Date: Mon, 19 Feb 1996 15:13:02 -0500
From: Marshall
Cranking compression is not a good indicator of internal cylinder condition on a British motorcycle because it is dependent on the cranking RPM, which with a kick-starter is obviously variable. Both cylinders being equal is however a good sign.
A far better test is the leakdown. The cylinders are pressurised with compressed air and the percentage of leakage is measured. Any good cycle shop (and most quality automotive shops) are equipped to perform this test.
Marshall
Motorcycle Enterprises
Chicago
Date: Tue, 20 Feb 1996 00:12:24 -0500
From: Mike Taglieri
>What's "good" compression for a Norton 850 MK III...Mines
>about105 psi in both cylinders...Seems I've read 150 psi is
>about right.
I need to put in new rings (I confess -- haven't done it yet), but I
used to get 150 psi under the following conditions:
Mike Taglieri
Raul -- '72 Commando Interstate
Date: Wed, 21 Feb 96 09:31:59 EST
From: Gregg Kricorissian.
I agree with Mike T about Compression ... I get 150 psi in both cylinders,
under the same conditions.
...Gregg
Date: Tue, 20 Feb 1996 13:14:41 CST
From: Rob Brotherston
Could someone tell me what the air pressure from the compressor is for a leak-down test. A ball-park figure will do as I just don't want to run the risk of destroying anything in the engine with air pressure.
Also what are good readings (expressed as a percent of the incoming air pressure)? Poor readings?
The reason I need to know is that I have an article that describes how to build a leak-down tester and I was wondering if it would be suitable. It uses 100lbs pressure.
Rob
Date: Tue, 20 Feb 96 16:38:58 -0700
From: Andrew Wolf
I have a very nice leak down tester, it is a Snapon. But it is just simply a pressure regulator with a gage.
Snap on uses 60 psi as 100% and of course 0 psi for 0%, they assume linearity in-between.
With this setup I typically don't get bent out of shape until I get to more than 15%.
This is a very valuable tool, much more so than a compression tester as you can listen to intake, exhaust, and oil filler caps to find the source of 'leakage'
I have never verified it but the Snap on man said that the ones for aircraft only used 30 psi for leak down.
This actually makes some sense as higher pressures will close up marginal seals and while a dead engine on the road a dead engine in 3 dimensional space is not so healthy.
Of course using the leakdown tester assumes that you are AT TDC on compression and that you have a LONG breaker bar to hold the crankshaft into position. If you are truly at TDC the breaker bar is not required, but this is VERY rare. A long one is required to overcome the torque of the engine. Short ones HURT, I know!!!
Once you have attached the leakdown tester and you use the breaker bar to put it at tdc, you can remove the bar and then listen for leak locations.
Now the final point
The leakdown tester will allow better diagnosis, but a simple compression tester will more quickly let you know if you are in the park,
Andrew Wolf
Date: Wed, 21 Feb 1996 08:38:07 -0600
From: John Goodpaster
>What's "good" compression for a Norton 850 MK III...Mines about
>105psi in both cylinders...Seems I've read 150psi is about right.
>Also, I'm taking down the forks today to determine why there so
>much stichon...any thoughts???...Thanks,
>marko
If you check comp. this way make sure you open the throttle all the way. There is no set number, you can only tell if both cyl. are fairly equal. (assuming you have a reading above 100 lbs.) The only sure way to pinpoint what the problem is, is by using a leak-down testing set of gauges, aircraft type. If you cannot obtain a set of these, ( have a friend who is an A&P ) you can take an old sparkplug beat the ceramic out of it, weld a airhose chuck adapter to it. The you can attach the air hose to it. Put the machine in top gear at t.d.c. on compression, use 80# of air or there about. Then listen at the ex. pipe, carb intake and crankcase breather. You will hear air escaping from the source that is suspect. You will hear a small amount from the crankcase depending on the condition of the rings, but should none from any of the valves................
Date: Wed, 21 Feb 1996 10:50:30 -0800 (PST)
From: Colin Bryant
>If you cannot obtain a set of these [Leakdown tester gauge set],
>you can take an old sparkplug beat the ceramic out of it,
>weld a airhose chuck adapter to it.
>bsajohn
Alternatively auto part/tool shops sell airhose to sparkplug adapters. They are used to hold valves open [ED: This must be wrong, the air would hold the valve shut] (with air) so that you can change valve seals or springs without removing the cylinder head or having the valve drop into the cylinder.
