Piston Crown - How much should it protrude above the cylinder muff

SteveF

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Non-VOC Member
I've got to the stage of preparing to put the pistons, barrels and head back on my C Rapide.
Previously the engine was, according to Maughans, stroked but I have had them return it to standard.
The beast originally had thick compression plates (about 5mm) between the barrel base and the crankcase and I am trying to decide if I should refit them.
The sidewall of the piston looks to protrude quite a lot above the muff (see attached photo) when assembled without the compression plate. I'm wondering what to do now.
Should I assemble with the compression plates, without the compression plates or get some other compression plates from the spares company?

Cheers - Steve
 

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nkt267

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VOC Member
Have you measured the height of the muff.
Know thy beast states 3 and 1/16th of an inch 3.0625"..John
 

Bill Cannon

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Put the heads on and fit pushrods (assuming valve timing is already set) and carefully rotate the engine to see if it turns over with no plugs in.

If it does then get the playdoh out and check piston to head and piston to valve clearances, from memory only 40 thou minimum.

If it doesn't turn over then repeat above process with compression plates fitted.

Bill
 

Bill Thomas

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Hello Steve, I think I said to you before, Was it stroked ?.
The photos you show , Look the same as my 12 to 1 pistons, Give or take ?.
For a soft engine, I would like to see the top edge of the piston , Level with the top of the liner.
Good luck, Bill.
 

Bill Thomas

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Just a thought, If someone has been "Playing" in there, They may have put Mk2 cams in there, Which needs the top of the lower valve guide to be shortened, Just in case you have had the head redone ?.
I have always ground the Valve pockets in the top of the pistons away, because the valves tend to touch on the edge of the pockets, Cheers Bill.
 

SteveF

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Non-VOC Member
Mmm, lots to think about here, thanks everyone.
I think I will replace the compression plates and see how it performs. It does have Mk2 cams so I'll check the pockets.
I don't care that much about high performance, if I was after that I'd buy my mates (modern) Triumph.

Cheers - Steve
 

Bill Thomas

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VOC Member
If you keep the compression plates, The pockets might be out of harms way?, Also if you take out the compression plates, I think your pushrods will be too long ?.Cheers Bill.
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Cheaper to buy push rods than liners, 20 years on 9.5 : 1 and 120 k miles Alfa pistons,& twin plugging regularly 65 to 70 mpg 200 + miles on 15 Ltrs. Prior to the WWII & after one could buy compression reducers because of poor fuel, just simply screw chamber into spark plug hole and fit plug on the top of it. Thought about it my self and make different sizes, with so much compression plate thickness the push rod tubes go out of alignment, also the rear cylinder head bracket into the UFM will be further to the rear of the expansion slot, that is why it's not tightened too much, I just nip them and tighten the lock nut. I could never determine what gave me the better mileage until I reduced to 8 : 1
 
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