also shows piston is eliptical by about .011 larger when measured 45* between pin bore and thrust face, than across thrust face.
And
On sides piston should be .000-.005 larger when measured across thrust face at bottom of skirt as opposed to top of thrust face just undrr oil ring.
CE
And
On sides piston should be .000-.005 larger when measured across thrust face at bottom of skirt as opposed to top of thrust face just undrr oil ring.
CE
mopowers
Well-Known Member
Thanks for the input guys.
nm9stheham
Well-Known Member
BTW, those out-of-round and taper numbers are SERVICE limits, not limits to use as new. Service limits are are used to say 'It is not TOO badly worn and has some life in it.' But that does not mean it will run good and evenly. You can rest assured that a .010" taper in a bore will flex and wear out the compression rings in a hurry, and will not seal well. And oil ring tension will vary hugely all over the place with that much taper. I would never put together an engine with anything close to .010" bore taper; I did it just once with tapers up to .008" when in college and poor and you could tell instantly in the uneven running sound and feel. It was an old horse truck and it just needed to be able to move around....Good new bores have practically no taper.
Borrow a bore gauge or some telescoping gauges (touchy to use well but adequate for this work if used carefully).
While you are checking the bores, check the ring lands and ring side clearance with a new ring (or the equivalent). If they are sloppy and out of tolerance, the rings will flip up and down as they ride down an up in the bore. Bad compression seal and poor oil scraping by the 2nd rings will result, as well as shortened ring life.
Borrow a bore gauge or some telescoping gauges (touchy to use well but adequate for this work if used carefully).
While you are checking the bores, check the ring lands and ring side clearance with a new ring (or the equivalent). If they are sloppy and out of tolerance, the rings will flip up and down as they ride down an up in the bore. Bad compression seal and poor oil scraping by the 2nd rings will result, as well as shortened ring life.
Chryjoe
Active Member
She should be 3 - 5 depending on how you want to set it up loose or tight?
MOPAROFFICIAL
If it has tits, it's a liabilititty.
Depends on how much more life you are trying to get out of it, it would run with .020 for a while.
Very true and most important in a fresh build with application considered as it will effect which clearances are necessary.
Eh not always.
Was gettin sleepy, but great points to a point and to the point I go back to.. that it depends on what you are doing.
When you exceed half the cost of doing it the right way or aka new pistons and a hone job. Most detrimental is the out of round if the rings leak like crazy... the motor will burn and feel flat.
Good info for the op, this he can go off of to get some better idea of shape of the bores.
famous bob
mopar misfit
I ran a hemi w/ .014 clearance one week end, rings lasted about 3 runs . LOL
Captainkirk
Old School Mopar Warrior
As of much concern as the piston/cylinder clearance is, is also the cylinder taper top to bottom (though one should be reflected in the other). The majority of the taper wear is goning to take place in the upper 1/3 of the cylinder, right where you need the oil control and sealing the most. My engine machinist says he considers (for a performance motor) around .005 to be max allowable taper. Not to say it wouldn't run and operate fine under normal conditions, but if you are going so far as to rebuild the engine, starting with straight bores and new rings is a good beginning.
Captainkirk
Old School Mopar Warrior
whitepunkonnitro
Well-Known Member
Rule of thumb, is that stock pistons are fine to reuse with up to a .010 overbore, so I would assume .005 is the most clearance you would want to see, over the factory recomended max.
Captainkirk
Old School Mopar Warrior
mopowers
Well-Known Member
Well, considering the stock pistons all came out with no problem and with only the tiniest of ridges, I'm hoping the block will clean up with a simple hone job and not need new pistons. We'll see. He comes down to pick it up on the 18th. We'll know shortly after that.
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