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Hi all,

A long story involving some special O/S forged pistons (with short skirts). I got some a while ago and it should be said I've not heard of anyone else having any problems. Anyway I have had.....................

As my 500 broke a ring and had the usual cracked skirt problem I bored some shagged barrels to what I thought were the correct tollerances for these pistons. While I was at it I rebuilt the bottom end with a new fancy, "that'll never go" roller bearing on the right, and put it all together before last years Cadwell. Result a top end which slapped like a very slappy thing indeed.

Bo****cks I thought, but luckilly I had some standard barrels (which I really didn't want to bore, but what ever.....) and was in a hurry to get on track (I'd missed 2010). So that I didn't make the same mistake twice I took old and new barrels, plus pistons, to NLM. They thought the tollerances a bit slack so said they'd bore them 'correctly' this time. 'Correctly' ended up being exactly the same as the first pair of barrels (I mic'd them they are identical) so on re-fitting (you guessed) they rattled just as bad as beofre. As such I now have 4 shot, O/S, barrels.

As such, and as I'm out of the country until the week before Cadwell, has anyone got a usable pair of 500 barrels and pistons for sale ? I'd pay to "borrow" some temporarilly but given my previous experiences on the track (500s really don't like going into double figure revs) that might be unwise.

Help !!!!!!!!!!!!!!!!!

Pete,
I think I mentioned to you before that pistons are made with the clearance already accounted for, so that if the size of the piston is 69.1 then the bore is taken out to that size. Based on Phil Irving expertise which is still good now, then the average clearance for pistons is about 0.005/inch but this is racing clearance, giving you a clearance of about 0.013". There is the possibility that the rebore chappy has mistaken the +0.1 as ten thou when it should be seven thou which would give noisey running. If NLM have produced the same result then I suggest checking the piston-bore clearance using feeler gauges or measuring.
Worn valve guides can also give clattering engines that sound like piston slap and if you have not replaced them after the mileages you do then they may be worth looking at. The valve stems also wear increasing the clatter. The guides tend to wear barrel shaped internally so the valve sets up a rocking motion. I make my own valve guides from PB102 which conducts heat away from the valve and is a superior material to the cast iron ones as fitted.
If the barrels are scrapped then I would suggest having liners fitted to the existing liners to bring them back to standard. You can get plain liners from a firm in Droitwich -can find the name for you, but I would recommend over your way, Coventry Metalling and Boring to do the work. They are located near to where the Triumph cars works were. I had them submerged arc a bent six cylinder crankshaft and bring the mains back to standard and in line for me cheaper than having a new one made. I do have, to me, a firm who can do the relinering etc.(they have a steady turn round of 1960's and before car grand prix engines) but C.M.B. would take care of your problems and offer advice.
Not sure whether you are in the country or not but if you can give me a ring I could offer better advice over phone.
Nigel

Morini barrels had tapered bores from the factory!

Plain boring is a comprimise between sufficient clearance and quiet running.