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After tinkering with my monza for a number of years, I had a realisation today that in all that time I'd never measured the compression. I dug the gauge out of the tool drawer, and first test gave a maximum of 75psi

* A small amount of oil into the bore raised that to around about 95psi;
* petrol in the intake port did not significantly leak past the valve (can't easily check the ex valve in this manner with the engine in the frame)
* Exh valve clearance was tight, so backed it off and reset it to 0.07mm
* Intake valve clearance was spot on at 0.05mm

Current measurement is a maximum of 105psi. When testing more modern bikes, I would expect to see 140psi+ - what should I be aiming for on this standard 250?

I'm currently suspecting that I've won myself a job of removing and inspecting the head, barrel and rings next weekend. The exhaust port is pretty gummy with, so the valve seat could probably do with a bit of a clean.

Well, quite a lot depends on what piston you've got in there, Mike, and therefore what the compression ratio is. For example, I see some Monzas had a compression ratio of only 8:1 (ref. Mick Walker). Which would give you a theoretical maximum compression reading of 117psi (see below).

So, given the fact that you've already diagnosed a bit of ring wear, maybe 100psi ain't so bad after all ...

Anyways, the important thing is, how does it run? Especially, how does it start? I see you've already sorted a tight exhaust valve, so that should improve things!

Now I check Mick Walkers book, I can indeed see that it's being quoted at 8:1, which given normal atmospheric pressure of 14.7psi, that would give a maximum possible pressure of 117.6 psi, so I suspect you're right - maybe it's not so bad. It'd obviously benefit from a stripdown at some point in the future, but probably not urgently.

> Anyways, the impotant thing is, how does it run? Especially, how does it start?

It has never ran particularly well, but then, I've just done a bunch of work (new carb & ignition), and it's definitely running better, but still to do a road test to confirm whether all of the prior badness is gone. I think I'll pop the tank back on and get it on the road for a test run, rather than worrying about the rings right now.


Edit - I only just spotted the table you helpfully posted after I did the maths there. Good to be able to check my homework

Ventodue,
Could you tell me the source for that chart?
I've heard all kinds of stories about the accuracy of using compression gauge readings to estimate compression ratio. Some positive -others negative.
Regards,
Phil

I think cylinder compression gauges can be confidently used on multi-cylinder engines, where one cylinder may be revealed to have a fault due to its reading being notably less than the others. The readings in themselves have little meaning, until compared to each other.
It may be that readings taken from a single cylinder engine could be useful, especially if logged over time. Then they can be compared to previous readings. A sudden drop in pressure could indicate that a valve has become leaky.
Otherwise, I don't think I'd like to read too much into individual measurements, on engines that may have been rebuilt many times to possibly non-standard specs. In these cases, comparing to other engines' readings could lead to wrong conclusions.

bikester250 wrote: Could you tell me the source for that chart?

You can find it here, for example.

http://www.healey6.com/Technical/Nomina ... essure.pdf

But please be careful. The values quoted there are, as I said, theoretical. The actual compression readings you get from an individual engine can be influenced by a number of other factors - valve timing, for example. Even relative humidity or altitude can come into play.

As Jordan has said, taking compression readings is a simple way of getting a basic idea of the relative well-being of an individual cylinder. But not much more.