Opinion about piston damage

[Roelof]

Hi guys,

After tree years of welding, bodywork and rebuilding my 140 '66 monza is almost ready to get it road approved and get plates on it. It was a lot more work than anticipated, due to other activities and life it took a lot more time then anticipated, but it was fun and the car looks good.

The car after finishing bodywork

But first, i wanted to fix an oil leak before i go to the test facility. So i started disassembling and now, a few evenings later, i am looking at a big chunk of greasy metal that is sitting next to the car. Why? First of all: i found out why compression was low on the left bank. The head nuts were on almost finger-tight. The squeeking fan bearing was not a bearing after all... And then I saw the greasy blok and thought the family would complain about the smell from the heater...

Engine

So far so good. Having fun. The block looks looks original and it seems like is has not been opened before. Wear is little and well within limits. There are some marks on the cylinder walls, but no ridge, nothing I can feel, and i think honing and new rings will do. I will measure. The pistons look good and It looks like I can reuse them.

Dirt in and out

Clean parts!

Some marks

Good piston

All but one.... When i was rebuilding the carbs I wondered where that one screw of the butterfly in the left carb went. Now I know. Cylinder number six. The screw that was missing has left some marks on the head and the piston. The cylinder wall is ok. As you can see there are marks on the head but i think i'll look how it cleans up and i can get away wihth reusing the head. (And i am on the other side of the ocean, so just finding a new 140 head around the corner is not going to happen).

Head damage

Good part of head

The piston worries me more. What do you think about this kind of damage? It is not really cleaned yet, but as you can see there is quite some damage. So what are my options?

Piston damage

Can I just replace one piston? Or do I get weight issues with mixing original and new pistons? Or is it better to find just a good used piston? And what is your opinion on the head?

I know the best option is just to get a new set of pistons and replace the lot, but then I might get cought in the "i might as well" loop..... When i buy new pistons i might as well get the cylinders bored oversize, and the old head on a new set of pistons is an accident waiting to happen, so i might as well replace the seats. And the valve guides and springs while i am at it... And then I might as well replace the camshaft,. The bearings....etc. etc. I was not really planning to get into that. I was fixing an oil leak...

So please help me out. I have some rebuilding skills and have done some engines but i am not a pro. By far. And i do not have any reference regarding corvairs, so I am in the dark here.

Thanks,
Roelof

[Jerry Whitt]

The top of the piston does not look too bad. The real concern would be the edge of the piston. Should the top be crushed down on one of the compression rings, bigger problem, and the piston would have to be replaced.

The pictures here look like normal wear and tear. Cylinder hone with fresh rings will probably be ok. Have you measured cylinder taper?
If taper is excessive bore to next size.

[Roelof]

Thanks for the fast reply. Here is a better pic of the piston. I just walked to the garage and cleaned it a little. It just seems to be surface damage and some damage around the edge. The edge is not crushed down. It was just the head of the screw.

Maybe just sand it down a little to take of the sharp edges?

A fast measurement shows the cylinders ok and not tapered, but i have to get the right tools to be sure.

Roelof

[66vairguy]

Just my opinion based on my experiences - You are at a point of either doing some minor repairs, or going for a full rebuild = $$$$$$$

If the car is only for fun driving and occasional use, then it won't see a lot of mileage. So I'd just make minor repairs assuming the engine ran reasonably well (no smoking, etc.). The stock cast pistons do have a reputation for breaking occasionally at the oil control ring(s) slot. If that area is fine, and you indicated the ring slot is good, then just knock off the sharp edges of the piston damage and take a chance. I'd clean the pistons to make sure the rings are free of carbon (DO NOT REMOVE THE RINGS). Make sure the head gasket area in the heads is good (a loose head can cause leakage at the gaskets and burn them) and do install NEW head gaskets. IGNORE the manual on head torque values as GM changed it several times. Now the accepted values are - first torque pass is 15ft.lbs, then 20, then 25, and finally 30ft. lbs. If a head stud pulls out (it happens), then don't panic as it can be fixed with a Time Sert (no helicoil).

Yes the pistons were balanced, but the factory ONLY matched opposing pairs for weight, so not all the pistons were the same weight in one engine. Yes a shop can machine a good used piston to weight the same as the one you are replacing IF the replacement piston weights more. Keep the rods where they go (they are numbered) and don't put a piston in backwards (dot indentation in top shows proper orientation).

