Compression Ratio & Gearing?

Information relating to the Matchless G85 500cc Heavyweight, AJS 7R, Matchless G45 and Matchless G50
Matchymarty
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Compression Ratio & Gearing?

Post by Matchymarty »

Hi all,

About to embark on full check over and rebuilds of my '56 and '55 (short stroke and long stroke respectively) engines. The '56 puffs a bit of smoke (maybe valve guides) and the '55 blows enough to look like a steam locomotive, even after a new piston, rings and cylinder sleeve.(suspect a broken ring or worse).

Q: The '55 G80CS was fitted with a JP 8.5:1 CR +0.020 Piston.
1. What is the weight of the standard piston supposed to be? (where do I find this information for all models?)
2. What do you guys think is a 'happy medium' compression ratio to use for general road use to balance engine longevity and power?
3. I'm running stock gearing. (18T Engine sprocket) What engine sprockets are you guys running for good road use for modern speeds?
3a. Is there any benefit in a lower CR for higher gearing? After all, the engine is designed to plonk along not, Rev.

I'm beginning to think, the higher comp piston fitted, has although given more 'power', but it seems to die on hills and doesn't really 'hum along' unless at 50mph. I'd like it to be happy to hum along at 60mph all day.

Q: The '56 G80CS is an unknown quantity until I pull it down. (Short stroke, AMC gearbox and clutch with cush drive) This bike has hideous vibration, so want to sort it (refer other thread) and am thinking it's either piston, balance, incorrect flywheels or something. Only way to sort it, is to pull it down.
1. What is the weight of the standard piston supposed to be?
2. Same as above #2.
3. Best road gearing? Running 20T Engine Sprocket.(19T standard?)

I'm thinking, I'd like to build these engines with longevity in mind, with the ability to be happy at 60mph.

I have a 2100 mile trip in planning to the 2017 Jampot Rally, so want the bikes to be WELL sorted and happy to plug along all day. :D
Cheers
Marty
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Rob Harknett
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Re: Compression Ratio & Gearing?

Post by Rob Harknett »

Learnt my lesson to mess with what was standard as a teenager and paid the price. The dealer that put my bike back to standard CR, after me changing it and blowing the engine, did me a favour. He give me back the 8.5 piston. Just said if you want another bill like I just gave you, put the piston back in. Not being able to go out with my mates for about 3 months, while I paid back Mother the money, taught me a lesson the hard way.
Andy51
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Re: Compression Ratio & Gearing?

Post by Andy51 »

Hi, I have a 54 18S fitted with scrambles cams and JP 8.5 to 1 piston. It goes well, and has done for 3500 miles since rebuild in this form. Vibration is the same as my 54 G80S running on original wire wound pistons and standard cams. 18S is rather noisier, pulls better and goes faster, not quite so flexible but hardly 'cammy'. The two compliment each other, one for 'hooning' (as an Aussie you'll understand!)' the other for gentler work. Hope this helps, Andy
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dave16mct
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Re: Compression Ratio & Gearing?

Post by dave16mct »

Standard roadster gearing is 21T on the engine with road tyres. Your bigger rear tyre will change that a bit.
Dave.
Matchymarty
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Re: Compression Ratio & Gearing?

Post by Matchymarty »

Andy51 wrote:Hi, I have a 54 18S fitted with scrambles cams and JP 8.5 to 1 piston. It goes well, and has done for 3500 miles since rebuild in this form. Vibration is the same as my 54 G80S running on original wire wound pistons and standard cams. 18S is rather noisier, pulls better and goes faster, not quite so flexible but hardly 'cammy'. The two compliment each other, one for 'hooning' (as an Aussie you'll understand!)' the other for gentler work. Hope this helps, Andy
Andy,
My '55 G80CS is running 'standard cams' (well, actually I don't know as there is no marks other than the timing dots) with the 8.5:1 JP. Noisy yes. Punchy? Not really. My '49 G80S ended up pulling up hills as good with no work to the engine. Vibration is not really an issue, though it is with the '56 G80CS. My '55 has done 3000 miles on the JP and has either broken a ring or something in the last 1000miles, as it smokes like Hugh Hefner at his mansion parties.

Marty
Don Madden
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Re: Compression Ratio & Gearing?

Post by Don Madden »

I must apologize to our members for not being more responsive lately. I lost my wonderful Lancashire Lass last month, born & raised in Rochdale, then Lancashire County before the Counties Reorganization Act moved the ancient town to Manchester County. I found her in Recife, Brazil in 1957 & we were married in the Church of England chapel there in 1958, then I brought her to the US. We produced three children, two granddaughters & one great-granddaughter in our 58 years together.

The original pistons were made by BMB & although very sturdy, were lighter than most others, partially due to the gudgeon pins being taper bored at each end. I have weighed many twin & single original & replacement pistons on an electronic scale I compiled the results. I know of no other source of this information.

The original 1956-59 G80CS/18CS piston I have is new, old stock, never installed in +0.040 size. It weighs 16.6oz/469.9g bare, 21.3oz/602.6g with original pin & rings. Standard size would be slightly lighter.

The early short stroke engine revved very freely with greatest output from mid-range to top end producing almost twice that of the long stroke CS, but not having much low-end "grunt". In 1963 the factory fitted the shorter G2 pushrod end rocker arm to the inlet rocker assembly which resulted in greater lift of the valve. This easy mod greatly increases the low end torque resulting is a power curve much like the touring engine but revving as the CS. I recommend this mod to any short stroke CS.

As I & my old "Desert Sleds" aged, I am 82yrs old now, I retired them from the off-road abuse & set them up for street use only. I used 22T engine sprockets on the 1956 CS & 23T on the more powerful 1963. I also used 23T in my 1961 G80TCS Typhoon, But with 12:1 CR piston my old knee, after kicking big singles for over 65 years, started crying "Enough!". I still have my first Matchless, my 1956 G3LCS that I bought from Bud Ekins that year.

Not so "Cheery" now, Don.
MikeM.
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Re: Compression Ratio & Gearing?

Post by MikeM. »

hi Don, sincere condolences to you and your family on you sad loss from not only me but I am sure every one of us not only on the forum and everyone in the club.
MikeM.
JEAN-NOEL
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Re: Compression Ratio & Gearing?

Post by JEAN-NOEL »

Hi Don,
sincere condolences to you and your family.
Congratulations for your amazing erudition regarding single and twin AMC engines.
Kind regards.
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robcurrie
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Re: Compression Ratio & Gearing?

Post by robcurrie »

Don, sorry to hear the sad news of you wife. Please keep in touch with us, as your experience and advice is appreciated by all of us here I'm sure.

Rob C
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Expat
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Re: Compression Ratio & Gearing?

Post by Expat »

Yes, sincere condolences from a relatively newbie here.

Regards,

Steve
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