Comp ratio and barrel height info please

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Fast Eddie

Fast Eddie

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Gents,
Further to my earlier posts, today a I re-assembled the motor (of my '74 MK2A) to properly measure the CR.
The motor is running a standard bore. I cannot see a liner, so I assume the barrels have been replaced by a PO.
The first thing I noticed is that a standard piston sits approx 0.030 below the surface of the barrel. This is with a standard paper base gasket in place.
With the standard pistons, the combustion chamber volume is 62cc. As it is a standard bore, this gives 7.67:1 CR.
I assume you'll agree that this is pretty bloody pathetic?!
Can someone help me understand why my pistons are so low in the barrel? I thought they were supposed to be flush with the top? Have I got some kind of aftermarket barrels that are taller than standard? Or is this how the factory nailed them together?
On the positive side, my high comp Omega pistons, labelled as 10.25:1, will actually give 9.44:1 or (an estimated) 9.8:1 ish if I run without a base gasket. Me thinks, sans gasket is the way to go!
 
Eddie, I think the combustion chamber is more like 50cc s on the 850, a bit less on the 750.
So 414 +50 =464 ÷ 50= 9.28 to one. This is without a head gasket. Add a thick head gasket, subtract approximately one full point, thin. 020 flame ring, subtract one half point, super thin JS. .003" subtract negligible.
Also need to subtract something for the valve pockets and about .8 because your piston is down the bore thirty thou.

Glen
 
Stock 850's were supposed to be 8.5 to 1 as I recall.

Most actually measure closer to 8 to 1 with a base gasket. Jim
 
worntorn said:
Eddie, I think the combustion chamber is more like 50cc s on the 850, a bit less on the 750.
So 414 +50 =464 ÷ 50= 9.28 to one. This is without a head gasket. Add a thick head gasket, subtract approximately one full point, thin. 020 flame ring, subtract one half point, super thin JS. .003" subtract negligible.
Also need to subtract something for the valve pockets and about .8 because your piston is down the bore thirty thou.

Glen
Click to expand...
Hi Glen,
My combustion chambers (RH10 head) are actually 52cc. Which, with standard head and base gaskets, would have given 8.03:1 CR IF the pistons had been flush with the top of the barrel.
It Is the fact that they are 0.030 down the bore that surprised me! I can only assume its because they're pattern barrels.
Does anyone know how I can identify whether or not they are pattern?
 
Fast Eddie said:
worntorn said:
Eddie, I think the combustion chamber is more like 50cc s on the 850, a bit less on the 750.
So 414 +50 =464 ÷ 50= 9.28 to one. This is without a head gasket. Add a thick head gasket, subtract approximately one full point, thin. 020 flame ring, subtract one half point, super thin JS. .003" subtract negligible.
Also need to subtract something for the valve pockets and about .8 because your piston is down the bore thirty thou.

Glen
Click to expand...
Hi Glen,
My combustion chambers (RH10 head) are actually 52cc. Which, with standard head and base gaskets, would have given 8.03:1 CR IF the pistons had been flush with the top of the barrel.
It Is the fact that they are 0.030 down the bore that surprised me! I can only assume its because they're pattern barrels.
Does anyone know how I can identify whether or not they are pattern?
Click to expand...

Average stock barrel height runs about 4.550.
Give or take .010. :roll: Jim
 
comnoz said:
Fast Eddie said:
worntorn said:
Eddie, I think the combustion chamber is more like 50cc s on the 850, a bit less on the 750.
So 414 +50 =464 ÷ 50= 9.28 to one. This is without a head gasket. Add a thick head gasket, subtract approximately one full point, thin. 020 flame ring, subtract one half point, super thin JS. .003" subtract negligible.
Also need to subtract something for the valve pockets and about .8 because your piston is down the bore thirty thou.

Glen
Click to expand...
Hi Glen,
My combustion chambers (RH10 head) are actually 52cc. Which, with standard head and base gaskets, would have given 8.03:1 CR IF the pistons had been flush with the top of the barrel.
It Is the fact that they are 0.030 down the bore that surprised me! I can only assume its because they're pattern barrels.
Does anyone know how I can identify whether or not they are pattern?
Click to expand...

Average stock barrel height runs about 4.550.
Give or take .010. :roll: Jim
Click to expand...

Going by what I've learnt so far on this bike... +\- 0.010 is bloody good for Norton !!
 
>.010 is on a good day, on a bad day it may be +- .010 from side to side.
 
comnoz wrote;
Most actually measure closer to 8 to 1 with a base gasket. Jim
Click to expand...

Prior to modifying my 850, with stock pistons, but with cut outs for a 4s cam, the C.R. came out at 7.9:1. Was I shocked :shock:

I now run at 9.8:1, with other mods, and the motor is much crisper and more economical, so Fast Eddie, yes 7.67:1 is pathetic but does not surprise me much.
 
