Compression Ratio

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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ELPS
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Compression Ratio

Post by ELPS »

Am in the slow process of rebuilding my 1949 G80 engine. Have decided to run an early 1950s G80 alloy head and am thinking of going with a 8.5 to 1 JP piston but thought I'd ask if 8.5 to 1 is too much compression for these old girls? I'd think there must be some power gain with the high comp compared to the 7.2 to 1 alternative piston or am I wrong??
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clive
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Re: Compression Ratio

Post by clive »

Yes there will be some power gain, but marginal. I ran a high compression piston for some years on my 54 model 18 with a heavy sidecar attached. Make sure you have cleaned out the thread for the head bolts all the way to the bottom so that don't bottom out rather than compress the head gasket. I would anneal even a new head gasket before you fit it, and remember to tighten the head bolts after the first 100 miles and to recheck the pushrod clearance when it's all back afterwards.
Downside is it will put more strain on the big end so if you have not replaced it be sure it's in good nick. . I think you will need a different exhaust pipe for the alloy head as the angle coming out of the head is slightly different to the iron head if I remember correctly.
clive
if it ain't broke don't fix
ELPS
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Joined: Wed Jan 04, 2017 5:11 am
Location: WESTERN AUSTRALIA AUSTRALIA

Re: Compression Ratio

Post by ELPS »

Good point about the barrel threads. Have ordered new head bolts as the ones I removed had stretched threads. Havent got the bottom end apart as yet. The little end has fair amount of play but no apparent play in the big end. Yes, thought I may need to change the header pipe. Can you recommend someone who can supply the correct pipe?
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Rob Harknett
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Re: Compression Ratio

Post by Rob Harknett »

Years ago I upped compression on my almost new 55 G3LS, it went a bit faster, but there were few long straight roads where I could hit almost 90. I thought should I had got a Triumph speed twin instead of my Matchless 350. Then came a hefty bill when the big end failed. ( 3 weeks wages ) The dealer replaced my 9.1 piston, to standard CR. I was not happy, he said up to you, we'll put it back in if you want another bill like this again. He showed me the chewed up big end. Lesson learned while still a teenager.
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dave16mct
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Re: Compression Ratio

Post by dave16mct »

Feked.com for the pipe. Recommended on here. Make sure they know it's for an alloy head.
Dave.
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Rob Harknett
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Re: Compression Ratio

Post by Rob Harknett »

ELPS wrote:Am in the slow process of rebuilding my 1949 G80 engine. Have decided to run an early 1950s G80 alloy head and am thinking of going with a 8.5 to 1 JP piston but thought I'd ask if 8.5 to 1 is too much compression for these old girls? I'd think there must be some power gain with the high comp compared to the 7.2 to 1 alternative piston or am I wrong??
Don't know what they quote for 1949 G80 comp ratio. The bottom end was still using a mixture 1937 to 1939 parts in 1949. The comp ratio then was 5.2-1 even on the G90 clubman model. ( source of info, parts lists and workshop manuals )
ELPS
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Location: WESTERN AUSTRALIA AUSTRALIA

Re: Compression Ratio

Post by ELPS »

OK, thanks for the info. If I do find the big end to be serviceable, then I think I will err on the side caution and run with the 7.2 to 1. Cheers
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