Strange one this:

New engine, new custom Wiseco pistons, freshly bored block. On start up there were no strange noises, engine sounded sweet, good oil pressure etc...

Was mapped on the dyno, made good power, did around 250 miles or so without any noises what so ever or any sign of a problem when due to some unlucky problems it got very hot and killed the headgasket. Since then it developed a light knock, which quietened after time. The car still drove well, didn't smoke and performed well.

We assumed the noise was the lifters being full of crap and it had got too hot, but after overheating again for the second time the noise got a lot worse, to the point where on a cold start up it sounded like it had spun a shell, but the noise got a lot better after some heat/time. A few people suggested it was piston slap, but it never piston slapped before so we discounted it.

We decided to pull the head off for a rebuild though and when we looked at the pistons all 4 have around 1-1.5mm of sideways movement I.e. Piston slap. They have lightly scored the bores too.

Now when building the engine it didn't have this movement, and it never made this noise before; so why now?

Machining error? Or related to the fact it overheated twice?

Properly confused!

 

if it's overheated it would have allowed the pistons and bores to expand, this has possibly forced the oil from the bores a little and the pistons rubbing and wearing.
it's a bit worrying as i also have wiseco pistons (oversized) however i don't suffer any piston slap, only just reaching a thousand miles but been running on power well.
1-1.5mm is well over tolerance and may be worth replacing the block as a precaution if the pistons can be cleaned up.

 

Sounds like a piston has picked up when overheated, this is backed up by the marking on the bore.

If your this far into it , I would drop the sump and lift the pistons out.

The only way your going to find out for sure im afraid

As for the overheating , why did it happen the first time ?? did you ever find out ?

 

Have you got to the bottom of your overheating problems yet? I'd take a guess that the oils got too hot and the rings have scored, making them thinner and thats where you're getting the piston slap. The only way to check is by measuring the ring gaps

 

Thanks for the replies.

I didn't think a block or pistons could distort easily or so evenly; all 4 pistons seem to have near enough identical amount of movement.

Rings sound more likely to be the culprit...

There seems to be only light scoring in the bores in small areas so it certainly wasn't excessive.

The engine is being stripped down for inspection and to measure clearances Saturday, I was just wondering what people might think at this stage.

 

WHat was original piston-bore clearance?

How was the engine run in, and for how many miles?

Forged pistons expand quite a bit under high boost applications, and also as they are aly, wear easily...could be a couple of reasons, if it had picked up a piston it would be fairly obvious depending on the severity, are the sides of the bore polished looking or scored? probably wise to remove a piston, check for wear and check piston/bore clearance is still within spec.

 

Had a problem like this years ago on a bike later found the bore was done wrong was bored way too large to start with.

 

p.s. - I have actually heard forged pistoned cars, from cold with piston slap for the first few mins until they get a bit of temperature in them...as I say bore clearances need to be larger than a cast piston.

 

Did you check the ring clearances when building?

 

Rings were checked, yes.

I know a lot of aftermarket pistons slap from cold; a lot of my mate's Zetec turbo engines have piston slap from cold. What's really strange with this is that it never piston slapped even in the slightest before, and it's been started/run a lot of times. It only did it when it got hot, unless that was a coincidence.

The fact that all 4 pistons are doing it the exact same amount. You would think if the heat had done anything it would have been slightly worse on one cylinder than another. It indicates it's a machining error.

I'm going to whip a piston out Saturday and measure it against the bore.

 

what material were the pistons made of?

wiseco are owned by JE, and they offer two kinds of material.

No freshly built engine should have piston slap if it has been built properly.

But your engine over heated and this could have done all sorts of damage.........

 

Standard 2WD Cossies slap from standard and I've heard JE pistons tend to slap after a few miles strangely, so the fact Wiseco are owned by JE is interesting.

 

JE/wiseco tend to slap because they make them from a softer material but people assume that they are just like any other forged piston.

They then machine the bore for a normal forged piston.

The end result is a bore that is far to big for the piston= piston slap.

the soft material combined with the over heating may have caused problems.

 

You would think it would slap from first start though.

I'd like to think it was just down to the way the pistons are but I don't think it's that easy...

 

measure the bore, pistons and the rings and then you'll know what's going on.

 

just to point out that my wiseco pistons were oversized to 86mm and bores machined to tolerances stated on instructions, no piston slap at all, cold or warm. done just shy of 1000miles now and no change, just a slightly tappy top end due to solid followers.
sounds like a machining error more that anything.