Apart from the barrels, have you checked the piston dimensions - i.e. the gudgeon pin centre? If that is correct then get the barrel top flange milled down to match. 850s are OK without a base gasket, but do use a quality sealant.
 
Reggie said:
comnoz wrote;
Most actually measure closer to 8 to 1 with a base gasket. Jim
Click to expand...

Prior to modifying my 850, with stock pistons, but with cut outs for a 4s cam, the C.R. came out at 7.9:1. Was I shocked :shock:

I now run at 9.8:1, with other mods, and the motor is much crisper and more economical, so Fast Eddie, yes 7.67:1 is pathetic but does not surprise me much.
Click to expand...
Hi Reggie,
How did you get 9.8:1? And (briefly) what other mods have you done, just out of interest?
 
ML said:
Apart from the barrels, have you checked the piston dimensions - i.e. the gudgeon pin centre? If that is correct then get the barrel top flange milled down to match. 850s are OK without a base gasket, but do use a quality sealant.
Click to expand...
As the error seems to be the same across 4 separate pistons, I'm thinking its the block that's at 'fault'. I'm intending to use it to my advantage though, and machine it as required for fine tuning to get a nice squish band clearance.
I just wondered how a barrel could be so far away from nominal really!
 
Fast Eddie wrote;
How did you get 9.8:1? And (briefly) what other mods have you done, just out of interest?
Click to expand...

I put a Steve Maney 920 barrel on with his pistons machined to suit. Also put one of Steve's stage 2 heads on and 34 mm Mk2 Amals and a PW3.

If I were to upgrade again, I'd use high comp pistons (such as you have) in standard barrels with a full auto head, not because I am unhappy with what I've got, but because the bike would look "standard," and probably have as much or possibly more low down torque, based on the Fullauto head reviews.

I have some 850 barrels in my garage if you would like to borrow them to compare barrel height. I live at Leeds.
 
Hi Jim, about those 4.550 , are they the height of the cylinder (saying between head face gasket and bottom face gasket), cause it seems a bit short, if it's 4 ,550 inches..............!
 
Fast Eddie said:
Gents,
Further to my earlier posts, today a I re-assembled the motor (of my '74 MK2A) to properly measure the CR.
The motor is running a standard bore. I cannot see a liner, so I assume the barrels have been replaced by a PO.
The first thing I noticed is that a standard piston sits approx 0.030 below the surface of the barrel. This is with a standard paper base gasket in place.
With the standard pistons, the combustion chamber volume is 62cc. As it is a standard bore, this gives 7.67:1 CR.
I assume you'll agree that this is pretty bloody pathetic?!
Can someone help me understand why my pistons are so low in the barrel? I thought they were supposed to be flush with the top? Have I got some kind of aftermarket barrels that are taller than standard? Or is this how the factory nailed them together?
On the positive side, my high comp Omega pistons, labelled as 10.25:1, will actually give 9.44:1 or (an estimated) 9.8:1 ish if I run without a base gasket. Me thinks, sans gasket is the way to go!
Click to expand...


It is my understanding that 1974 850's were assembled with NO base gasket (as per workshop manual) from the factory. I used a .020" head gasket to come up a bit from 8.5:1 originally designed. Loctite 518 at the base.
 
concours said:
Fast Eddie said:
Gents,
Further to my earlier posts, today a I re-assembled the motor (of my '74 MK2A) to properly measure the CR.
The motor is running a standard bore. I cannot see a liner, so I assume the barrels have been replaced by a PO.
The first thing I noticed is that a standard piston sits approx 0.030 below the surface of the barrel. This is with a standard paper base gasket in place.
With the standard pistons, the combustion chamber volume is 62cc. As it is a standard bore, this gives 7.67:1 CR.
I assume you'll agree that this is pretty bloody pathetic?!
Can someone help me understand why my pistons are so low in the barrel? I thought they were supposed to be flush with the top? Have I got some kind of aftermarket barrels that are taller than standard? Or is this how the factory nailed them together?
On the positive side, my high comp Omega pistons, labelled as 10.25:1, will actually give 9.44:1 or (an estimated) 9.8:1 ish if I run without a base gasket. Me thinks, sans gasket is the way to go!
Click to expand...


It is my understanding that 1974 850's were assembled with NO base gasket (as per workshop manual) from the factory. I used a .020" head gasket to come up a bit from 8.5:1 originally designed. Loctite 518 at the base.
Click to expand...

Well that's interesting Concours, and it would remove most of the difference, actually leaving only an extra 0.010, which as Jim says, is probably within the normal tolerances.
Locative 518 is a good tip too, thanks, I'll refrain from using bathroom sealant this time!
 
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