AJS & Matchless Owners Club

I'm just about to do a compression test on my G2, what readings should I be looking for?

Woodland wrote:I'm just about to do a compression test on my G2, what readings should I be looking for?

Roughly a reading of about 120psi and above. Any lower and there will be running problems.

You will get a more accurate answer off someone soon though.
Regards

Granny...eggs...etc. but whilst I'm here browsing...........
Open the throttle wide when you do the test.
If the reading is low and you put some oil in through the plug 'ole and the reading increases it indicates worn bore/rings. If it doesn't increase it's likely to be a poorly sealing valve.
If oil comes out from the head to cylinder joint...........well you know the reason for that

Just a thought. Upon doing a compression test should you expect to see the compression ratio value. Ie 8:1= reading of 8bar.
JohnT

electrajohnt wrote:Just a thought. Upon doing a compression test should you expect to see the compression ratio value. Ie 8:1= reading of 8bar.
JohnT

I may well have misunderstood your point but the compression ratio is a calculated ratio of relative volumes; (combustion chamber volume at TDC vs cylinder volume at BDC) and doesn't relate to pressure.............or maybe it does?

Upon a search it states the g2csr is 9.5:1 compression ratio the standard is slightly lower.
If you wish there are calculations to work this out based on these figures.
As Spriddler said it is based on stroke length, cylinder volume and combustion chamber volume etc.
This will give you an estimate of the cylinder pressure needed based on the ratio.
I'm no engineer just basic knowledge and google
Regards

Thanks guys, will let you know the outcome and readings when I do the test
Gary

Spriddler is correct, compression ratio is simply the ratio between the total volume above the piston at BDC compared to the total volume at TDC. However, in a real engine at low rpm, e.g. during a kick-start compression test, compression doesn't start until the intake valve is closed and so we'd need to know conrod length and intake valve timing to calculate ‘effective' CR.

But once air is quickly compressed it heats up, greatly increasing the pressure. 8 bar from an effective CR of 8:1 is true only if the air is compressed slowly enough for the temperature to remain constant.
Assuming no heat loss, air pressure during fast compression rises at the 1.4th power of compression ratio, so an ‘effective' compression ratio of 6:1 would yield up to 12 bar peak pressure.

Enough tech, let's see what pressure Woodland reports ...

Just done the test and get 120psi, as the engine does not smoke or burn any oil, etc with this reading I think all is ok.
Also i run on normal unleaded fuel, should I be thinking about getting the head converted and adding hardened valve seats at some time?
Many thanks in advance of any replies
Gary

Woodland wrote: should I be thinking about getting the head converted and adding hardened valve seats at some time?

If you've got some brass to chuck away, Gary, I could use a bit Don't waste your money. Conversion isn't needed though you'll find plenty of 'engineers' who will happily take your money and do the job.

Steve