Bevel twin question

[IanHood]

Apologies for being off topic as far as singles are concerned. I’m refreshing the top end of my 750GT (mainly to replace guide seals and to check valve to guide clearances). I thought I would pull the cylinders and check things even though compression on both pots was about 120 psi which seemed OK.

Both cylinders are on standard 80mm bore and show no appreciable wear and the same for the pistons which are well within wear tolerances. So far, so good, but on checking ring gaps I have 0.8mm for both top and second ring and significantly more than that for the oil scraper.

That seems far too much to me but does anybody here with knowledge of the twins know what they ought to be? This engine was getting quite oily so I’m thinking that may well be contributing.

It also seems standard bore rings aren’t available so I’m not sure what to do.

Ian

[ranton_rambler]

According to what I have read, the top ring should be 4 thou per inch of bore, so I think yours are a bit big. Second ring and oil control ring gaps should be slightly bigger than top ring.
Have you tried Cox & Turner or F W Thornton. Both seem to be good at finding rings which are otherwise difficult to get.

[Jon Pegler]

Some of the Porsche 914 and 911 models use an 80mm piston.
It won't completely help as you need rings 1.5mm top, 1.75mm 2nd and 4mm scraper and the Porsche rings are 1.5mm top, 2mm second, and 4mm scraper.
Porsche rings will only help with the top ring and the scraper, but may get you out of trouble.
They can usually be bought for £30-£40 a piston

Jon

[Eldert]

Hi Ian

some Alfa romeo's have a 80 mm bore with 1.5 , 1.75 and a 4 mm oil ring .

i think its the 1750 or 1800 cc engines

Regards Eldert

[IanHood]

Many thanks for information gentlemen. Eldert, you are absolutely correct, it’s the 1750 Alfa engine that has an 80mm bore and a full set of rings can be had for £85! An absolute bargain in Ducati terms

Ian