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Compression ratio choice


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Hi folks,

Having seen the success of others posting questions, I thought I'd have a try.

I have a series C Rapide engine which is fitted with new 9:1 specialloid pistons. I have not run this engine. I am wondering if I am going to run into problems with the increased compression ratio. Increasing the CR is clearly going to change the characteristic of the engine but I don't know how significant this would be. For instance as the CR changes at what ratio would I notice a loss of bottom end torque.

My quandry is should I reduce the CR or not. Stevens in KTB states that there is nothing to be gained by exceeding 8:1, indeed he says its advisable not to exceed 7.3:1. I also remember reading somewhere that higher CRs can eventually cause failure of the kickstart mechanism. Will I need to rebalance the crank? Paul Richardson says no, it seems although the pistons vary in weight the engine doesn't rev enough in normal use for it to be noticable. Will the engine be more difficult to start?

So I can leave it at 9:1 or I can try to reduce it. If I reduce the CR I have either to resort to skimming pistons which I don't particularly want to do or I can use compression plates. I've calculated crudely from Tuning for Speed that I would need approx 0.112" of compression plates to go from 9:1 to 7.3:1. PEI states that a maximum of 0.062" can be used else problems will be encountered with rocker adjustment. 0.062" packing should give approx 8:1 CR.

So is this all resonable? Has anybody out there got any practical experience of all this. Its over to you.




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Hi Chris, from a personal point of view - no pretense to professional expertise, I would stick with your 9:1 pistons, for the amount of miles youre likely to do in a year I dont think you would notice any change in bottom end torque, ( Ive run 9's for years & if Im determined I can still abuse the shock absorber!) As to the kick start mechanism, surely if you use the valve lift in the correct way there should be no difference in wear rates. ( Len or someone will now dismantle my theory!!)


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Ahhh - now there's a topic for a chat....

Evening All,

Couldn't resist it - let's certainly continue to answer Chris's question on CR - but how about some views on the correct way to use the valve lifter......that should merit a few different opinions I suspect...


As to the kick start mechanism, surely if you use the valve lift in the correct way there should be no difference in wear rates. ( Len or someone will now dismantle my theory!!)

Robert Watson

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I have been running forged pistons at 9.4:1 for several years and several 1000's of miles. The Maratimes in 2001 around 5200 in a month, Australia -- well over 6000 and close to 7000, California last month 3200. 700 in the last 2 days. Mk 2 cams, electronic sparks (as opposed to those old non electronic ones???) funny carbs. It will run fine on 87 octane - although I usually run it on 89 or 91, pulls like a feight train. Much prefer it to the Black B with 7.3 and Mk 1's

My advice -- Keep the 9's in it.



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I agree. Stick to the 9:1s. If you want to reduce to 8:1 you might find it more tractable in traffic but it is completely the wrong thing to do to use compression plates as you end up with an inefficient combustion chamber shape. Better to change the pistons before you use them if you really want to go lower.

Regarding kick starting; I am continuously appalled at some of the ways I see people starting their bikes. They fiddle around with the valve lifter and gently turn the engine over until they think that they are on the cylinder with the longest dwell before it fires. They then let go of the valve lifter and try to kick the engine over against the compression, hoping that they have enough momentum built up by half way down the stroke of their kick. No sir. Prime your carbs or use the chokes or whatever you have, turn the engine over gently with the kick starter a few times to get the oil started on its journey, then turn on the ignition, unless you still have a magneto in which case it is already on and the slow turning of the engine will have prevented it firing, then with the valve lifter lifted, kick down and half way down the stroke let go of the valve lifter. That way there is no fiddling about trying to find the correct position of the engine. You have lots of inertia built up because there is no resistance to the turning of the engine and hence the kick starter and you have given the whole kick starter mechanism a much easier time of it. Trying to boot the engine over against compression might be good for your right leg muscles but it is an abomination for the kick starter mechanism. If the bike does not start first or second kick with the above approach then there is probably something wrong.

Be gentle with it and it will serve you well.


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Hi all. I have been starting my twins as timetraveller described since 1962. We have had the Shadow since 1968 and it doesn`t show any problems with the valve lifter gear.
9.1 pistons. I have these in the Prince but thought it might be a bit high for todays petrol and my 11 stone to kick over, so I put one compression plate under each pot. I am happy with the result and have no probs starting and the bike runs well and is very pleasant.
The same Prince has not had the crankcases parted since leaving the factory, 43,000 miles, and the engine seems smooth so the original balance seems ok.

Peter Stokes

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Valve lifters

Hello folks.

It is a while since I had my Rap.,but I remember the starting procedure well.

Lightly down on kickstart to engage ratchet, Valve lifter up, long slowish swing on the kickstarter to spin the engine and drop the lifter at some point (not critical) towards the end of the swing. It did not matter what position the crank was in to start with. It was an easy starter, had a good mag., I suppose.

No brute force was needed, unlike the quick flick of a kick to get a Velo, for example, going.

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Ken Tidswell

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Starting and high CR

I ran for many years on 10:1's, with valve pockets filed down to remove the sharp edges. It was easily started provided that the correct starting procedure was employed , this was with leaded fuel and 36 deg advance.


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Can I just use this reply to thank everybody for responding to my thread. It seems that I should keep the pistons as they are which is a relief.

I bought the pistons many years ago from Roger Slater who did the rebore as well. Unfortunately after reboring the engine he informed me that he had no standard Shadow or Rapide pistons only 9:1s. I readily agreed as being young I had visions of travelling at even faster speeds not thinking of the consequencies. Thanks to your replies I can now progress to the next stage of the rebuild with something less to think about. Thanks again.


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I, too, have always started Vins by the method described by timetraveller, including the point about turning the engine over a little bit before the first real kick. Not so much to get oil moving, but more just to get myself and the bike 'primed'. I just slowly kick through one complete travel of the kick start lever, with the valves lifted the whole time.

But bikes vary, and if you have a different method that works for you, great.

One other thing that seems to work for me is to only tickle the carbs until the threads on the bottom of the carb body get wet, I don't tickle them until it squirts out the float bowls. With the climate here in So Cal, I only recall needing the chokes one extra cold morning in December many years ago.
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