Does anybody have a Hepolite piston catalogue they could look up a piston for me please? The box says Norton Type C/- Ref.4874, M 38, 82m/ +60

I realise it's an 82mm +60 piston but it doesn't look like a WD Big 4 piston. It could be a later Big 4 piston or Model 19, but it's low compression as it has a concaved top instead of flat or domed. Any help appreciated, thanks.

I've just found a slip of paper in the box which confirms M 38 is the packer number in case of complaints.

email (option): horror@blueyonder.co.uk

Horror,

The piston you have is for a 82x100 model 19 the latter short stroke 19 these were concaved pistons. If you come across a 79mm concave Norton piston let me know as i need one for my long stroke 79x120 model 19.
Sure the big 4 had a flat top piston as i have a used one in the shed.

Tim W

email (option): t.j.walker@btinternet.com

Is the piston that Horror has the same as that used for the 19S...at 82 X 113 (the 19S stroke) that works out to 596cc...Ian

email (option): ian@wright52.plus.com

Your correct Ian, the bore and stroke for the 1933 model 19 engines were changed to 82X113 from the older long stroke engines of 79x120. The piston that Horror has must be a model 19 with the concave crown and 82mm.
Still if anyone has a 79mm Norton concave crown piston please let me know.

Tim W

email (option): t.j.walker@btinternet.com

Thanks very much Tim, that's a great help. I'll look out for a piston for you.

email (option): horror@blueyonder.co.uk

So the 113mm stroke is the 'short stroke'?... ...Ian

email (option): ian@wright52.plus.com

Yes, that's the hi-revving engine Ian

Tim, can't you use an 79mm Model 18 piston? I know it's flat topped but there's lots of meat on the crown and it could easily be skimmed to bring the compression down a bit if that's the problem. Or cut-aways for the valves if needed.

email (option): horror@blueyonder.co.uk

Horror,

I have a modifiyed model 18 piston in it at the moment, i have machined more than i want from the crown i also had to shorten the skirt. I have used the bike this summer its very rapid for a 1930 motorcycle and handles well, also sounds awesome with its open can. But the engine is lumpy at low revs, i have not measured the compression ratio but know its too high, i plan to fit a compression plate over the winter but it would be nice to locate a original concave piston.
120mm strokes rule!


Tim W

email (option): t.j.walker@btinternet.com

I like the long stroke engines as well...I have fitted M21 flywheels into my B33 and an oversize piston to give 712cc...

The 'grunt' of this motor is incredible. With an engine sprocket 5 teeth over standard it will still easily pull away in second and will accelerate up most hills in top...A real slow revving 'big banger'...Quite addictive...Ian

email (option): ian@wright52.plus.com