Ariel Owners Motorcycle Club

Hello Ariel People

what is the Fully Advanced Ignition timing

for my Ariel SB31 550CCM side valve i have found it should be 9,5mm but im not sure about it

kind regards

Søren Mortensen

The owner's guide is available free to members on the club website, you will find this information and much other useful stuff. The figure is 7/16" but is not critical.

Thanks Roger

we have started with 14,5 mm then reduce it to 9,5mm and thats not far from 7/16 11,11 mm

Roger Gwynn wrote: Mon Jul 06, 2026 6:14 pmThe figure is 7/16" but is not critical.

+1. Old manuals give timing specs to the nearest 1/16", often 7/16" (11.1125 mm) or 3/8" (9.5250 mm) for most engines. Although the piston position depends on both the connecting rod length and stroke, at the BTDC timing of the engines we care about, a resolution of 1/16" (1.59 mm) is equivalent to a timing difference of nearly 3°. I find it isn't too difficult to set the timing to ~0.5 mm using a ruler in the spark plug hole, which is a bit better than 1°. Given that we seldom even know to 1° (or even 2°) what the "correct" value should be for modern fuel in a given engine, it's probably not worth obsessing over tenths of a mm.

thanks cmfalco

so if im between 9.5 and 11.11 then it should be all good