Performance upgrade info needed

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Location: BUCKS UK

Re: Performance upgrade info needed

Postby Jdraper » Thu Sep 26, 2019 8:14 pm

After my teenage attempts to rebuild my '55 18S engine in 1976, Ken DG rebuilt it a few years ago, including a custom big end bearing assembly. Not too many errors by myself, but the cumulative effect of many small errors meant a difficult and an 'unhappy' feel to it. It is still on the +60 piston I fitted all those years ago, still got the same valves, cams & carb... but the effect of of being put together correctly - with a good magneto now makes it feel 'happy'. It almost feels cammy when it comes on strong at around 50-60. Why do I need more when the engine is clearly now better than the brakes & suspension (try going over the potholes in Bucks!).

My advice is put it together really carefully, get the parts to work together - KDG spotted some bits that worked, but not optimally - slight twist in con rod, slight play in the big end, balance the flywheels assy, mis-matched oil pump parts. Respect.

Following the KDG model my recent interest has been getting the ignition to work optimally, hence the thread around auto advance curves a few months ago - still a work in progress.


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Re: Performance upgrade info needed

Postby Cjay59 » Fri Sep 27, 2019 6:46 pm

Another good approach, which hasn't been mentioned, is shed weight, my white silk scarf ended up wrapped around the back wheel, didn't help the breathing...

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Re: Performance upgrade info needed

Postby Martin.S » Mon Sep 30, 2019 8:41 am

"But I'm guessing that that's not what you're asking. If you mean what the highest compression ratio"

The old adage 'There's no sustitute for cubes'
The only drawback that I can see there is a little more vibration. Depending on how much your capable of depends on whether it's worth it.
You'd need to source a piston with the same deck hieght and similar combustion profile.
You can look into opening out the bore in the crankcases and put a larger diameter sleeve in the barrel otherwise the barrel skirt thickness would be your limit.
I know that this is done with the Norton singles and if it was possible to do this for your machine then it would have already been done by now - you just need to search out who had done it.

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Re: Performance upgrade info needed

Postby Pharisee » Mon Sep 30, 2019 2:43 pm

There was a certain gentleman, sadly no longer with us but known to a good many in the club, who grafted a Velocette barrel and head onto a 350 heavyweight crankcase. If memory serves me correctly, that was bored out somewhat and a Triumph piston of some sort fitted to give a capacity in excess of 400cc. He toured all over Europe on it, mostly two up. It was also fitted with a belt driven car alternator and hydraulic TLS front drum brake amongst a host of other modifications.
It's only limited by your own engineering skill and facilities!!

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Re: Performance upgrade info needed

Postby Cjay59 » Mon Sep 30, 2019 4:42 pm


my point about weight is based on machines like the '66', I do think in terms of motocross, but surely the Matchless squeezed out all the power that could be found from the 500, and every effort was used to shed weight, this would make a good machine to inspire improved performance.

"Too little (or possibly too much), too late. That could probably be said about the Matchless G85CS Scrambler. As one of the last models of Matchless to be made, it was the last (and best) attempt by the British to build a 4-stroke scrambler capable of beating the light-weight 2-stroke machines that were dominating the European Scrambles scene."

"It’s easy to see the Rickman brothers influence in the design of the frame. The G85 was a duplex design with lightweight forks, machined front hub, magnesium rear hub, and as many lightweight fiberglass and aluminum components as possible utilized. Though Matchless claimed 291 lbs., actual weight was nearly 320 lbs, much heavier that the 2-stroke competition."

Matchless G85 1966.gif

"The G85CS looked and sounded magnificent, but was not particularly fast and when combined with the weight; it just wasn’t competitive at the top level."

"Of the 99 produced in 1966, this is #15 and is one of about 10 left in the United States. Matchless seized to exist at the end of 1966 and no further models were manufactured."




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Re: Performance upgrade info needed

Postby spookefoote1956 » Mon Sep 30, 2019 6:03 pm

Put it on a trailer! Not only will it go faster but it will also save fuel! :D
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