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Poorish compression and piston leak by

nkt267

Well Known and Active Forum Website User
VOC Member
I bought a new toy for myself and as I have thought that the compression on my Comet has not been what it should be I would use it to take a look through the plug hole.
I have 3 Vincent singles in my garage that I have rebuilt and mine has the worst compression.
I had the magneto fill with oil causing my last trip home on the breakdown truck and following various ideas put forward I have investigated further.
I don't have the equipment to do a leakdown test,but listening to the sound of air passing the piston from the timing case was good enough for me to realise that the problem is most likely excess crankcase pressure. So fix it properly or fit a big bore breather:rolleyes:.The bike runs well enough and I know that the engine will have to come apart,at least the head and barrell,but I bought this bore scope/microscope and decided to have a look.

http://www.vincentownersclub.co.uk/photos/showphoto.php/photo/9811/title/comet-cylinder-bore/cat/906

The bore looks quite scored to me,I hope it's not an optical illusion as I hate stripping and engine for no reason.
This was a new liner,muff,and piston about a 1000 miles ago. I really took it easy running it in,about 600 miles at a maximum of 40 ish, before I started to increase the loads and speeds. There is no sign of nipping up whilst riding and the bike still does about 80 ish flat out accelerating well.
I suspect that this is a ring problem but it will still cost me a new piston and rebore.
 
Last edited:

Monkeypants

Well Known and Active Forum Website User
VOC Member
That does appear to be badly scored, although it is difficult to get an idea of the depth of the score marks. Are you running airfilters?

Glen
 

davidd

Well Known and Active Forum Website User
VOC Member
Wow, a bore scope, but no leak down tester! We know which one is more fun. I am sorry that it looks so dire so quickly after the rebuild. I would presume that grit at sometime went into the cylinder. This is a problem that I used to have on the racer, particularly when running at Daytona where there is sand in the air almost all the time. It is also difficult to use air cleaners because of the performance issues. The cylinder would score very quickly.

I hate to suggest my solution which was to use a Nikasil coated cylinder. I know it is costly, but so is the alternative. If you use an aluminum sleeve, you can also use the old muff.

I think you did the right thing by doing a simulated leak down test. The if you can supply compressed air to the spark plug hole by some method it will give you the same result. For this type of diagnosis, you do not really need the data supplied by the gauges.

It has been said before, but the singles do need a lot of piston clearance. There is a lot of uneven and rapid heating when using a single. If the piston clearance closed to much it would probably produce similar results. It will be interesting to see what you find on tear down.

Best of luck,

David
 

nkt267

Well Known and Active Forum Website User
VOC Member
When the liner was bored I went with the recommendation of the borer and had it With 5-6 thou clearance as against the 3 thou that is suggested for Omega pistons.
I have never used Air filters as I never thought that we needed them much in this country, I may have to think again..john
 

timetraveller

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VOC Member
I do wonder whether keeping the speed down to 40 for 600 miles might not have caused some slogging of the engine which is what I was always tought to avoid whilst running in, rather than too many revs. Having said that I do not see how slogging could have caused the liner to be scored. Before going too far is it possible to insert some really thick oil through the plug hole, turn the engine over a few times to try and get the oil all round the piston top and rings and then do the compression test again? It might just give you some idea as to how bad the leak is and as to whether more high speed running in might wear down some of the scoring. Good luck with it. A thousand miles is just not acceptable and suggests something more than normal airborne grit. ? winter salt and sand off the roads.
 

vibrac

Well Known and Active Forum Website User
VOC Member
5-6 thou is my figure still had a slight nip when hauling trials s/c lets face it a quick strip to the cylinder base is not a big deal (get a good bolt set up on side of bench to take the front end complete wheel it in screw it down then back to the engine and and carry on stripping) better and quicker to hold the piston and barrel in your hand and make a good assesment than fiddle about and worry and if you havent its a good chance to add those electrical push connectors twixt front end and the rest of the plot
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
I have also had experience with a "nipper" as covered in some of my previous posts. I'll bet your rings are seized in the lands- be interesting to confirm. This modern "petrol" seems to burn hotter and require less advance as well. I only use 98 octane, because I've been told it burns cooler. I read somewhere of an old gasper (Bentley or Rolls-Canardly? setting off the sprinkler system in a car garage due to engine heat- query modern fuel. Rolls- Canardly- rolls downhill and canardly-------------
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
I have also had experience with a "nipper" as covered in some of my previous posts. I'll bet your rings are seized in the lands- be interesting to confirm. This modern "petrol" seems to burn hotter and require less advance as well. I only use 98 octane, because I've been told it burns cooler. I read somewhere of an old gasper (Bentley or Rolls-Canardly? setting off the sprinkler system in a car garage due to engine heat- query modern fuel. Rolls- Canardly- rolls downhill and canardly-------------
G'day Chankly, supposing the rings were seized in the lands, why would that cause the scoring?
Regards
Kevin
 

Howard

Well Known and Active Forum Website User
VOC Member
Because I'm on holiday and feeling good, I don't think the scoring is as bad as you think. Judging by the curve of the piston there's a lot of magniifcation, and comparing the scratches with the 5 thou piston clearance it doesn't look too bad. Also, on the middle piston photo, there's a reflection of the piston crown on the bore and the scratches don't seem to distort the reflection.

Oh well, back to my half full glass of beer and a whinge about knackered kickstarts on a sunny bank holiday. :(

H
 

aldeburgh

Well Known and Active Forum Website User
Non-VOC Member
Hi John
I think you need to try to get your thumb nail down the plugole and have a feel of the bore, the endoscope may be not telling the whole story.
As to crankcase pressure , I have recently experimented with the breather valve timing on my Comet, the procedure is extremely technical :eek: and involves blowing up the breather pipe while feeling the piston crown with a rod and slowly pushing the kickstarter, the results were very surprising as for most of the downstroke the valve was shut and for most of the upstroke the valve was open!
I have now timed the valve to my own preference and ignored the B timing mark.
The engine now revs more freely and I may have found at least a half of a horse in the process:cool:
 
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