Technical Piston ring oversize selection for stock piston reuse in refresh

Hello, looking for a guide in selecting piston rings for a refresh, not a rebuild. I have a 303 olds 1951, factory rod and main bearings are all perfect and in spec. The bores are within taper spec which is .0000 to .001 using a ring to measure change in gap in several places which there was none. The piston pins are play free, and piston fit in the bores are within factory manual specs which is .0005 to .001 and I cannot get a .0015 feeler between the skirt and bore which is the smallest feeler I have. I did have to take out a slight ridge to get the pistons out so I know there was some wear. I picked up a cheap $20 set of old stock Perfect Circle standard bore rings to check their fit and the new compression ring to piston groove vertical fit is .003 which is perfectly between the .002 and .004 spec. But its .021 to .026, depending on which cylinder, end gap exceeds the .008 to.016 spec. The new oil rings have an even worse end gap fit by a few thousandths though the manual calls for the same .008 to .016 fit. BTW the rings that came out of it were so worn that the gaps were literally 3/32" !

I have seen these old stock replacement rings labeled "STD to .019". Is this how they closed up the gap in the old days? File to fit I ***ume.


I'd prefer new to old stock (they can have a bit of shelf rust) as I can get new rings from Hastings in .010 and .020, which would be correct? Would the .020 be the same shape/size as the above ones, or is there a difference? I would hate to order .010 and there still be to big a gap and conversely if .010 is perfect I would not like to have to grind down a whole package of .020 down. I am not opposed to getting the old stock ones and cleaning them up as needed if they would be the best choice. Boring and new pistons could technically be possible but Id rather save the money for something else needed and on top of that I like to reuse stuff and fix it whenever possible.

 

At trade school we were taught .004" per inch of cylinder bore.

.bjb

I have seen these old stock replacement rings labeled "STD to .019". Is this how they closed up the gap in the old days? File to fit I ***ume.
View attachment 5660059

I'd prefer new to old stock (they can have a bit of shelf rust) as I can get new rings from Hastings in .010 and .020, which would be correct? Would the .020 be the same shape/size as the above ones, or is there a difference? I would hate to order .010 and there still be to big a gap and conversely if .010 is perfect I would not like to have to grind down a whole package of .020 down. I am not opposed to getting the old stock ones and cleaning them up as needed if they would be the best choice. Boring and new pistons could technically be possible but Id rather save the money for something else needed and on top of that I like to reuse stuff and fix it whenever possible.[/QUOTE]

 

Are you checking the ring gap at the bottom of the stroke?

 

Nope, I checked it at the top like the picture in the factory manual about an inch down.

 

With the tapper in the cylinders you want to check them at the bottom of the stroke or just above.

 

Just checked on, it is .020 at the top and a tight fit on .019 at the bottom, so probably .0185 with a difference of .0015. I know there is a way to figure taper from those numbers using cir***ference but its too much math for me. Either way, placed in the top or bottom the STD rings end gap is still too big.

 

I might be misunderstanding, but I believe the rings you spoke of about STD to .019 refers to the bore size, not the ring gap. If I’m correct (may not be) they would be filed to fit for a STD bore, filed less at ie a .010 oversized bore.

 

If you order 10 overs, the oil ring won't work. You can file the 2 top rings for end gap but the oil ring expander could be a problem. I wouldn't hesitate to use the stock rings as you have now. You be a little loose and nothing detrimental will happen. If you get the gap a little too tight and the ring ****s when the engine is warmed up under load, you might find out what "black death" looks like. When the availability of parts is limited, choosing the safest option is often the end result. I want to let you know that unless you use a dial bore gauge to check the cylinders, you will always be suspect in measuring. I have done the same as you before getting the dial bore gauge and my accuracy was never as good as it is with better tools. Good luck and have fun with your project.

 

I think that is correct, they are for bore sizes worn out to up to .019, unrelated to end gap. And I think also to be filed to fit. I am mostly curious if they are identical otherwise to a .020 ring or there is something special about them? Nobody makes new ones labeled as such with a STD to some oversize. All the new ones are simply labeled standard, .010 etc.

 

Are you sure you have the decimal point in the right place in your posts? In the first post you said that you can't get a .0015" feeler gauge between the cylinder and the piston skirt, and this is a engine that's ready for a rebuild/refresh? I don't think I've ever heard of piston to cylinder clearance as small as 1 and 1/2 thousandths of an inch.