Colin Bryant
Date: Mon, 26 Feb 96 07:02:59 -0700
From: Andrew Wolf
Yes I know that several sources state that leakdown % should be less than 15%. However in an earlier life I was a mechanic and a customer would NEVER notice the performance difference between 6% and 15%.
My customers were not out racing, rarely hit the drag strip and they would have lynched (rightfully so) me if I recommended a top end at 6% leakage.
The leak down tester is a fine diagnostic tool, but a compression gage used correctly will tell you all you need to know in terms of most folks ability to detect the difference in performance.
If you want to rebuild an engine based on 6% leakage, I suspect I don't own an engine that doesn't need rebuilding.
Andrew Wolf
12.4 Compression Test Vs Compression Ratio.
Date: Tue, 24 Sep 1996 21:16:04 -0500
From: David Paddock
OK. Re "compression ratio": It's the total cylinder volume divided by the combustion space volume, and it's done using a burette and thin oil. It's a measure of the amount of squish and has little real-world value because of valve timing, etc. "Pressure ratio" is the more realistic measurement since it tells you something about the condition of your engine, particularly if the measurement is taken with the engine hot. For example, if the pressure gage reads 152 psi (gage reading) and the ambient atmospheric pressure is 14.7 psi (absolute, also equal to 29.92 Inches of mercury - phone your local weather station or airport for the real-time value), then the pressure ratio is (152+14.7)/14.7=11.3. This is NOT the "compression ratio": it's only an indication of how well the compression process is being carried out. The variation from one cylinder to the next reveals how the various pots are probably contributing to overall engine output. For any engine, it's a relative measure of wear/deterioration.
Next, cylinder head sealing. Note that gaskets are the means by which fraying surfaces are sealed in the absence of adequate mutual conformity. It is always better to make the surfaces snuggle up to each other and avoid the need for gasketing (which is kinda like making love with your clothes on...). Many years ago I learned that the only true way to produce the ultimate seal is to try for a Johanson-block fit. (A mentor was building an alcohol-fuelled injected XK-120 engine for a sprint car he had built, and a buddy and I spent two evenings lapping the head to the block using increasingly-finer valve grinding compound and a figure-eight motion. The engine was modified for 16:1 compression ratio, and the probability of a blown head gasket had to be zero. It worked.) To drive this point home, I've done the same to my DBD34GS - no head gasket, although the figure-eight lapping motion was replaced by gentle oscillating rotary action. It works.
Re the odd "S" character on the Goldie case: Contact John Gardner, the ex-sec of the UK Gold Star club. He's a wealth of info and very knowledgeable in Goldie matters. He's not on e-mail as far as I know, and you ought to send him some sort of postage for reply. His address is
23 Wellington Close
Dibden Purlieu
Southhampton
Hants SO45 4RL
England Phone:01703 842669
Lastly, thanks to Charlie Falco for his informed and helpful contributions to this Digest. It's always a pleasure to read his offerings.
12.5 Measuring Compression Ratios.
Date: Thu, 14 Nov 1996 18:21:43 -0800
From: Vernon Fueston
Actually the best way to measure compression ratio is with the head installed. I seal the top ring with a little grease before bolting the head down. Make sure it is on top centre, then carefully fill the combustion chamber with kerosene from a burette through the spark plug hole. You have to wiggle things about a bit to get all the bubbles out. Fill it to the bottom of the plug threads, divide the amount used into the swept volume of the cylinder and your done. In the late 50's AMA class C had a compression limit. After a National they would measure the ratio of the first three. Since all but Harleys were 500 cc they just divided the ratio into 500 and got the amount of kerosene that should fit into the combustion chamber and poured it in. If it didn't run over you were legal. I don't think they checked Harleys as you could not exceed the limit on a flathead.
So this fellow was late to the BSA club annual dinner. When he got to
the door they were selling raffle tickets and being in a hurry he just
bought some without asking any questions. After the dinner they held the
raffle and as luck would have it he was the winner. When he got up front
to collect his prize he found it was a 441 Victor! Although shocked and
surprised he was able to collect his wits in time to ask what's the second
place prize?' Answer "two Victors"
Vernon Fueston "Better a bottle in front of me, than a frontal lobotomy."