Engine gone bang !

[CycleSi]

Not happy. Engine decided to let go on me - pulling out of a junction in 1st gear and then suddenly it sounded like a machine gun going off inside.

Turns out to be a broken piston. Not sure how the rod and crank are but I wondered how much I can have the 400ci block overbored.
It is currently +.040" so I think that's it - either liners or new block for me.

What do you all think?

[dave]

You might get away with +0.060.
I'm sure an engine guru will be along shortly.

[wilf]

Sorry about your news.

Two things that need to be done - establish why it went bang to begin with, and only then how to repair. Pistons rarely just "break" - they get overstressed for some reason first - could be over-revved at some time, over heated, wrong clearances, detonation, etc.

Most types of blocks have a "recommended" max overbore - I can't quote chapter and verse for your specific one, but I have seen engines out to +.060" when everyone said +.040" max. It may well depend on cylinder wall thickness on your particular block. And the availability of oversize pistons.

How many miles on this engine?

[keane]

Hi, it really does depend on that particular block, see if you can find anyone to sonic check the block, not sure what the min cyl wall thickness needs to be on a 400, 429/460 blocks generally go +80 no problem, + 100/110 also possible, but a sonic check will give you the reading of the thinnest wall so you can go from there, search the US sites for more info, hope that helps. Dave

[CycleSi]

Wilf,

You're right - the main question is why did the piston break?
Certainly not over revving, but it could be over heating as it has been getting hot recently and l thought a head gasket was on its way out or more likely to do with the nitrous kit l have. Although it is only a 75 hp jet, and l do have a progressive controller to feed the nitrous in, it seems likely that has weakened the pistons. Forged pistons are the only way to go with nitrous it appears?

Apparently I cannot overbore the block anymore so now I'm thinking if it's going to have to be lined at the cost of £800 to £1k then l might as well buy a new 350 block, use my 400 crank and make a 383ci from the parts left over.

The engine itself was rebuilt 5 1/2 years ago and has done around 10k miles.

It was on a rolling road recently where it made 446 bhp and 587 lbft so it "sees some action". The fuelling was spot on and there is no hint of detonation although it has always used oil.

[wilf]

Nitrous? - I think that is our biggest clue!

If you had cast or hyper pistons, they are not going to stand up to nitrous as well as forged. Also, if, when you strip it, you find a piece of top piston ring land missing, then the ring gap was too small, and the extra heat generated by the NOS butted up the rings. Ring gaps need to be bigger for any kind of power adder.

Your 383 stroker approach sounds good. But you could always go to one of the Dart/Merlin blocks that allow bigger bores, and go for more CIs? You might miss the torque you have now otherwise.

At least it's nearly winter.

[mikey]

You will need to be lucky to get a safe 0.060 out of a 400 block, its as much about porosity as thicness. The Merlin and Mowtown blocks as Wilf said are more capable and this is as much down to thir higher Nickel content which adds to the block material density and they are cast with a thicker wall also.

If it was me I would be looking at a replacement block and I think the 383 is a better proposition as whist the localised heating issues of the 400 can be overcome with a bypass it is only a partial solution. If you have the spare cash, do not dismiss the Dart or Motown Blocks where you can achieve 427 cu in safely in a small block configuration. Also the aluminium blocks are nice, but you are talking big bucks.

Good luck with it
Mike

[Andy302]

Originally Posted by wilf leek

Also, if, when you strip it, you find a piece of top piston ring land missing, then the ring gap was too small, and the extra heat generated by the NOS butted up the rings. Ring gaps need to be bigger for any kind of power adder.

Also, if you were running KB's, then the top ring gap has to be bigger than stock or you will break a land. (from what Ive been told)

[CycleSi]

Thanks for the feedback Guys.

Firstly, we now know what happened - there is a crack in the bore of the piston that broke. That would explain why it's been overheating with no leak as such - the coolant was disappearing into the block. Also, the oil in the sump is 'mayonnaise' ish and yet the head gaskets are still fine. This reiterates it is best to avoid 400 blocks in future.

My plan to use the existing crank in a 350 block to get a 383 has gone out of the window having been reminded it won't fit due to the journals being a different size. So, assuming the crank and rods are OK l am back to lining the block, fitting new pistons and taking it from there. The good news is that there is no sign of detonation and the rest of the engine looks good.

I've had mixed reactions to using forged pistons next time as some still think KB Hypereutectic pistons are fine. What advice can you give regarding type/choice of rings and what gaps do you recommend bearing in mind it will see the occasional shot (75/100hp) of NOS?

Thanks again.....

[wilf]

If you go onto the Keith Black (Silvolite) pistons site, there is a table somewhere giving piston ring gaps in terms of thou per inch of bore for various applications.

Forged pistons are: heavier, require larger skirt clearances (piston slap when cold), but are tougher, and will stand more abuse.

Hyper pistons are: lighter, run with lower clearances, so quieter and potentially better oil control, but will break easier with mistreatment. Cheaper too.

In a real world, you could use NOS occasionally with hypers, but you better have your mixture, spark retard, and ring gaps right first time, or bye bye piston (s).

Of course, now will follow umpty-three posts from people using NOS, blowers, turbos and 15:1 CRs that detonate like bast*rds , with hyper pistons, who never had a problem.................

My own engine (never designed to have any kind of power adders added afterwards), has hyper pistons, and I "exercise" it fully regularly, and have done for 18000 miles.

However, if I had built it with NOS in mind, I would have used forged pistons.