Compression plate

ville · Post by **ville** » Sat Mar 08, 2008 5:09 pm

Hi,

I have 1949 AJS 18S and I'm wondering should I have compression plate between cylinder and crankcase? Spares list says there should be one, but I don't have. I checked JSL and noticed they don't have it on the list. Is it necessary at all?

Cheers,
Ville

Biscuit · Post by **Biscuit** » Sat Mar 08, 2008 5:23 pm

Looking at a 1952 Owners manual, the compression plate reduces the compression ratio from 7.24-1 too 6-1, both pretty innocuous, so I think you will be safe without it.

Come in Ken!.

Samuel · Post by **Samuel** » Sat Mar 08, 2008 5:45 pm

I have a '49 G80 - when I re-sleived to standard and replaced the piston I also removed the compression plate - It was harder to start with the increased compression but not appreciably more powerfull. On having to replace the head recently following discovery of a hairline crack I reverted to the compression plate and easier starting !

ville · Post by **ville** » Sat Mar 08, 2008 7:18 pm

Thanks for the responses.

Samuel, did you notice any knocking with higher revs when you didn't have compression plate? I had some knocking and also piston ring breakdown. Could the missing plate be the reason?

Biscuit, I think 1952 model cylinder was longer. Do you know are these ratios counted with longer or shorter type cylinder?

Cheers,
Ville

paul knapp · Post by **paul knapp** » Sat Mar 08, 2008 8:01 pm

If you had a knocking at higher revs coupled with ring damage, sounds to me as if it could have been a 'mechanical' noise, perhaps the rings hitting the ridge in the top of the bore, hence the damage.

Biscuit · Post by **Biscuit** » Sat Mar 08, 2008 8:02 pm

I don't know whether the '52 barrel was longer, but it has a different part number. In the Haycraft book, when speaking of tappet adjustment he refers to 'No compression plate fitted (1949 onwards)' and 'Compression plate fitted (1945-48)' He then goes on to give different tappet adjustment methods for either case, the reason for this escapes me. Are your ring problems due to small gaps? if it is anything other than the top ring, it cannot be due to lack of compression plate.

ville · Post by **ville** » Sat Mar 08, 2008 9:04 pm

You are probably right. There can be many other reasons. Damage was with both of compression rings. I dismantled the engine first time after this breakdown and before that gaps could be anything. Now it is rebored and it has new piston from JSL. Maybe I try without compression plate at first (because I don't have any ). I suspected lack of compression plate caused knocking because when I retarded ignition fully, knocking disappeared.

I just noticed this compression plate issue today when I was looking at spares list. I also noticed Haycraft book says 1949 doesn't have the plate, but Bacon book and some other which I don't remember says that 500 cc engine has compression plate up to 1951 and 1952 having longer cylinder doesn't have it anymore. Complicated.

Cheers,
Ville

itma · Post by **itma** » Sat Mar 08, 2008 9:10 pm

I think your rings broke simply because you did not remove the ridge at the top of the cylinder?

Biscuit · Post by **Biscuit** » Sat Mar 08, 2008 10:10 pm

He had it re-sleeved Ken.

itma · Post by **itma** » Sat Mar 08, 2008 10:39 pm

That was Samuel, Alan
the lack of a compression plate should not, at these ratios make a noticable difference to ease of starting.
What is important is that if removing the plate you also remove the washer at the bottom of the tube, under the seal.
Some of these plates are up to 1/8 thick, and the pushrod seal washers should be jiggled about to suit.