LCGB Forums

So I blew my GT240 up in the summer. I was pushing along nicely at 70mph ( much faster than I normally go - just ask my riding companions!)…. Then silence …. Seemed to just cut out , no bang or nasty sounds… clutch in , coasted to the hard shoulder to find it seized…









I’ve had it rebored and gone up to the first oversize piston.
Whilst it’s apart I’ve decided to renew main bearing and flywheel side seals and gaskets, and fit a sluk head cowling and vortex fan for supposedly better cooling ( I’m a sucker for new bling )


So what caused the seize? The plug and piston crown do not indicate a lean issue or air leak judging by the colour. I pressure tested it prior to strip down and it holds pressure with no leaks.
There was a dark black oil stain on the main bearing retention plate. (Sucking air and oil in from crankcase?)
The carb was jetted professionally on a dyno.
Fuel tap is fast flow and measured rate is OK
A club member has suggested looking at the float valve size - I’ve ordered a 3.5mm fast flow valve from MB. Feasibly at that speed the engine may have been using more fuel thank the float bowl could supply and it drained..?

Also I gather that compression ratio will increase too with the rebore - should I add a headgasket to compensate for this - the gt240 kit doesn’t usually need one …

Once rebuilt I’ll get it redyno'd and dialed in
Any thoughts on the cause, or rebuild hints and tips appreciated

Good thread!
I'll check the pictures when I'm at home, as work blocks the hosting site URL

Any thoughts on the cause, or rebuild hints and tips appreciated

Rich T told me Saturday that the 'squish needs to be between 1.2mm and 1.8mm to aid kicking over'
I'll achieve this by ditching the 0.4mm base gasket, instead using a 1mm packer and Threebond 1215 for gasketing

I've seized on the A34, just at that point before I went onto reserve. what was your fuel level like at that point. were you far from reserve?
Worth checking your fuel cap breather hole to confirm a vacuum wasn't being pulled restricting fuel flow.

On the possible main seal - have you been losing gearbox oil?

Hi Andy - thanks for the reply and suggestions

A: fairly full tank, and breather ok,
I have previously suspected its losing a small amount gearbox oil, however no white exhaust smoke or smells.

Is it known if the ring peg had failed and caused the seize - I'm guessing not 100% known?

I'm only mentioning as I'm aware excessive ring wear can potentially cause the ring peg to fail

I'm only throwing this out there as a possible explanation if nothing obvious is found..
good luck with the rebuild and keep us updated

That doesnt look like it got hot so possibly not a heat size, maybe just the ring peg comming out and jamming the piston? I think fuel issues would have made it run lean.
Re the squish, I had my GT240 bored to plus 0.5 after severe wear cused by a contaminated tank and didnt change the squish at all, just put it back together as per the build instructions. I've done a few thousand milrs on it since including many at 65-70mph with no trouble at all.

Thanks fellas
What exhausts are you using ?
I’ve currently got a TSR EVO which in very happy with