Also you mention no taper can be detected in the cylinders, yet you had to cut a ridge out of the cylinders to remove the pistons.

Maybe I'm just mis-reading things and everyone else is on the same page with you?

Lynn

 

The factory manual has you inverting the piston in the hole, skirt up and measuring 90* to the pin at the tails of the skirt to cylinder wall. The clearance should be .0005 to .001. I cannot get a .0015 in there so it is up to spec. There is a second test using a .0015 ribbon 1/2" x 12" long attached to a scale, put ribbon in first and the piston shoved in past it and you pull out the ribbon with 10-18 pounds removal force required on the scale is acceptable, I do not have such a tool. This motor has everything looking great inside not counting the m***ive amount of sludge and flaky non-detergent oil residues. The ridge was just enough to catch a nail on and I was wanting to save the pistons if I could so rather than smack them out and risk damage I cut the ridge. It is measuring up well, its one of those high nickle blocks you hear about, but had nearly no compression due to excessively worn out rings with an end gap of .090 which presumably resulted in m***ive blow-by judging by the fact that the PO smashed over the road draft tube to seal it up!

 

@wfo guy the oil control ring may still work, standard to oversized. They have tension in the bores and the little s****er rings over lap as I recall. Like you mentioned you can’t file one, but my guess (could Ofcourse be wrong) is the same oil control ring is used on standard and say .010 rings, etc.

 

You have to remember,,,,,with the pistons inverted,,,,if you are still measuring off the top ridge .
The ridge will be standard bore,,,,,,no wear on the ridge .
If you have any ridge at all,,,,I’m sure the clearance will not be .0015 .
But,,,,you say you cut the ridge out .
Since the ridge was minimal,,,,I wouldn’t have been concerned about damage,,,,,many times more damage is done by using a ridge reamer,,,,,,they will cut into the bore at top and cause extra clearance .

Check the top grooves in the pistons,,,,,many times they are badly worn and will not let the rings seal .
The top grooves in mine were about.080 in some cases,,,,,not the gap,,,,,but the side groove clearance .

Pics would be a great addition to this situation,,,,,,there are guys on here that are the best I’ve ever seen .
I’m sure someone can help you .

Tommy

 

Yes, everybody likes pictures.
Standard rings I got off ebay, $20

Cleaned original pistons. Olds used forged pistons and rods in the early years, good parts worth saving if you can.


Original rings in bore, showing HUGE .090 end gap. These were so thin from wear and they had a bur all around them making them too thick to fit into the ring groove when put in wrong side in to measure groove wear.



Gaps top to bottom almost identical showing very little bore taper, again old rings with huge gaps.

Testing piston to bore with a .0015 feeler, no go except just the tip.

This shot really shows well the cut ridge marks, the tool, old style Snap-On lathe like cutter, did an excellent job still has factory bevel at the top.

New rings in bore, much better than old but gap is .021ish varying a bit hole to hole and top to bottom. And no I have not yet honed the cylinders but do plan to.

 

Looks pretty darn good,,,,if the ring grooves check good,,,,,,I would hone it and run the daylights out of it .
That little extra ring clearance will be minuscule in the big picture of things .
Better a little loose on the gap,,,,than getting up to temp and the rings ****ing solid .
Go for it man ,,,,,,those Olds are not too common anymore .

Tommy

 

When I was still in high school (cl*** of '72) you could still buy oversized rings up to .010, in increments of .001. Often when going to point .010 we honed the cylinders to get them as straight as possible and then knurled the pistons.

When you are freshening an engine and not boring you are always better off to buy file to fit rings. Then file them to the proper ring gap. Actually if I am freshening a low mile engine and am using drop in rings I still check ring gap. But I am a picky *******.

I actually suggested that piston to wall clearance could be checked using feeler gauges, as a rudimentary check. IE you are taking your engine to the machinist to be overhauled and want to be mildly knowledgeable when you talk to him. To got a proper idea of taper and piston to wall clearance you really need to use a micrometer. For just a quick freshening that is not usually done.

 

Don't know why I bother to bring it up, but this old-school legend won't die. At the machine shop, we hear it from just about every obsolete marque which is brought in. Back when, we'd argue with the customer. Now, when a Hudson, Studebaker, Packard, Olds, Cad, et al, guy says, "This is one of them high nickle blocks, ya know." We say, "Yeah, gonna have to charge you double for the machine work because it wears out our tooling."

