Top of piston to top of barrel measurement

[Radlan2]

Hi Guys
      Can somone answer this, on an A10 or A7 standard build engine with the piston at TDC is the "top" of the piston flush or below flush with the top of the barrel, if it is below flush by how much? I think this is called "deck height" in the states.
       Whatever variant of compression ratio there is the first step above the top ring, that is what I mean by top of piston. Anyone got a motor with the head off?
      Many Thanks
                   Chris

[bsa-bill]

 might depend on the year but my Flash is standard bore and compression, and the top of the piston is flat and level with the top of the barrel

[Swarfcut]

   Deck Height, according to Mr Google is the distance between the top face of the block and the centre line of the crank axis. Piston areas above and between the rings are known as "lands"

 Generally the outer edge of the top of the piston is flush with the face of the block at TDC. The crown of the piston can be dished, flat as in Bill's case, or raised. These variations give the different available compression ratio's.

 If the pistons are too low or too high in the bore at TDC there is something amiss with the parts selected, or the parts have been machined  in a previous life.   Sure you've got the right unmolested crank, rods. barrel and pistons?
 Needs a bit more information to give a meaningful answer .

 Swarfy

[RDfella]

Normally a piston would be below the 'deck' or top face of the cylinder by at least .020" to ensure clearance to head when hot and running. When setting up diesels, this is known as the bumping clearance. This gap is usually the 'squish' portion of the combustion chamber, promoting swirl. Having said that, engines with a hemispherical combustion chamber that's at least as big as the cylinder bore clearly wouldn't suffer the consequences of a slightly higher piston height. However, hemi heads are a fairly poor design shape from an efficiency point of view and often require quite large amounts of ignition advance because of their poor burning characteristics.

[Radlan2]

Thanks for the replies and you all deserve some explanation. Some time ago pre pandemic I did a post about the short rod conversion I had done to an A10, but despite following the magazine article fairly closely the motor did not want to really rev freely but I was using flat top pistons. One thing which I thought looked wrong was that the piston TDC was about 1.5mm+ short of the top of the bore so my next experiment was using super rocket high comp pistons, not the raised pimple type the flat top type.
        The piston still was short of flush by 1.5mm but due to the raised top compression was increased, yet still it was not a spectacular success performancewise.
         I spoke with Martin Russell, an ex factory guy who I had met through the TR3oc he told me they used to fit (71mm) Triumph pistons to Shooting Stars back in the day but as the pin height is different they would marry it to a longer rod.
         Armed with this info I made a special barrel to bring the piston now a Triumph 650 piston, flush to the top. Now the problem has changed, the BSA head has a shallower hemi shape so it would be better with this arrangement if the piston was 1.5mm below flush.
         I think I shall have to go to a 2mm head gasket (or try 2 standard ones) before I run this up.
       Best Regs
                  Chris

[muskrat]

G'day Chris.
When racing my A7SS I used standard T160 pistons (67mm) and shortened A10 rods by about 2.5mm. The top edge of the piston was level with the cylinder deck. This gave me 14:1 compression 
I think Rocket Racer in NZ had done a short stroke A10.
Cheers

[Radlan2]

Thank you Musky
      Yes that is another permutation, 14:1 is a bit rich for a road bike these days with E10 and all that but I do have those pistons after over 40years of Trident owning. So maybe if I shorten the rod by a couple of mm it should put the comp ratio into the useable category....
       What I have not told everyone yet is that I I managed to put holes, or more precisely splits in he top of a pair of pistons, one went first during a spirited ride and I tried to gently limp home and the other one went to. I am convinced this was due to poor quality pattern parts, but to be fair I was using a low compression piston in a high compression environment and it simply caved in due to the pressure.
            Cheers
                  Chris

[Swarfcut]

 Chris. All is revealed...I was sorta on the right track. A cylinder base spacer  plate could be a quick answer, raises the block so lowers the relative piston height. I'm sure that I've seen them on ebay from time to time. Otherwise time for the fretwork skills or your local CNC laser cutting outfit.
 
 Swarfy

[Radlan2]

Thanks again Swarfy
                I had wondered about that but it does change the cam follower geometry, my followers are re-grounds so along with a 1mm base plate they could be a bit short? Plus because they are angled it would effectively put the wipe area back a little, for all that it is a simple way out of my problem and a good idea, has anyone else used one?
          Best Regs
                     Chris

[Swarfcut]

    Longer custom pushrods should restore the geometry, as moving the barrel upwards moves the followers away from the cam, in effect retarding the cam timing. Alternatives are a higher lift cam or experimenting with the timing gear relative setting. Followers may settle on the locating pegs, but from your expertise all these are minor glitches in the grand scheme. Barrel and head/rockerbox geometry is unaltered, it's a case of making sure the cam followers can still reach the back of the cam unimpeded. As a suck it and see scheme, use washers between the barrel and crankcase to get an idea of how thick a base spacer needs to be and determine if this is an easy workable solution before spending.

 Swarfy.

[muskrat]

G'day Fellas.
From memory 2mm is the max compression plate with std followers and cam before the followers ride on their keepers. I tried that before I went down the rod shorten route.
Cheers

[Radlan2]

All great points fellas
           I feel encouraged to have a try with the base plate and will set it up using the washers to check things first, a good idea. It will be an over the winter thing and I will let you know how it goes.
         Best Regs
                     Chris

[Rocket Racer]

my a10rr motor is short rod pulls nicely to 7500 which is when i chicken out. if you need any info pm me

[Rocket Racer]

my a10rr motor is short rod pulls nicely to 7500 which is when i chicken out. if you need any info pm me

the crux of the short rod motor is 1/2” off barrel and rods then fly cutting pistons to clear flywheel. pistons normal bsa range. swapping pistons adds in variables above this. the motor was on 9:1 but latest incarnation on 10.5:1.