E: Engine New member, and comet question

[nkt267]

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.

Yes, by reading and re-reading a lot of other posts I came to the same conclusion. I bought a new muff and faced both ends parallel. I got a liner that was oversize on the o/d and turned the muff to a good interference fit and then placed under my press with a light pressure till it cooled so that there was no chance of the liner lifting clear of the liner.
I also found from this that there were problems with the head recess and corrected that at the same time. This took me a while as I am not a very skilled machinist, getting better all the time.
I think that I've got it right now. I'm running 8:1 with Mk2 cams.

 

[Gene Nehring]

8:1 with a nice chrome top ring made by Total Seal in the US. You can't go wrong. I have 4K miles on a recent rebuild and runs great.

 

[davidd]

James,

There have been many Comet owners that have seized their Comets very quickly. There are some major traps. One is to use recommended clearances that are often based on twins. The second is trying to get tight clearance in general when it will rarely produce beneficial results. If the rings fit properly you will rarely suffer from generous piston clearance in a single.

The single is different because it can build up heat quite quickly, compared to the twin, and that heat will be mostly in the piston. I think CP recommends .003" clearance in general for Vincents. That works fine for my Nicasil aluminum liner, but if I were using the stock Vincent liner I would go with .006" clearance. If you go with .003" it will likely seize as you ascend your first hill.

As for the amount of compression you want, it is based on what performance you are looking for. If you wish to go with the Mk2 cam I would go with a higher compression piston because the Mk2 when running looses a lot of compression, at least one point. On the other hand, the Mk1 or Mk3 will preserve most of the compression when running and will give you a higher running (or dynamic) compression ration than the Mk2. This is one of the reasons Mike 40m's bike starts so well when the Mk2 bikes can be balky.

If you want performance go with the Mk2 and the 8:1. Don't forget to check the cam and rocker clearances because the Mk2's will ruin your summer and your wallet if you don't check them.

David

 

[CoreyL]

I am replacing the seized CP 8:1 forged piston that had been in my B Meteor with a low expansion 8:1 Omega piston from VOC spares. I live in South Central Texas where temps can easily be in the 90s much of the year. From what others have said, I think 4 thou is the minimum clearance I would want. Does 5 or 6 thou seem like a more prudent choice? I have no intention of flogging the beast on the track (or elsewhere).

 

[davidd]

Corey,

Personally, I think 4 thou is on the tight side. At one point in the Factory's life I believe they set the clearance of all their builds at 4 thou and I was told that there were a series of seizures on all the bikes and the clearance figure was upped. Even if this is not true, I would ask the question, what benefit do you derive from reducing the clearance from 6 to 4 thou? The piston is unlikely to live a longer life, particularly if it seizes or partially seizes. If the rings are fit properly, they won't protest the extra clearance. I think it is good insurance.

David

 

[vibrac]

I always raced with a 6 thou clearance I live near T&L and thats what I asked for but in fairness they said they usually go a little less they certainly do a lot of engines and have top class machines for their work

 

[timetraveller]

CoreyL, I think you are one the right track with five to six thou. However, do check that the base of the cylinder muff is at right angles to the bore. Many are five thou out and if that is side to side, rather than front to back, then five thou piston clearance will still not do it. If you want a nice soft engine then use Mk I or Mk III cams rather than Mk IIs. Enjoy those temperatures once you have the bike on the road.

 

[BigEd]

I always raced with a 6 thou clearance I live near T&L and thats what I asked for but in fairness they said they usually go a little less they certainly do a lot of engines and have top class machines for their work

Who or what is T & L?

 

[vibrac]

Who or what is T & L?

Oh Eddie you surprise me! they are in the classic press adverts etc
http://vintage-engine.net/
They do tons of car and motorcycle work vapor blast to shine even my R soaked crank cases cylinder boring,valve seats etc etc

 

[BigEd]

Oh Eddie you surprise me! they are in the classic press adverts etc
http://vintage-engine.net/
They do tons of car and motorcycle work vapor blast to shine even my R soaked crank cases cylinder boring,valve seats etc etc

Never heard of them!
Blast to shine? Don't do shiny. You being the #1 "R" fan I'm surprised that you want to remove all that baked on character.
I was poking about in "the shed" and found a some "R" left over from the old racing days. I must get a few drops smoking on the stove to remind me of those great times.