Hitchcocks Forum

Hi

Had a bit of a problem with my Continental GT250 this weekend.

I could not start the engine (it has been running OK before this incident apart from making a ticking sound at tickover) so I removed the spark plug (a nearly new Champion L5) to clean it and noticed that a chunk of the ceramic insulator around the electrode was missing.

I therefore took the head off and discovered that the inlet valve has kissed the top of the piston. The inlet valve itself and the seat are perfectly OK and unmarked, and there are no marks whatsoever anywhere else in the head. A crude test using the old plasticine trick on a partly reassembled motor indicates that there is a clearance, and the bike turns over perfectly OK by hand.

I don’t know why the valve has hit the piston (it’s never been anywhere near the redline) so in the absence of other evidence, I have reached the conclusion that the damage was caused by the piece of ceramic jamming the valve. I just hope I am not jumping to conclusions and misdiagnosing the cause.

Problem now is the re-assembly of the top end. The strip down (it’s the first time I’ve had the head off this particular bike) revealed a bunch of first class bodges. You name it – from stripped threads to copper washers under the head nuts which have been compressed into the alloy and everything in between.

I spent most of the weekend fixing the majority of the more serious bodges, but I one major issue remains to fix. And that is one of the studs in the head securing the rockers has been helicoiled very, very badly at some point (for example the tang has been left at the bottom of the hole) and even if I manage to remove the helicoil I’m pretty sure the thread in the head has been damaged so will not accept a like for like replacement. The joker is that the stud hole is very, very close to the oilway feeding the inlet rocker so going up a helicoil/imperial thread size would risk breaking into the oilway. So I am toying with the idea of tapping a metric thread (which would require a smaller drilling than imperial ) directly into the head and making up a new metric stud. Anybody got any better ideas?

Whilst the motor is partially stripped I’m also contemplating using a couple of compression plates fro our hosts in order to increase the valve/piston clearance. Any thoughts on that?

Regards

It is possible that the piece of ceramic jammed in the valve causing you valve and piston to meet, using compression plates will move the barrel up and cause the piston rings to jump any ridge left in the barrel due to wear, without seeing the helicoil it's difficult to judge, but the bottom tang is removed after insertion and can still be removed if you wish, why not try fitting the rocker box and see how it tightens.

I can't really comment on the helicoil, it's your call but if it's solid I'd leave it alone even if offends your engineering standards! Re-tapping will almost certainly be riskier than leaving it in place. As to the compression plates I don't think you need them, my GT (actually my wife's but I built it!) has been regularly revved to the limit and I've never had problems, I don't thing valve/piston tangle is an issue. One tip, when replacing the head, use our Hosts hardened steel washers under the nuts, it saves them being crushed into the alloy. They are actually listed as sets for the Bullets but if you phone them they will sell them individually for other applications (they are the same size,) including the Twins for anyone interested.

REgards, Mark

Thanks for the replies.

I've managed to rescue three of the 4 rocker studs - the 5th centre stud is also fine. But the 4th stud is totally shot in my opinion.

So it seems to me the only options I've got are leave it (and run it with 3 out of the 4 studs) or risk retapping/drilling the hole to accept a metric stud. Not a brilliant choice. Still unsure what to do but maybe run with just 3 studs (and the 5th central one) is the least worst option? I'm just concious that the pressures on the rocker carrier must be significant.

Mark. Thanks for the tip. I'll email Hitchcocks and enquire about the hardened washers.

Regards