12 replies to this topic

[insamoufonyx]

Hello all, Just got my new 1380 built and running, had a banging coming from the engine so i took the head off to find out that the pistons are wacking the head gasket,

Any ideas why?
Any posible fixes?

Cheers

[Guess-Works.com]

I was reading an article about this on ( I think Minispares ) the other day...

It's all down to the offset of the bores, the crown to deck height and the 'overlap' of the gasket into the bore...

I'll go find it.......

here ya go...

http://www.minispare...x?ty=ad&aid=576

Edited by GuessWorks.co.uk, 16 October 2008 - 05:39 PM.

[mini13]

what gasket are you using?

if its the black BK450 then the AF 460 is worth a go, as i have it on good authority that the fire rings are a bigger diameter.

also cometic (sp) list 2 different gaskets for the 1275 A series one for the mini and one for the midget, the midgit one is listed as being for 74mm (rather than 73.5 for the mini one) and has a different thickness.

[danny2009]

is the gasket on the right way

[insamoufonyx]

Gasket it the right way yeh, mini13 il have a look at that cheers

and guesseworks cheers for the link, just had a quick read of it and now severly *Drunk or yellow human water* off, looks like im going to need to take the engine back out and strip it to get the piston machined, Great!

Edited by insamoufonyx, 16 October 2008 - 09:40 PM.

[insamoufonyx]

after a quick read of the link guesseworks posted seams its jsut pot luck wether its ok or not, seams i was unlucky

[Strontium Dog]

Was the block bored to a gasket or just offset a given amount?

I would pop the pistons out and machine them down in the lathe so they don't touch plus a small clearance. A chamfer is better than a step as any sharp corners can lead to cracks. Not a difficult job, just a PITA to have to pull the engine down!

[insamoufonyx]

no idea about the block previous person got it bored. dont really want to strip it unless i have no other option as ive only jsut rebuilt it and if ya work it out its a suprising amount of wasted money on gaskets and instant gasket if i have to replace them all after 5 miles of use lol......

[GraemeC]

Not really pot luck - as part of a dry build the piston to deck height should be measured, if the the pistons protrude from the block you are asking for trouble. It is recommended that they are 10-15 thou down the bore, if the build is so specialized that they need to protrude, or even run flush, then suitable measures must be taken.

Have you calculated you compression ratio? It must be quite high unless you have a head with big chambers.

[insamoufonyx]

right, just done some acurate mesurements and the pistons dont protude, there exactly 4 thou down from the deck of the block so why the heck are they hitting the headgasket?

[Guess-Works.com]

beacuse, if you read the article again, when the head gasket is compressed, if there's any which overhangs the bore it will be pressed into the bore.

[insamoufonyx]

yep defintly the problem, the gasket over hangs by a few mm into the bore and after running about half a mile there is already heavy marking on the headgasket

[insamoufonyx]

lol any other ideas?

Just a quick update, i know quite well Ian Slark (thouse of you into mini motor sport may reconise the name) he owns a resto place near me, hes the brother of Neil Slark (also in the racing scene) whos the owner of Slark Race Engineering - which as many of you will know they are pretty much the dogs bolloks lol. Neil is having a look at the head gasket and compairing it with some hes got lying around to check all is ok with that, if not theres the posiblity of having a custom head gasket made to suit.