Compression Plate (again)

[TommoT]

One thing to check on the piston in order to determine which came first,heat then seizure or vice versa, would be to accurately assess the discolouration. If you look inside the piston, and check if the inside of the piston crown and perhaps down the sides and maybe even the gudgeon pin bosses are brown and discoloured, then heat probably came first. If not expansion came first, caused heat and seizure. If you end up needing a new piston and a rebore, then avoid Italian GPM pistons if at all possible. They require extra clearance in order not to seize, which in turn will cause piston slap and premature wear.

[jackstringer]

Well turns out the barrel has been Re-sleeved at some point. The piston is a Alpo? and from what the Piston Broke can see he thinks its a mixture of crap piston and a bike that has been run for to long. He mention there were water marks in the barrel. So now I have to try and find a +20 Piston plus rings.

[Duncan]

Jack, I believe that the piston that you had fitted was an earlier type post 1947 pre mid 50’s which would have a lower compression ration (CR in the mid 6’s?) than the later ones with the valve cutaways (CR in the mid 7’s?) , combine that with a compression plate and your available power must have been somewhat reduced.

Can someone confirm what the compression ratios are?

[jackstringer]

I thought it might of been a army mod,making the bikes softer for use off road.

From a quick look[1],
Part no. 13379 +.8 MM AJS - 348cc OHV (16MCS) Year 1956-59, compression ratio 9.9:1 (bore 72mm +.8mm) compression height 2.23/64, piston length 3.31/64 - 1 cylinder (dome head with valve pockets) Pin diameter .7/8. 6 - pistons available (PHC).

Part no. 10524 STD AJS - 347cc OHV 16, 16M, 26, 26SS, 26T, Silver Streak. Year: 1935-46. CR 8.5:1
Part no. 10524 +.020 AJS - 347cc OHV 16, 16M, 26, 26SS, 26T, Silver Streak. Year: 1935-46. CR 8.5:1

Part no. 10465 STD AJS - 347cc OHV (16M) Year: 1948-55, compression ratio 6.3:1. Matchless - 347cc OHV (G3/L) Year: 1948-55, compression ratio 6.3:1.
Part no. 10465 +.020 AJS - 347cc OHV (16M) Year: 1948-55, compression ratio 6.3:1. Matchless - 347cc OHV (G3/L) Year: 1948-55, compression ratio 6.3:1.

Part no. 10276 STD Matchless - 347cc OHV (D3, G3, G3/L, G4, Clubman & Special) Year: 1933-46, high compression 11:1
Part no. 10276 +.020 Matchless - 347cc OHV (D3, G3, G3/L, G4, Clubman & Special) Year: 1933-46, high compression 11:1

Part no. 10277 +.030 Matchless - 347cc OHV (D3, G3, G3/L, G4, Clubman & Special) Year: 1933-46, high compression 12.5:1

Part no. 13265 STD Matchless - 347cc (G3/L) high compression. Year 1948-55 - 347cc OHV (G3/LS) Year: 1956-58, compression ratio 7.5:1
Part no. 13265 +1.6 MM Matchless - 347cc (G3/L) high compression. Year 1948-55 - 347cc OHV (G3/LS) Year: 1956-58, compression ratio 7.5:1


Jack Stringer
[1] Took descriptions from a website. But I have found that some sites use cut and paste too much so you can never rely of the figures they supply. I also have not checked to see which pistons fit mine yet.

[itma]

All these below will fit
the others are all long con-rod high pin pistons
except for the Part no. 13379 which is too high a comp ratio.
The army [any of them] did not modify bikes; they specified what they wanted and used them.

In part you are providing your own answers; its been relined, and probably bored badly, thats normal: its been owned previously by the sort of dipstick [trade to boot]that knows all about old brit bikes.
A sure recipe for trouble.

You are stuck for choice of piston; various different makes are about, A lot have had trouble with some, a lot have had no bother at all.
It's all down to finding a reborer that knows what he is doing.
Originals are few and far between; Rarely a Hepolite one pops up on ebay.
There are even some buggers with a shed full of new genuine AMC parts that won't part with them.


Part no. 13265 STD Matchless - 347cc (G3/L) high compression. Year 1948-55 - 347cc OHV (G3/LS) Year: 1956-58, compression ratio 7.5:1
Part no. 13265 +1.6 MM Matchless - 347cc (G3/L) high compression. Year 1948-55 - 347cc OHV (G3/LS) Year: 1956-58, compression ratio 7.5:1

Part no. 10465 STD AJS - 347cc OHV (16M) Year: 1948-55, compression ratio 6.3:1. Matchless - 347cc OHV (G3/L) Year: 1948-55, compression ratio 6.3:1.
Part no. 10465 +.020 AJS - 347cc OHV (16M) Year: 1948-55, compression ratio 6.3:1. Matchless - 347cc OHV (G3/L) Year: 1948-55, compression ratio 6.3:1

Be wary of what is on offer on ebay. Right now there are several pistons on there which the seller knows eff all about and are wrongly dated.

That's tonights piston bulletin from my part of very exclusive expensive hell


PS here you are

http://cgi.ebay.com/ebaymotors/NEW-NOS- ... db57Edited by - itma on 22 Mar 2010 9:27:53 PM