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

[Duccout]

Hi Ian,

The Ducati 860GT manual states the ring wear limit as 1.20 for the compression rings and 1.00 for the oil scraper, so you are probably still in spec, but now the engine is apart....

Cheers,

Colin

[IanHood]

Thx Colin,

Bloody hell - 1.2mm is a huge gap! In any case I have sourced some Alfa 1750 rings as per Eldert’s suggestion. They have a three piece oil ring which I think is always a better design than the original one piece scraper. I now have gaps of 0.4 to 0.6 which I’m happy with and just waiting on a fin repair on the vertical barrel before putting it back together.

Ian

[Duccout]

Hi Ian,

After I had my 750 rebored I could never get the engine to run clean - the piston crowns were always oily. After some years I rebored the barrels again and fitted Vee Two pistons that had those three piece oil rings and I followed the fitting instructions of assembling them with WD 40 and not oil, and from then on the piston crowns ran clean and dry and the engine ran noticeably better.

Cheers,

Colin

[IanHood]

That’s interesting Colin. The bevel twins all have a reputation for being quite oily (and fouling countless plugs), particularly if they are doing mostly slow speed stuff. I never had the problem in Australia where I am originally from and I suspect that the generally higher speeds and distances travelled had something to do with it.

Fundamentally though, I think the three piece ring design is much more efficient at clearing excess oil from the bores and your experience seems to support that.

Ian