E: Engine New member, and comet question

[James B]

Hi,

Just joined the VOC, following the realisation of a 30 year intention to 'own a Vincent'.

I'll post a proper introduction to myself and the bike shortly - but for now, lets just say that thread will become a rebuild documentary since my Vincent came 'in large bits and various boxes'.

I'm currently going through itemising what's there, and more importantly what's missing - and building up a frightening large list on the Vincent spares website.
One question that the learned members of this site may be able to assist with: I see pistons are available in 7.3:1 and 8:1. Which would best suit a '51 comet? Intended usage best described as, ahem, 'fast road', I believe is the term.

J

 

[BigEd]

Dear James,
Welcome to the club and the VOC forum.
I'd go for 8:1 and in fact I've just rebored my Rapide and "upgraded" from 7.3:1 to 8:1.
If you are on 8:1 and find it too fast you can always shut the throttle a bit.

 

[timetraveller]

I agree, 8:1 are the way to go. Remember that the lower ratios stem from a time when the octane rating of fuel was much lower than it is now and even then Vincents provided 8:1 for the USA market. Good luck with it.

 

[CoreyL]

Building on the initial question: is there a consensus regarding old piston design (cast, heavy, generous skirt) versus new design (forged, light, skimpy skirt)? I've read that the newer pistons don't do as well in older, air-cooled engines. I'm in the middle of replacing a seized 8:1 CP piston in my '50 Meteor.

 

[Colin]

I much prefer a "skimpy" skirt, but that has nothing to do with pistons!!

 

[CoreyL]

I much prefer a "skimpy" skirt, but that has nothing to do with pistons!!

I must admit, that reply put a smile on my face. There's one in every crowd...

 

[nkt267]

I have an omega piston in my Comet but i disregarded the low clearance and went with 5 thou instead of the recommended 3. it's been in there for a few years now and the sweet spot for cruising is 60-65mph 1 or 2 up.
I got a bit more cautious after this happened, admittedly I had a 16 tooth gearbox sprocket(too small) riding 2 up on a hot day with and indicated speed of 85 mph for a sustained period.

 

[timetraveller]

It might or might not be significant that the piston seems to have suffered worst at the side, not fore or aft where the valve cut outs are. One thing that became clear after a few years of the (then) new low expansion and low clearance pistons was that three to three and a half thou was not enough clearance. It might be alright if everything is correct but the base of many cylinder barrels/muffs is not perpendicular to the bore. I, and others I know, have checked several muffs and generally find that the base is off by several thou. Re-machining the base is the way to go using specially made plugs with centres to go in each end of the liner.

 

[vibrac]

I have just come down in compression ratio after all I am always starting it but rarely exceeding 70 not with todays surveillance and if I was it wouldn't be on my Comet

 

[Mike 40M]

Welcome. I have 7.3 in my Comet. At 68 kilograms it's easy starting. Once upon a time sold a Goldie Clubman cheap, cause I seldom managed to kickstart it. Nowadays have other bikes if I want to go fast.