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New pistons

Puddle jumper

Well Known and Active Forum Website User
Non-VOC Member
Hi Gang, Another quick question for you, I am just fitting 8 to 1 compression ratio Omega pistons to my twin, and want to know how much clearance is required between the valves and piston cut outs. cheers
 

Tom Gaynor

Well Known and Active Forum Website User
VOC Member
The rule of thumb (as applied to Norton Manx engines) is 40 thou minimum. I should have thought a Vincent, with a combustion chamber like the Albert Hall, would have feet to spare.
 

van drenth

Well Known and Active Forum Website User
VOC Member
Depends which type of camshaft is fitted, with the mk 2 you better check if the pocket for the exhaustvalve in the piston crown is deep and wide enough to the outside of the liner.

Regards, van Drenth JC
 

clevtrev

Well Known and Active Forum Website User
VOC Member
Hi Gang, Another quick question for you, I am just fitting 8 to 1 compression ratio Omega pistons to my twin, and want to know how much clearance is required between the valves and piston cut outs. cheers

Put some plasticene on the valve pocket areas, and turn the engine over by hand. Look and see what happens to the plasticene. Take a blade and section the plasticene while it is still on the head, that is make a cut along the centreline of the engine. Peel off one half of the plasticene, then you can see exactly what the valve is doing. Aim for a minimum thickness of 2 mm. That will allow for any stretch of the rod and valve at speed.
 

timetraveller

Well Known and Active Forum Website User
VOC Member
And be particularly careful of the area just outside the cut out towards the outside of the piston. Years ago it was this area of the piston which used to foul the valve head as the cut outs on top of the piston did not have a large enough diameter.
 

vince998

Well Known and Active Forum Website User
VOC Member
And be particularly careful of the area just outside the cut out towards the outside of the piston. Years ago it was this area of the piston which used to foul the valve head as the cut outs on top of the piston did not have a large enough diameter.

Something else to watch out for here are the later die cast muffs!!
They tend to compress on one side than on the other.
Not a problem if you bore and hone the cylyder using the top lip of the liner as a datum, but in a moment of (not uncommen) wisdom, i got my guy to use the bottom of the muff as a datum thinking this would bring the bore back to 90 degrees with the crankcase.
This it did, but moved the axis of the bore within the liner and caused clearence problems between the (in my case) exhaust valve and piston cut out. :confused:
The change in bore axis was so large that you could see it on the taper at the bottom of the inside of the liner. (the taper was smaller on one side than on the other)
 

clevtrev

Well Known and Active Forum Website User
VOC Member
Something else to watch out for here are the later die cast muffs!!
They tend to compress on one side than on the other.
Not a problem if you bore and hone the cylyder using the top lip of the liner as a datum, but in a moment of (not uncommen) wisdom, i got my guy to use the bottom of the muff as a datum thinking this would bring the bore back to 90 degrees with the crankcase.
This it did, but moved the axis of the bore within the liner and caused clearence problems between the (in my case) exhaust valve and piston cut out. :confused:
The change in bore axis was so large that you could see it on the taper at the bottom of the inside of the liner. (the taper was smaller on one side than on the other)

I told your Dad years ago, that diecast muffs are to be thrown away, and never put anywhere near your bike.
 

vince998

Well Known and Active Forum Website User
VOC Member
I told your Dad years ago, that diecast muffs are to be thrown away, and never put anywhere near your bike.

Cheers Trevor, i´ve since changed them. (and put them in the box along with 10 other liner/muff/piston combinations)
Them where different times then, and money was tight (especially with three kids to bring up)
 

methamon

Well Known and Active Forum Website User
Non-VOC Member
Regarding the correct datum from which to bore a cylinder. At one time it had not occured to me however I believe that I read in MPH maybe 4 years ago that the datum should be the base flange of the cylinder as this then makes the axis of the cylinder perpendicular to the crankcase mouth / deck. Armed with this information I asked my re-borer which cylinder end he intended to bore from. The look was withering and he proceeded to tell me that he had been re-boring for the last 50,000 years; using the base as a datum so in conclusion I don't think using top lip of the liner is correct.
 

clevtrev

Well Known and Active Forum Website User
VOC Member
Regarding the correct datum from which to bore a cylinder. At one time it had not occured to me however I believe that I read in MPH maybe 4 years ago that the datum should be the base flange of the cylinder as this then makes the axis of the cylinder perpendicular to the crankcase mouth / deck. Armed with this information I asked my re-borer which cylinder end he intended to bore from. The look was withering and he proceeded to tell me that he had been re-boring for the last 50,000 years; using the base as a datum so in conclusion I don't think using top lip of the liner is correct.
That assumes that the deck is square with the mainshaft, there are one or two like that !
 
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