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MG Midget and Sprite Technical - What does 1500 engine piston slap sound like?

Having put new pistons and rings into worn and un-honed bores, I'm getting a lot of top end noise, which I know isn't rocker assembly, cam shaft or cam followers (all have been changed/swapped).

(and with compression figures of
1 12.5 bar
2 13.0
3 12.5
4 12.0)

Oil pressure good, new shells/thrust washers on un-ground crank.

I think the noise is a little like some kinds of diesel engines I've heard.

Obviously, with all these changes, I'm aware that the engine needs running in, and did a 30 minute run the other night limiting revs to 2750. But actually, that was all my (musical) ears would have wanted anyway...

Any thoughts?

John
John Ball

Piston-slap is most audible when the engine isn't under load. The best way to here it is to blip the throttle to around 4K and listen as the engine slows. It sounds like a muffled hammering - at higher freqency than e.g. b/e bearing noise.

It's best heard on something like a high-milage Essex V6.

A
Anthony

BTW - the gudgeon pin is normally offset from centre to reduce the changes in load as the piston goes over the top.

Having the piston in 180 deg out (rotated) will bring on piston slap...

A
Anthony

Diesily noise can also be a loose exhaust manifold - don't jump at a full stripdown unless you've check this first!
Will Munns

John, the noise goes something like this:

£ £££ £££ £ ££ dum de dum. £ £££ £££ £ ££ dum de dum. and the chorus....

Sorry, just my take on typing noise on here... lol

Perhaps Anthony has the clue.

Mark.
Mark T. Boldry

Thanks for all contributions ;-)

I carefully made sure that the pistons all had their little arrows pointing to the front of the engine.

Next?
John Ball

Small ends weren't replaced, but the (new) gudgeon pins were a tight sliding fit with no wobble.

And the exhaust manifold is done up tight...

J
John Ball

So what else? Would rings not-bedded-in make any kind of noise? I know they're not broken, btw.

J
John Ball

Hello John

Your compression readings indicate that your rings have bedded in. You fitted new pistons IIRC so you shouldn't have ring chatter. Top-end possibilities that remain are piston slap and contact with the head gasket. The latter is unlikely in what is basically a standard engine. How does your noise line with up or differ from the descriptions above ?
Paul Walbran

John, how is your oil pressure? Can you be sure its not a dry crank bearing?
Bob T

John,

You mentioned that you did not hone the cylinders. Do you also mean that you did not run a glaze breaker through them? (Sometimes the terms are used interchangeably, though they are not quite the same).

What I am wondering is if there was a ridge at the top of the cylinder where the rings do not ride. Seeing it is easier when you run a glaze breaker through the cylinder. Unless you use a ridge reamer, ridge dodger rings, or pistons with a lower top ring, the new rings hit the ridge and may break. How did you determine that the rings are not broken?

Also, you mention new thrust washers on an unground crank. Have you checked the crankshaft end play? Besides the thrust washer wear, there can be thrust surface wear on the crank.

Charley
C R Huff

Thanks for these -- endplay on crank seems ok, although I haven't measured it accurately. The bores were hand-sanded with 800 in a spiral action to roughen the glaze a little. The ridges at the top of the cylinders were ground off. Oil pressure comes up pretty quickly: 80+psi on start, around 55-60 max when hot, 20-ish on hot tickover. This isn't a bearing rumble sound. I determined the rings weren't broken by stripping down and taking out the pistons all over again the first time I heard the (same) noise, so logic says if they weren't broken the first time they aren't now.

Cheers, J
John Ball

John,

Did you drive this engine before you did the work, or did you get it dead on arrival? If you drove it before, did it make the noise then?

Under what circumstances does it make the noise? Like Anthony said, piston slap is most pronounced when backing off the throttle. If it is making the noise under acceleration, is there any chance it is spark knock or pre-detonation? Did you use higher compression pistons?

It might be worth measuring the crank end play, if only to rule it out. If you don’t have a dial indicator, you can do it well enough with some scrap metal, vice grips or c-clamps, a pry bar, and feeler gauges.

There is a chance that you can narrow your search by driving it on three cylinders (alternating the disabled cylinder) to see if one of them makes any difference. I have "cured" rod knocks to avoid a tear down on cross-country trips by pulling a plug wire.

Charley
C R Huff

Thanks, Charley. this is the same engine I've had since I took over from PO in 1997. I did all the work above with the engine in situ - I don't have the facilities to pull the engine. The noise started after I replaced the pistons -- I had to do that because a chunk of #4 had gone through its exhaust valve! I replaced that valve and re-lapped all of them. I hadn't thought of your idea of pulling a plug alternately, which makes sense. The noise is continuous, from almost nothing at tickover, rising as the revs rise, not more noticeable as you accelerate/decelerate.

Everyone: it may well be that we can't get to a solution online. You really have to hear it for yourself, don't you? Since starting this thread, I have done a deal with my good friend and much better engineer, Geoff. He and his ex-RAF engineer son are taking the midget over at the end of the month and won't be at all fazed by any of this. It'll be tough to say goodbye, but it's a sensible option. I don't have big bucks to throw at my hobbies, so I was not prepared to pay out for someone else to put in a replacement engine for me. Thanks for all the tips!

J
John Ball

John,

Did you check the piston were actually the right size before fitting. I just remembered that over the years I have heard of a few cases of the wrong sized pistons being in the boxes and in one case one wrong piston supplied in a set. In at least one case the engine was built and gave yours symptoms.
David Billington

John,

You could get one of those stethoscopes for automotive work. They are pretty cheap and it may help you locate the noise. You could also listen (either with or without the stethoscope) while the valve cover is removed.

You mentioned that you made sure to get the pistons faced the right direction. Did you also get the rods faced the right direction? I don't know if it matters on that engine, but it does on most. And, did you get the rod caps oriented correctly to the rods?

Charley
C R Huff

This thread was discussed between 27/06/2008 and 01/07/2008

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