Your other choice is a full rebuild that is time consuming and expensive. It looks like you have the 140HP heads, if so then new deeper seats are a must for a proper head rebuild. Forged pistons are recommended, unlike cast pistons most forged pistons require a larger 0.030" clearance (wall to piston clearance 0.015" EACH side).

Good luck

[toytron]

How did that butterfly screw come loose? Was it a rebuild? Did someone forget to stake the screw? I would check the carbs and make sure this doesn't happen again.

Ed Stevenson

[Roelof]

Looking at the state of the secondary carbs i think the primaries were rebuilt at some point. One of the screws in the left primary was missing the head.

The car ran reasonably well (with the heads loose:)) but It smoked and had leaking carbs. It is an original and complete 140 hp PG. The studs are all ok. And I already removed the rings. I was thinking of a basic rebuild/overhaul. Replace all seals, new valve stem seals, check and lap valves, hone cylinders (if within specs, but it looks like it) and new std rings. The crank is clean and seems within specs. I am not sure about replacing the main and rod bearings. The old ones are worn evenly, but have been used quite a while. So i might go for a new set of std bearings.

I tend to change the piston for a good used one. If i can find one. Or buy a single new one, but I don't know if i can mix and old and new. But then again: I am not a rebuilder and do not know what is acceptable or not.

I am aiming for stock. The car will be a weekend driver. But not a trailer queen.

Roelof

[66vairguy]

If you re-use the old pistons, then check the gap between ring and piston slot (ring fits in) to make sure it is within spec.

The 140 nitrided crankshafts area usually within spec. The bearing surfaces are harder due to the nitriding process and only suffer if the oil is not changed over long periods of time, or the engine is run without oil pressure. If the bearing surfaces look smooth it should be within spec. You can plastiguage the bearings to see if they are within spec. If you decide to install new bearings then the crankshaft bearing surfaces should be polished by a professional machine shop (occasionally the bearing surfaces have a film that builds up and must be removed when new bearings are installed to insure proper clearance).

The 140 heads are notorious for dropping valve seats, which can cause some costly damage. For reasons not understood it has been reported that machine "lapping" the valve seats tends to lead to valve seat failure. It might be a heat issue and may not be an issue if you "hand" lap (slowly) the seats - not sure. Several valve seat "fixes" have been tried over the years and the ONLY fix demonstrated to work over time is to install "deep" valve seats, new guides, and valves. Costly and you should find someone experienced do it, a problem with your location.

NOTE: if the valve guides are worn beyond spec.. then this results in valve movement that is damaging to the valve seats as the valves close. Also only the intake valves use seals on a Corvair engine.

[toytron]

Looking at the state of the secondary carbs i think the primaries were rebuilt at some point. One of the screws in the left primary was missing the head.

Roelof

Hi Roelof,

What might have happened is that someone tried to either remove or check the tightness of the screws on the butterfly and unknowingly corrupted the tensile strength of the brass screw. In order to remove the remaining screw thread you might have to try and work it out from the staked end (after removing the carb). If you feel that this is too much then have someone reputable rebuild it. You can find them on this site. If you do try and tackle this yourself make sure you stake the new screw threads so the screw does not fall out after it is replaced. Staking involves "damaging" the end of the screw after it is installed so it does not work itself out. Some people use a pair of side cutters and lightly but firmly squeeze the end of the screw others use a punch to the area where the screw and shaft meet. I am sure there are other ways to accomplish this too. Just make sure that if you use the side cutter method you don't fracture the end of the screw leaving it to dangle. If that is the case then clip it completely off and make sure you don't leave the cut off piece in the carb.

Ed Stevenson

[66vairguy]

Clark's sells replacement throttle carburetor shafts that come with the butterfly screws. Sometimes folks replace the throttle shafts during a rebuild. If the both levers on the throttle shaft are held on with screws, then the throttle shaft was replaced.

[Roelof]

Thanks for all the good advice.

I already rebuilt the carbs and replaced the screw. The shafts were not worn and seem to be original. So someone might have tried to tighten the screw and broke it. Or it is just old and fell off. I staked the end of the screw, but having seen the damage i'll double check the other ones. Without breaking anything... :)

Overall I am happy with the condition of the engine. I do not find much wear. Bearings look a bit old but smooth and i am lucky that all seems within specs. I cleaned the pistons up last evening. Five look good and can be used. Number six has some damage around the edge and i don't want chips of metal coming off. And if i file the sharp edges off the gap will be to big. So i am in for another one.

Given the overall condition of the engine i'll go for a used one. If i start replacing a lot of parts i am in for an expensive rebuild and i don't think that is necessary at this point.

Roelof