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New pistons

Puddle jumper

Well Known and Active 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 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 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 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 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 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 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 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

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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 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 !
 

Puddle jumper

Well Known and Active Website User
Non-VOC Member
Hi Guys,
Before I had the cylinder’s bored I turn up a mandrel (it was left in the chuck so running true at all times) to the exact size of the existing bore then mounted each cylinder on it using the tail stock with a pad to clamp the cylinder whilst machining, then faced the base (removing minimum amount to clean and match the cylinders). After this I also machined in an ‘O’ ring groove to help seal the base with out using a gasket.
The base is now true to the bore and can be used as a mounting face for boring.

Cheers Paul
 

Puddle jumper

Well Known and Active Website User
Non-VOC Member
No, the bores were very good to start with no lip at the top and on a trial hone only took 0.001" to clean up, as long as the bore is perpendicular to the base it will be correct, there is more chance that the muff has moved over the years than the liner!!!!. And in my case the base was running within 0.003”( and that was a slight hollow ) before I machined them
Cheers Paul
 

timetraveller

Well Known and Active Website User
VOC Member
If your comment about muffs rotating was a 'reductio ad absurdam' then fine as there is no chance of the muffs rotating due to four studs passing through them. But for those with less experience I have seen lots of bikes where the liner has rotated, easily spotted by the lubrication hole on the rear no longer being on the rear. Machining out the muffs and fitting thicker liners to regain the original six thou interference fit is the answer to those with that problem or, of course, alloy liners with a ceramic coating inside.
 

craig

Well Known and Active Website User
VOC Member
Comet piston fit

I have 0.100" piston protrusion with a new piston fitted to a new liner. Is this okay? Are there any standard limits for this?
Craig
 

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timetraveller

Well Known and Active Website User
VOC Member
It is the same answer that I gave for the clearance between valves and pistons. Put some Plasticine round the top of the piston, put the cylinder head back on (you do not need to fit the tubes etc which cover the push rods) and turn the engine over a few times. Remove the cylinder head and see what has touched and where. Provided that you have at least 50 thou clearance every where you will be alright. Note that if you fit cylinder heads and pistons designed to give a squish band there must be at least 35 thou clearance to allow for expansion and stretch etc and not more than about 45 thou if the squish band is to do its job. (If the term squish band is not familiar to you then I think that the American expression is quench band but am not totally sure about that.)
 

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