The V8 Owners Forum -

As u may of read in my other post i had a mishap wiv missin a gear and over revving the motor, has broken piston 5 right thru to the ring as u can clearly c in the pic below but valve is fine, what u guys reckon?

If the bore looks ok I'd just fit another one. I'n fact, I'm swapping a set of those out now and fitting a set without valve cut outs. Thats exactly where mine let go. I was on nitrous at the time, but they are brand new. Theres hardly any metal left above the ring land and under heavy use they seem to let go in the root of the cutout.
If I'm just replacing one, I machine it a little to match the weight of the old one from the weight sheet I have.

Just looking at the photo I reckon that if you replace the damaged one another will let go. Too much material removed above the ring groove at that location.
It would be a false economy not to remove all the pistons and crack test them with a penetrant dye - magnaflux or similar.

I think the proper solution is to keep at least 060" of material above the ring and choose your piston, vale lift etc to match. I am assuming that those are low compression pistons with a deep bowl and 100" or deeper valve reliefs, I have built an engine running 460" lift with 9.75:1 pistons and nothing touched so know you either have a seriously high lift or long duration..

Should go without saying that you have length records for your rod bolts and if any have stretched you replace the lot.

Dave

If its a 4.6 I've got seven spare ones here. They are not much good for nitrous use. I re read your post and realised that you over revved. So as Dave says, look out for anything else that may have gone out of shape.
If they are +10 or +20 pistons I would change the lot anyway as they are pretty cheap over here. Dont know about prices in your neck of the woods.

Thanks guys, I will measure the depth of the cutouts once removed. They are standard bore, i didnt put the valve reliefs in, As its a TVR motor i pre-sume they do? I presume they just use 4.6 pistons, will find out. I have offset bushes in my rockers so makes them slightly longer giving 502thou of lift. Times like this i wish i still lived in the UK, we do have a rover specialist in Perth that builds 5.2/5.3 motors so maybe he can help. The headgaskets were 30 quid a throw from V8 devs( they get them made), add postage 2 here is getting alot, maybe the bloke in town can get them here?

Also as u can imagine, it has peppered the head face, what is reccomended to do there, C/R of the 5.0ltr tvr is 10.5-1, dont fancy removing any material, would it be ok to just remove the high spots?
You can c how oily that bore is compared to the others beside it.
I probably drove 200 miles with a broken piston, it inially rattled after over revving it for about 20 seconds then was fine, payed now attention 2 it after that as was runnin fine, no rattles or poor runnin, was only on my way home from a car show, i nailed it past a line of slow movin classics and it then started 2 misfire, after checkin pushrods i pulled the head.

Hi
with the face of the head I would clean it up to remove any high spots, clean off anything that looks as if it come loose and finish it with a flat scraper and emery cloth. You could use a fine stainless steel wire brush in a dremmel if you have one but I think the scraper/ emery cloth will do fine.
Mike

Yep!
I just use a stanley blade like a paint scraper. I keep it at a real acute angle to the head and gently scrape the high spotsoff making sure to stay inside the gasket ring so as not to touch the area that would be under the gasket.
Two of my heads have peppered areas like this and I have no probs at all.

Agree on smoothing it off. You want to get rid of any loose bits and high spots that can heat up and cause detonation.

Dave

Ring lands don't break from over revving imo, if they do I've never heard of it I'd expect a cap failure or rod bolt failure. To me it looks like you detonated the piston, compression to high, poor fuel, incorrect timing for those revs or hot plugs etc? Maybe it was just high cylinder pressure and a weak top land but wouldn't pressure lessen with revs?

I run it on 98 octane, C/R is 10.5-1 as standard, timing was set at 28 degree's full advance from memory, this was what rob at v8 dev recomended. No signs of detonation on the other pistons, when its hot and u start it, the starter does turn slow initially and makes a noise like ignition knock as if timin is 2 far advanced. Will check caps out when engine is out.

98 octain and 10.5 to 1 compression on 28 deg should be safe as regards detonation even on a standard cam, also no sign of pitting in the other chambers in the photos. But you are dead right it would be a good idea to check for rod caps and bolts. But looking at it there is very little material below the valve cut out.
Mike

Boosted LS1 wrote:Ring lands don't break from over revving imo, if they do I've never heard of it I'd expect a cap failure or rod bolt failure. To me it looks like you detonated the piston, compression to high, poor fuel, incorrect timing for those revs or hot plugs etc? Maybe it was just high cylinder pressure and a weak top land but wouldn't pressure lessen with revs?

I agree that normally they would not break but "normal" RV8 pistons are not pocketed. If the pocket is cut too deep then the piston could break.

The pistons in my 4.6 are also pocketed

Pete

Pockets aren't good then and it's probably cylinder pressures but if that were the case all RV8 engines built like this would fail hence my detonation possibility. Also, what outside air temps have you been getting in Perth? Has it been especially hot?

Outside air temps are low now, were in winter here, around 20-23 degree's.