Compression plate

[limeyrob]

OK I'm trying to get inside a previous owner's mind and I'm struggling.  The engine looks to have been rebuilt and not run and parts quality so far is good.  But!!  Engine has 1/8" compression plate under the barrel (well made, longer studs, slightly domed pistons (8:5:1?) a 26mm Concentric and an ally head 67-1126. The push rods have been cut and extended to match the plate.  Cam is high lift (357?) won't know until I can read the end.  Checked piston travel and they did stop 1/8" short.
Its like the PO got a tuned engine then lowered the compression and put a small carb on.  Anyone seen anything like this before?
Bike looks to be a 59 Super Rocket but log book is vague "A10" and it could be  Golden Flash with a lot of later parts.

[muskrat]

G'day Rob.
1/8" comp plate  . Max is 2mm, anything over and the lifters won't reach the cam and will bottom out on the retaining screws! Then your missing lift and duration of the 357 and have a very noisy motor. I wouldn't trust the extended push rods.
Yes the A7 & A10 motors can be a mix & match recipe. Lots can be changed over the 60 or so years.
Cheers

[limeyrob]

Agreed, the extended push rods are going in the scrap bin with the compression plate.  Pistons look to be 9:1 which should be OK.  I don't trust the bottom end - it all looks rebuilt and not run, but after seeing the top end I'm not taking any chances.  It looks like the cylinder base studs were specially made over length (even with BSC threads), I've just found a bag with a set of std ones.  What a lot of effort to mess something up!

[muskrat]

  What a lot of effort to mess something up!

G'day Rob.
When someone inherits my two A's they might say "WTF". 
Cheers

[Joolstacho]

With an 'unknown' motor like this, I would certainly do a cylinder volume / compression test using burettes and fluid, that's the only way you can be sure what you've got.
When I got my motor it had been running on alcohol and about 11 to 1 comp ratio, I got it to a saner 9 to 1 using compression plates.

[muskrat]

G'day Jools.
11:1 is a bit low for alcohol! I was running 14:1
Can you remember what pistons & rods? What thickness plate did you use?
Cheers

[limeyrob]

The bad boys are on e-bay at the moment, look like 10 or 11 to 1 :
bsa A10 pistons 15638 69.989 mm 0.20 mm
https://www.ebay.co.uk/itm/314957848121

[bikerjohndavies]

The Draganfly chart of pistons shows 15638 to be 10.5:1

[RichardL]

Rob,

Maybe you said it between the lines, but what I think you should do is toss the plate, check the rod length and replace if necessary, get standard push rods, keep the 9:1 pistons and, therefore, end up with a reliable engine. Others here who more into experimenting and hot rodding (maybe you're one of them) may think I'm being too conservative.

Richard L.

[limeyrob]

I'm well along that path.  Lengthened push-rods and plate in the scrap bin.  Pistons were 1/8" below cylinder face at TDC with the 1/8" plate so I'm confident that will work.  I don't trust the engine builder so I've got it on the bench.  Its a nice engine with some decent parts - ally head, new rings, honed bores, crank grind and new T/S and cam bushes.  But why oh why screw a metric nut on the cam or assemble a rebuilt engine without a single end float shim and 12 thou of end-float?  And the D/S roller on numbers inside?  I'll put it all back together properly and it has the makings of a decent engine.  How hard is it to put the nuts on a crank case stud equally, rather than thread bind one and the other on 3/4?  They are new studs and nuts too!!  Looks like the sludge trap is new - result!
In my first post I mentioned trying to get in the mind of the engine builder, I've given up.

[RichardL]

In my first post I mentioned trying to get in the mind of the engine builder, I've given up.

Yes, very dark. Don't go there.

Richard L.

[Joolstacho]

G'day Jools.
11:1 is a bit low for alcohol! I was running 14:1
Can you remember what pistons & rods? What thickness plate did you use?
Cheers

Musky when I bought the motor I wasn't dealing with the owner (I think he had croaked) so perhaps I got wrong info. The monobloc did have a very big MJ however.
Pistons are very lumpy I'd say 9:1. The comp plates I made are 1mm thick, I fitted 2 to get a shade under 9:1 measured. Not sure if the PO had skimmed the cylinder base to raise the comp ratio. Hard to accurately measure the flange - It's around 0.46" so maybe the PO skimmed the thick flanged barrel flange to raise the ratio.
Running Dural pushrods.

[limeyrob]

I think 9:1 are a gentle dome, for really lumpy ones click on the ebay link in my earlier post.  These have now been IDed as 10 1/2 : 1.  Aside - I ran 10 1/2 : 1 in an A65 and it was smoother and better balanced that at 9:1.  I never really understood why but I did wonder whether BSA had all along planned for some high compression for competition.  The Spitfire cam and high comp seemed to go together too naturally.  This was in the days of high octane petrol.

[Joolstacho]

Rob, from memory, the highest comp A10 (SR & RGS) pistons are 8.75:1. Not 9:1.
They are pretty 'lumpy' looking with a high crown.
(Like the ones linked to the Evilbay sale)
Correct me if I'm wrong.

[muskrat]

G'day Jools.
The evilbay link are 10.5:1. I had them in the Cafe.
The 8.75:1 are a lot lower crown.
Cheers