ET: Engine (Twin) ET35 to valve guide clearance

davidd

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Bill,

Frankly, I don't remember spending so much time fitting these bolts to earlier projects, but it looked to me like the parts were exactly the same. I don't like grinding away the top cord of the rocker arm, but I was very careful to try and preserve as much strength as possible. The Comet engines have always run to high RPMs without problems.

I have always used the Ian Hamilton designed piston. Carleton Palmer started racing before I did, so I purchased spare pistons from him. He encountered a few problems with the design. He moved the ring lands lower and closer together. He could do this easily with the use of thinner and better rings. Second, he enlarged the valve pockets.

I would have expected that the same could be done easily for other Vincent pistons. I am a little surprised that there is any problem with the valve to piston clearance as the stock pistons are so low in compression. The Hamilton design is 11.23:1 and is quite top-heavy. The pistons tend to scuff as a result. Surprisingly, I have never had a valve touch except when the big end failed.

DSCN1296 91.jpg


David
 

Bill Thomas

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Never seen a Piston like that before, I spoke to Ian many times, And He never told me :D .
I used standard 12 to 1, But I took the pocket right out, Right to within a Nats of the top ring,
I thought I might be taking a risk, But it never gave trouble, I think I was running 24 or 26 ign',
Used it on the road too !.

I think how deep the valve seat is can make a big difference with the rocker arm movement.
But the last rocker feed bolt I fitted about 5 years ago, I was shocked how much movement was lost after
Putting it together, just looking at it in my hand.
I think I have an Omega for my new Comet, But don't know if I dare fit it, I love the old Specialloids.
 

timetraveller

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That's an interesting piston David. Is that a ceramic coating on the crown and skirt? The compression ratio used is also interesting. Another tuner, I know, used a different shape of squish/quench band and found that 10.5:1 was too high and the engine was happier on 10:1. I take it that you are using petrol/gas intended for road use rather than alcohol?
 

nkt267

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David is correct and it is a bit of a can of worms. It not only depends upon the static figures but also how aggressive the cam is and what valve springs are being used and are they still in good order. If the time and interest is there then I would still recommend checking valve head to piston clearance as well. I suppose we should have asked whether the engine is going to be raced or subjected to high revs for some other purpose. If the owner is not going to exceed four thousand revs and the springs are in good order the there is probably nothing to worry about anyway.
My thought exactly about the amount of revs being used.
This is a road bike and my thoughts initially were that if there is clearance and the valve travel is not restricted by the guide then all should be well..
We are going to check valve/piston clearance,that goes without saying really , especially considering that the cams are not standard. I even checked that in my Comet when I fitted the mk2's, I had new valve seats at the same time so I was being extra cautious.
 

nkt267

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Never seen a Piston like that before, I spoke to Ian many times, And He never told me :D .
I used standard 12 to 1, But I took the pocket right out, Right to within a Nats of the top ring,
I thought I might be taking a risk, But it never gave trouble, I think I was running 24 or 26 ign',
Used it on the road too !.

I think how deep the valve seat is can make a big difference with the rocker arm movement.
But the last rocker feed bolt I fitted about 5 years ago, I was shocked how much movement was lost after
Putting it together, just looking at it in my hand.
I think I have an Omega for my new Comet, But don't know if I dare fit it, I love the old Specialloids.
what about this for a 7.4:1 in my 1932 Sunbeam 90 engine
 

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timetraveller

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Just think of the flame front during combustion. It has to climb up one side of the piston, slither across the top and then tumble down the other side where it sits panting and wondering what happened. Interesting to see how things have changed in ninety years.
 

Bill Thomas

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It didn't do my Bike any harm, How many Road Vin's do you know that did over 140 mph ?.
I don't like these squashy pistons.
 

timetraveller

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Well mine did at the end of a half mile sprint at Long Marston. Ridden to and from the meeting. Not a speedo reading but a rev counter reading which would have marginally lagged the real value.
 
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