Minimum Compression ratio

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
tippi11
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Posts: 72
Joined: Sat Dec 01, 2018 9:59 pm
Location: Germany

Minimum Compression ratio

Post by tippi11 »

Hello engine experts,

After sorting few other possible reasons (e.g. magneto repair, thank you Bill) for my non-starting engine, I measured compression to get basic information for the mechanical side/condition of the engine. The reading was 84 psi or 5.8 bar at 4-5 full turns, throttle fully opened, but no extra oil in the cylinder. I believe this compression ratio is too low; what do you think and which psi value or range can be seen as minimum? Engine number is 56/16MS28374. I don’t know the exact history regarding overhaul works or parts used by the previous owner, but hopefully internal mechanics are standard, such as valves, piston etc. My workshop manual (1957-1964) lists a compression ratio of 107 psi or 7.4 bar for 350cc Heavy – is this, as per your experience, the ratio to go for? :roll:
Thank you very much. Regards, Peter
Mick D
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Location: Aberdeenshire

Re: Minimum Compression ratio

Post by Mick D »

Hi

Your compression is low but if the bike hasn't been run for a considerable time I'd attempt to start it as it is. If all else is OK it should run. Allowing it to run for a while could free up a stuck ring or improve the sealing of the rings etc and consequently improve your CR.

Regards Mick
tippi11
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Posts: 72
Joined: Sat Dec 01, 2018 9:59 pm
Location: Germany

Re: Minimum Compression ratio

Post by tippi11 »

Hello,
Thank you Mick - unfortunately I wasnt able to start it as it is. I think I will repeat the compression test, but this time with extra oil in the cyclinder to compare compression ratios without/with oil injection. And may be its a good idea to remove the head and check the condition of the valves/valve seats. What do you think? PS: I am still confused about compression ratios because I found values of 6.3 and 7.5 when looking at piston offerings; and what is the truth or correct setup for head gaskets? I found an article elsewhere saying that some owners run their singles without head gasket...
Regards, Peter
Mick D
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Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Minimum Compression ratio

Post by Mick D »

Hi Peter

It would help if you said more about the bike, what year and model type is it? Some history would also help, when was the last time it has run, have you ever started it, do you have a spark, fuel, etc?

If the bike is known to you as a runner I would concentrate on the ignition and fuelling to get it running. If you've never seen the bike running I'd carry out a full strip and inspection for my own peace of mind.

Some competition models were designed without head gaskets, road models were all fitted with gaskets.

Regards Mick
SPRIDDLER
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Location: WEST SUSSEX UK

Re: Minimum Compression ratio

Post by SPRIDDLER »

Hi Peter. From your thread title and comments it seems you are confused between 'Compression pressure' and 'Compression ratio'.
Compression pressure is what you have been measuring and needs to be around 100psi. It has no connexion with or relevance to compression ratio which is the ratio between the cylinder volume above the piston at BDC relative to the cylinder volume above the piston at TDC.
I've tried to find the year and exact model of your engine from your earlier posts without success so you need to find the compression ratio details from the Owners Manual in the link below, under 'Books', although I repeat, it will have no practical relevance to the compression pressure.

http://archives.jampot.dk/
'There is a tide in the affairs of men
Which taken at the flood............'
Shelby-Right
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Posts: 33
Joined: Thu Sep 02, 2021 7:09 am
Location: New zealand

Re: Minimum Compression ratio

Post by Shelby-Right »

On my 49 500 , i had to do the pushrod seals , and when i pulled the head off , What a surprise , new piston good bore , but that's where it ends , at TDC my piston was 1/2" down the bore, yes wrong compression height piston, it had a 1.312 instead of the 1.812 and after pulling the barrel and i checked the ring gaps , wait for it ........ .120" yes 3mm gap on all 3 rings , i calculated my compression ratio at 4.1:1 and it actually ran ok, i rode it around my paddock , I never checked my cranking compression . :-) . Cheers .
tippi11
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Posts: 72
Joined: Sat Dec 01, 2018 9:59 pm
Location: Germany

Re: Minimum Compression ratio

Post by tippi11 »

Good morning,

Thank you for your replies. Some info regarding type, year, history (Mick) - about a year ago I contacted the club´s dating service and got this: “The bike is a mixture of parts, the frame A85536 is a 1963 AJS 16 and the engine 56/16MS28374 is from a 1956 AJS 16MS”. So, it is not for the purists, see picture below ...
And yes it is a runner (incl. road registration), or better say WAS a runner. Trouble started in June this year when the engine suddenly stopped running during a short ride. This happened without any signs of tech. problems, e.g. fuel supply, ignition, bangs or noises indicating upcoming troubles. Knowing from my Enfield that our old bikes are sometimes a bit self-willed, I thought “ok, this can happen, lets try again”. But I experienced a dead engine, no chance to get running neither on the side of the road nor in the garage when back home. Dead, dead, dead over the next days, engine ignoring my efforts incl. some basic checks such as fuel supply, carb. jets etc. Despite having a spark, I decided to get the SR1 magneto tested and reconditioned (now completely tested and with new modern cap). But unfortunately the reconditioned magneto did not impress my engine! That is what I am facing today and which leads me to the idea to remove the head. Before doing so, I wanted to know more about the compression and did the test mentioned earlier. SPRIDDLER: I understand what you are saying about “compression pressure” vv “compression ratio”. Nevertheless I wonder why they published the ratio but not the pressure in the workshop manual, technical data table. For a normal workshop or owner a true ratio measurement would be a tricky thing (piston head shape, combustion chamber volume etc.), whereas a pressure test is quite easy. Are there any names/specs for different pistons related to certain ratios, e.g. piston A for 6.3, piston B for 7.4?
Regards, Peter
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Mick D
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Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Minimum Compression ratio

Post by Mick D »

Hi Peter

In the light of the information you have provided, I would first check the ignition timing and renew the spark plug.

As a rough guide you can assume the air fuel mixture at BDC is at atmospheric pressure, (1 bar / 14.7 psi), multiply this by the compression ratio to get an idea of what the compression pressure could be - this is a rough guide ;)

Regards Mich
tippi11
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Posts: 72
Joined: Sat Dec 01, 2018 9:59 pm
Location: Germany

Re: Minimum Compression ratio

Post by tippi11 »

Hello,
Thank you for the information such as the rough guide for compression (pressure). Here is my "checklist" for tomorrow:
- 6 mm spark on bare HT lead to cyl. head at kick-start speed?
- ignition timing, 1/2" BTDC fully advanced (expanded weights on A/R unit)?
Cross fingers, please. Regards, Peter
tippi11
Member
Posts: 72
Joined: Sat Dec 01, 2018 9:59 pm
Location: Germany

Re: Minimum Compression ratio

Post by tippi11 »

Hello,
First test with (very) good result: there is a bright spark at a 6mm gap, see picture below.
Even ok with roughly 10mm, no picture taken.
Next test, ignition timing, to follow as soon as possible – I will let you know…
Regards, Peter
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