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pistons wrong way round

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tonyk

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Hi, '67 BJ8. First query here, although I've been working back from page 340 or so for the past several weeks.
Anyway, I took the cylinder head off last week to check valves, seats guides etc and replace disintegrated seals. The head is in the machine shop for light skimming and flattening (minor blow-through in the gasket) and a basic valve job.

However, I noticed 2 pistons are in the wrong way round - #2 and 4 have the "front" stamp facing backwards, the others are OK. I haven't noticed any rattles or slaps. Compression on cold cranking earlier gave all about 170psi rising to 190psi with oil.

The pistons are 30 thou oversize but I don't know when the "rebuild" was done.

Q. the machinist says I should turn them round, is this necessary? I took the sump off this morning but have not yet taken the offending pistons out.

Can I simply take them out, take them apart, turn them round, maybe rotate the rings and put them back or will they have worn in to the configuration of the cylinders?

Should I replace the rings?

Thanks.
 
Hi Tony,

Very interesting question, I will be looking forward to other responses.

In my opinion it shouldn't matter - stock pistons are not asymetrical. I just pulled a new one out of the box and can find no measurable differences from 'front to back'. Having said that, I do always put them in with "front" in the front (no reason to temp fate). I've got a set of forged pistons that have no 'front' marking on them.

Rods are another matter and do need to be oriented right, so if the motor was running, my bet is the rods are in right.

If this engine has some miles on it with good compression & no bad noises, I would leave them be.

I would be worried about the rings sealing properly if you pulled them and turned them around and put them back, especially if there are many hours on them. If you do decide to turn them around (assuming no big ridges at the top of the bores), I would pull all, glaze break, and install new rings.

BTW, If you re-ring it, I am a big fan of Total-Seal gapless 2nd compression rings, they really work well.

Side note, now that you have the pan off, I would plastigage a few rod bearings just for the comfort it will give you. Besides, if the rod bearings look bad or are scored, it's an easy thing to slip in a new set (can't do the mains without a tear-down).

Hopefully some one else will chime in with their thoughts.

Good luck,
Dave
 
Awaiting the definitive answer from Randy. I can't imagine a piston manufacturer bothering to mark "Front" on the piston unless the wrist pins were offset and it made a difference how they were fitted. OTOH, if the pins were offset, they never should have fit in the bores at all if reversed. As far as the rings go, don't they rotate around the piston so that their original orientation shouldn't make any difference?
 
Geez I dunno...the valve seals are cooked the head needed to be skimmed, its been bored, the fact that the pistons are marked to orient toward the front means it was assembled by someone who didn't read the shop manual ( I hate guys like that ). I'd wonder what else wasn't paid attention to. Either leave it alone ( you'll probably never have a problem) or be prepared to disassemble completely and recheck everything. You make no mention of if any ridge is present on the cylinders or any crosshatch visible or what oil pressure you have. You only have one original engine. I've done a lot of engines and I'd never re use old piston rings once they've been pulled out of a cylinder with any miles on them, they'll never re seat. If your compression readings increased when you squirted oil in the cylinders you have some blow by the rings already. My two cents mate ! Kevin
 
I'm just posing an idea cause I don't know but could the under side of the piston be off set. I've never paid attention and just put front in front. If any one has a piston the could check this would be interesting. People don't go to the trouble marking them that way for no reason.
 
My question is how long have you owned / driven this motor ? If it's been several thousand miles I'd leave it alone. Whatever was going to go wrong has had a chance and didn't. Why make a lot of work and expense for no particular gain? If on the other hand it is a recent buy with no history good or bad, it might be worth making it "right" for the next dozen years. I am not a fan of "project creep" and would be inclined to do nothing outside of the necessaries. Bob
 
I agree with Bob, if it has been working for some time/miles as is there would appear to be no major issue with the piston install, if not assymetrical as far as shape (must not be or it wouldn't work for long if at all) maybe there is a slit on one side, which I think may have something to do with oil distribution expansion or heat dissipation??
 
Thanks for all the replies - always good stuff here.

I bought the car last year knowing it would require some renovation. It came from the estate of a guy who'd owned it since 1974, it had been in dry storage for some time and I don't think the relatives were interested. Hence, the lack of paperwork. I agree with Brinkerhoff, it's easy to tell that work has been done - some badly and some skimped and I have been wondering what else might be poorly done as I've gone round it. (eg I've taken the wheels of while it's up on bricks and just noticed that the front hub oil seal caps are missing, no wheel slackness but I'll have to take off the hubs and check). The oil pressure previously was 50 running/25 idle.

The valve guide seals are the useless "O ring" type. I've got a set of the AH spares "improved" ones.

I quite agree with Bob. I don't want to end up with "mission creep". There has been a strip down respray, from white to red and I suspect it was put together with bits missing, but leaving many original parts - this was probably in the '80s. The bodywork is intact very little ,if any, rust and I have yet to find a seized bolt or stud.

To glemon, the Haynes manual says to put the pistons in the right way round - but doesn't seem as didactic as with some instructions , although it does say that the split should face away from the camshaft side. My machinist tells me these parts were not very well balanced in the '50s and 60's. It has an MOT certificate and looks like it's done a few hundred miles since early last year.

It's nice to know that the rings rotate of their own accord, so i needn't rotate them.

BTW The cylinder bores are quite smooth and I can't see or feel any ridges. No visible cross hatching, so I guess that it has done some mileage since being bored out.

So I suppose the real question is whether to put in new rings, if only for peace of mind.
 
