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Muff to Crankcase shims and gaskets

Phil Baker

Well Known and Active Forum Website User
Non-VOC Member
Hi all

Stumpy advised me to post this in order to decide what to use when I reassemble the top of the engine : the muff height is 3.059 inches; the piston has 9010 on it and
Conway Motors say that's 9:1 compression; the previous master-engineer has fitted 1 x 1.63mm, 1x.93mm aluminium plates and 2 x 0.17mm paper gaskets which, by my reckoning, reduces 9:1 to about elastic-band strength. What does anyone feel is the correct shim/gasket combination to aim for, please, to get the compression down to about 8:1 and prevent my beating the big end to death in the first couple of miles?

Regards

Phil
 

Jim Richardson

Well Known and Active Forum Website User
VOC Member
I have just put my twin back together with 9:1 pistons, only because they were there, previous owner had fitted 2 decompression plate, I think they were 1.63mm and 2 gaskets which he felt would get it down to 8:1.
I have had no problem so far at 9:1 although Dave Hills thought it might be a bit much round town.
my thinking was that as I was trying to reduce oil consumption by fixing leaks and fitting new valve guides, I may as well try them and if it doesn't work out I will replace them with a rebore and new 8:1 pistons next winter.

JimR
 

BigEd

Well Known and Active Forum Website User
VOC Member
VOC Forum Website Moderator
Maths was never my strong point. (If I'd ever had a strong point that is.;-))

Here goes.

By calculation a 9:1 head volume would nominally be: =55.42 cc
By calculation an 8:1 head volume would nominally be: =62.35 cc

Increase in head volume from 9:1 to 8:1 =6.93 cc

On an 84 mm bore 1.25 mm of stroke =6.93 cc

Total thickness of shims and gaskets under each barrel for 8:1 =1.25 mm

Does this make sense or is my maths as bad as I think it is?

Hi all

Stumpy advised me to post this in order to decide what to use when I reassemble the top of the engine : the muff height is 3.059 inches; the piston has 9010 on it and
Conway Motors say that's 9:1 compression; the previous master-engineer has fitted 1 x 1.63mm, 1x.93mm aluminium plates and 2 x 0.17mm paper gaskets which, by my reckoning, reduces 9:1 to about elastic-band strength. What does anyone feel is the correct shim/gasket combination to aim for, please, to get the compression down to about 8:1 and prevent my beating the big end to death in the first couple of miles?

Regards

Phil
 

Phil Baker

Well Known and Active Forum Website User
Non-VOC Member
Well, BigEd, it all sounds very technical - shall I throw in the fact that it's 40 thou oversize? That'll hurt the maths brain!! :eek:
 

BigEd

Well Known and Active Forum Website User
VOC Member
VOC Forum Website Moderator
Dear Phil,
That gives you approximately 10 cc more capacity on each cylinder so yes it will make a difference but not enough that we would notice.
Re Maths brain: My pencil lead has broken and my calculator is in melt down.;-)

Well, BigEd, it all sounds very technical - shall I throw in the fact that it's 40 thou oversize? That'll hurt the maths brain!! :eek:
 

Pete Appleton

VOC Hon. Editor
VOC Member
VOC Forum Website Administrator
A point of pedantry

Sorry Eddie, Your maths might be worse than you think.

CompRatio = ( SweptVol + Head Volume) / Head Volume


By calculation a 9:1 head volume would nominally be: =62.35 cc
By calculation an 8:1 head volume would nominally be: =71.26cc

Increase in head volume from 9:1 to 8:1 =8.9cc

On an 84 mm bore 1.6 mm of stroke =8.9 cc

Total thickness of shims and gaskets under each barrel for 8:1 =1.6 mm

I agree that the oversized piston makes about 10cc of increase in swept volume but as the compression plate works by lifting the oversized bore the thickness needs to stay the same.

One of us must be wrong!

Peter
 

roy the mechanic

Well Known and Active Forum Website User
VOC Member
As you do not actuallty know what your start point is, I would reccomend the following, assemble both top ends minus the valve gear, measure the compression space at top dead centre on BOTH cylinders, then refer to tuning for speed for the neccesary equasion to establish what you are starting out with, only then can you make an accurate measurement to acheive your desired comp-ratio.Anything less will only be guesswork. O-k it sounds like a lot of fafing around, but that's what it takes to build a "good" motor!
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Hello Phil, I have not been in the engine for some time, But from what I remember, You want the top side of the piston about level with the side of the top of the barrel and you won't be far out, I used to run 12-5 pistons, They stuck out the top by miles !! Other than that do what norman says, He knows a thing or two, Cheers Bill.
 

timetraveller

Well Known and Active Forum Website User
VOC Member
The volume of a standard Vincent cylinder head is 103.5 cc. The swept volume is 499 cc. Total is therefore 602.5 cc. If one had a flat topped piston the compression ratio would be 602.5/103.5 = 5.8:1. That is why pistons have domed heads. Each millimetre of swept volume is 499/90 = 5.54 cc so if the total depth of the compression plates and gaskets is 2 mm there is an increase in unswept volume of about 11 cc bringing the effective cylinder head volume up to 114.5 cc. The aforesaid flat topped piston would then give a compression ratio of 5.36:1.
In order to have a compression ratio of 9:1 one has to have an effective cylinder head volume of 62.5 cc. Increase this by the 11 cc above and one gets a compression ratio of 7.79:1, that is, near enough to 8:1. The combustion chamber shape is not ideal but lots of us have done it and been happy with the results. Also lots of us have also used 9:1 and been equally happy. Enjoy!
 

BigEd

Well Known and Active Forum Website User
VOC Member
VOC Forum Website Moderator
Dear Peter,
My maths is always worse than I thought.;-)

I forgot to add the volume of the head to my calculation.

Sorry Eddie, Your maths might be worse than you think.

CompRatio = ( SweptVol + Head Volume) / Head Volume


By calculation a 9:1 head volume would nominally be: =62.35 cc
By calculation an 8:1 head volume would nominally be: =71.26cc

Increase in head volume from 9:1 to 8:1 =8.9cc

On an 84 mm bore 1.6 mm of stroke =8.9 cc

Total thickness of shims and gaskets under each barrel for 8:1 =1.6 mm

I agree that the oversized piston makes about 10cc of increase in swept volume but as the compression plate works by lifting the oversized bore the thickness needs to stay the same.

One of us must be wrong!

Peter
 
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