Nope, cast pistons.

jack vines

 

That's not a forged piston.

 

I could be totally wrong on both points. I heard both things from various sources over the years. But looking in the manual I cannot find a reference to either point. All that is said about the pistons is that they are 8.0 to 1 ratio.

 

You can tell by the construction of the piston, it has steel plates cast into the wristpin bosses on each side.

 

Your new ring fit is just fine. Do a quick glaze breaker hone or a carbon ball clean up and run it. The best thing you can do is retro new positive seals on the valves. Last year I had the heads off my Y Block and bought the cutter and seals from Comp Cams and did the work myself. The wear on the stems were within spec and I lapped the valves after cleaning up everything with light wire brushes.
Adding a PCV system and the engine is smooth and quiet.
Under no cir***stances use 10 over and file them their tension will be too high. Call Grant Piston rings in Anaheim Ca. and ask them.

 

Crusty, just for kicks have you seen these
New 1949 1953 Oldsmobile Pop Piston | The H.A.M.B. (jalopyjournal.com)
with rings
I'd be tempted to think hard on these but I can be foolish in my old age

 

Man,,,,those are beautiful ,,,,,,,make a really nice mill,,,,,,,look like they are forged as well .

Tommy

 

Thanks, I am easily tempted too by shiny new bits and sent him a PM asking for more details. I think they may be .060 oversize though. I have been known to be a collector of bits for future use. I turn 51 this year so I have plenty of time left in me to wear this one out and build it again.

 

Thank you, this is the stuff I want to hear, good or bad.

My heads are obviously off and I did intend to actually bring them to the machine shop for going over as I just do not know what a home shop build should or should not attempt to do on them. Might need a new thread on that! I know the valve stems are a bit worn on the tips and ***ume the guides and valve stems themselves possibly as well. Seems to be the only real point of wear on the whole motor, I ***ume this is from weak oiling up there relying on mist inside the valve cover coming out a small hole at the push rod end of the rockers, most that were clogged. I will mention about using the new type seals.

I will fit a PCV into the back of the block at the original road draft tube hole. I have already tried and a standard SBC valve cover PCV grommet is a very tight fit in the hole and once you shove the valve in it is really stuck well in there. If the factory WCFB turns out to be good to reuse I will tap the intake for a vacuum port or otherwise use a Edelbrock 500CFM carb like I have on my 56 Pontiac 316 that performs fairly well and has the vacuum port on the backside.

 

Yes, a PCV is a good idea, but it will require a bit of tuning, as '50s carb idle circuits were not designed to expect unmetered air being added below the throttle plates. Beg, borrow or buy an O2 meter to tune the idle and transition circuits.

BTW, those pop-up pistons are a steal deal on price. Can't know for sure from the photo, but they appear to have a big enough pop-up to get your compression beyond street use on pump gas.

jack vines

 

Just put the standard size rings in, you'll be fine. btdt

 

That’s true they did not plan on a leak but the leak is metered with the use of a correct PCV valve. I use 2 Carter WCFB’s from 1953 Oldsmobiles on my 296” Ford Y-Block and they performed perfect before and after my system was installed.
If you seal your engine and take a vacuum on the crankcase, a pvc system was designed so there should be 1 to 3 inches of vacuum at idle. The system I choose mimicked the 1962 Ford/Mercury car including the correct valve. The reading was 1.5 inches of vacuum and I did not increase the idle mixture screws.
Worn throttle shafts, and choke heaters are all planned for as vacuum leaks by manufacturers. I’m sure our 70 year old engines will be fine with the added system. Maybe mine having 8 throttle plate leaks just worked out.

 

How sealed does the engine need to be? I know remember I tried to add a PCV to our 56 Pontiac 316 by sealing up the breathers in the valve covers and putting a PCV valve in the valley where the road raft tube went connected to the Edelbrock carb. It kept blowing out the dip stick and oil spray from the dip stick tube as there was too much positive pressure. I eventually just put the road draft tube back on. On the Olds there is only the single fill tube with breather for incoming air at the front and the road draft tube hole at the back for outgoing.

 

You need a vent, in addition to the PCV valve, because at higher RPM even a very good condition engine will produce more blowby than the PCV valve can deal with. On that Pontiac, you should have left the valve cover breathers alone, and just replaced the draft tube with the PCV valve.