High Compression Piston and High Lift Cam

[Yves.Astein]

Hi fellow Motorcyclists,

I would like to request some advice from the experts.
I want to give my Matchless G3 1951 a bit more steam on the hills. I have found and bought some high lift cams. And I was thinking to also purchase a high compression piston. https://www.classic-motorcycles.co.uk/c ... b_pid=4665

Does anyone have experience with this kind of combination ? Is there anything else that I will need to change to make it work? Carb jetting is currently standard 76AE/1AK. Will the valve clearance be sufficient?…

Thanks for any input

Yves

[clive]

With high lift cams and an earlier engine I think you should check that there is sufficient clearance between the foot on the tappet and the guide when at full lift. I have had a guide drop down slightly and with the higher lift and earlier engine there is not a lot of clearance if indeed there is enough, Someone here will know if any mod is needed or if it will fit.

Difficult to say if the valve clearance will be sufficient as the piston illustrated in the supplier you reference is clearly marked "the piston shown here is not the actual piston" if it is not flat topped then I would expect it would have to have valve pockets cut into it.

[Andy51]

I did a similar conversion on my '54 18S. When you say high lift cams, do you mean '54-on 'H' cams or full blown scrambles ones (SH)? I used SH cams and they have a smaller base radius which means for pre '54 motors the top of the alloy surround for the cam spindle bearings needs to be machined so the cam follower can maintain contact with the cam at all times. If you are using H cams, I believe they are a straight swap with no machining needed but check on your '51 model. With regards to the piston, I run an 8.5 to 1 JP piston without any problems but I carried out a simple check. Put some plasticine or blutack on the piston, particularly in the valve pockets, assemble everything, turn the motor over a few times, take head off and measure thickness of the plasticine - you need 1/16 inch minimum clearance under valves. If you go the SH route you will lose some of the 'sweetness' of these machines; H shouldn't affect it so much. Carbs - use standard settings. According to the parts books, the pre '55 longstroke CS models use the same carbs and settings as the standard models. Good luck, Andy

[Yves.Astein]

Hi Clive, Andy,

thanks for the valuable feedback. I have purchased ‘H‘Cams not SH. I was planing on using blutack to check clearance. I just wanted to be somewhat sure that I am not trying something stupid, before purchasing the new High Comp. Piston…

Your Guy’es feedback makes me confident that I have a good chance to get it to work.

Andy: I am taking the H route, but out of curiosity, what do you mean by „loosing sweetnes“ when using SH ? I mean I love my old single with almost a straight pipe its just a joy to ride to work almost every day in the summer. So do you mean the thumping sound gets lost or does the acceleration curve change radically?

Cheers Yves

[Andy51]

Hi Yves, by 'sweetness' I mean the low speed flexibility and (relative) mechanical silence, and exhaust noise. My standard G80S is happy to plonk along at 30 mph/50 kph in top, and accelerate from that speed. The 18S isn't happy much below 40 mph in top, so riding about town means more gear changes and 3rd gear use. The SH cams do not have quietning ramps incorporated (these open and close the valves slowly over several degrees of crankshaft rotation) so there is a lot of top end engine noise. The earlier exhaust valve opening point means a LOT more exhaust noise, to the point that it on a 500 it can become uncomfortable on a long run - wear ear plugs! You do get a bit better performance, but that's not what these old thumpers are about, as a wise man once said, there's no such thing as a free lunch. Hope this helps, Andy

[Yves.Astein]

Ah, thanks Andy, for pointing out the trade of‘s.

Yves