If the pistons have split skirts they will eventually disintegrate if they are the wrong way round.

I'm a neurotic so the engine would be out and in bits by now!

I've reached that stage in life when I no longer trust anyone to put these old cars together properly. They usually get some of it right and a lot wrong.
 
Piston rings do not rotate around the piston when the engine is running. If that were true why would it be so critical to stagger the ring gaps during assembly so the gaps were not lined up? Cylinders are honed so that slight wear will occur between the fresh ring and the cylinder then create a seal. That's why an engine needs to be run for at least a few thousand miles before the rings will fully seat in these engines ( longer or never if synthetic oil is used ). Re using old rings is false economy in my opinion as you are creating new wear patterns on the ring. I'm not an advocate of just tearing an engine apart for no reason ! If you can live with it like it is it will probably outlive you! These aren't complicated engines by any means and only an idiot would put pistons in backward if they obviously were made differently and anyone competant enough to re bore and fit them would surely see the difference. Just smile and put it back together and enjoy :smile: . I have a 4.2 e type that uses a little oil but runs fine other than that and has great compression and oil pressure . I can hold off for now..
 
Tony, Put rings in it ! If that engine was tired enough that it had a blown head gasket and needs a valve job it needs rings. In doing the valve job and new head gasket, you will seal up the top end and that will magnify the worn piston rings. You will use oil like crazy ! When you have the pistons out turn the two around that are in wrong. Not the end of the world, but it has to do with splash for pin lube in most cases. Good luck ! :hammer:
 
Brinkerhoff said:
Piston rings do not rotate around the piston when the engine is running. If that were true why would it be so critical to stagger the ring gaps during assembly so the gaps were not lined up?
Click to expand...

Apparently, they do rotate even though we were all taught to stagger the gaps upon assembly. Here's a good thread on ring rotation: https://www.clubcobra.com/forums/fe-talk/108003-do-piston-rings-rotate-piston.html and some searches on Google will bring up similar info. I still wouldn't tempt the engine gods by not staggering the gaps though.
 
Turn them round. They're running with the non-thrust side taking thrust, and may well self-destruct.
Me, I'd have the whole thing torn down and rebuild it, but if the budget and timing aren't right for that, then at least try to ward off damage. If you only turn 'em round, you can leave the rings until a sunnier day.
 
Food for thought, I would also turn the 2 pistons around, but to do this you will have do it on the wrist-pin, rods are off-set on the crankshaft(only fit one way)plus rings are cheap(check end-gap) hone w/ball style hone. cheers Genos2
 
Hi
To Roger You're right. I don't want to be stripping down the engine and rebuilding it at the moment. This is mainly why I haven't removed the engine (+ mission creep).

Flex hone sounds like a great idea - gently,just to deglaze the bores. Would this alter the cylinder dimensions - they're already 30 oversize and I don't want to end up having to get an expensive professional rebore. Basic compression test of these cylinders was OK at 170psi before I started on this.

From reading the ads, before using this device, I think the engine would have to be out and stripped down to avoid debris in vital parts. Is this right?

BTW. We've been hit by Siberian weather this week and it's far too cold to be spending all day in the garage. (And the ratchet broke on my torque wrench).
 
Tony,

You run the risk of "fixing it until it's broke". From your compression readings, it seems the bottom end of your engine is in decent shape. This may be a good place to stop taking things apart.

If you don't, when do you stop? To pull pistons, or replace the rings, you'll have to pull the oil pan, undo all the connecting rods, then you'll be looking at the rod bearings, then main bearings, thrust washers and on and on.

Unless there is a significant ridge at the top of the cylinder bores, you might have a good place now to stop and start reassembly.

As far as the pistons the wrong way round, these engine parts wear together, and if you remove them and turn them around, the wear patterns with the piston skirts, etc. will be wrong. A simplified example is your most comfortable shoes. You have worn them and they feel good. If you buy a new pair, just like the old pair, they won't fit exactly right and it won't feel so good. I fear you'll create more problems than you solve.

It has been a long time since I've had a Big Healey engine apart, but seems wrist pins were lubricated with pressure from the oil system, not a splash system, so they should be getting good lubrication. I'm reasonably sure the wrist pin bushings had a hole to line up for an oil hole in the connecting rod. Based on that recollection, it won't matter which way the pistons are oriented as far as oil. If there were other issues, such as fit to the bores or connecting rods, the engine would have not made it this far.

Good luck whatever you decide. Jeff
 
No pressurized oil lube system on wrist pins on Healey 6 cyl. engines ! There aren't too many engines in the world with gun drilled connecting rods for pin oiling. :cheers:
 
Like Rodger says, I would worry about the thrust more than splash. Thrust side isn't quite as important on a inline 6 as it is on a V-6 or V-8 where the piston is laying on one side so to speak. Save some money, and when you have enough have a good mechanic friend help you pull the pan and bump the pistons out. Mic the crank journals and if good, hone cyls. and put new rings and rod bearings in. Check timing chain and replace if worn also. This is kind of a shade tree job, but I have seen older engines does this way run for years and years. Good luck :hammer:
 
The difference made by gravity to a piston in a V or Slant engine is irrelevant when compared to the thrust of combustion, and is probably immeasurable anyway.

If a piston is made to go in with a certain orientation, and there's nothing asymmetric, as in this case, then it's because one side is meant to take thrust and the other isn't. If you apply thrust to the side that isn't meant to take it - like a split-skirt piston for example, it may break up.

There's some really bad advice on this thread, in my opinion.
 
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