Anyone got ideas regarding valve seat widths?
Inlet/outlet same or different? Optimal width and tolerances? M20 and B31 are in question.
Also is there any benefit with these motors using a 46° interference angle rather than 45°(been reading the Neway manual )
Cheers
Pete

email (option): petercomley@web.de

Let’s start with the exhaust. An air cooled side-valve motor is a harsh environment for the exhaust valve, which has to be made from high quality steel and even so needs to shed heat quickly. The primary way in which it gets rid of heat is through the valve seat, and for this reason it is not normally recommended to reduce the seat width or, if you must, reduce it only by a little. The beneficial effect of a narrow seat in terms of increased flow at very low lift is in any case less important for the exhaust valve because when it opens there is still a pressure of several atmospheres in the cylinder to get the gases moving. BSA specified 1/8” (0.125”) for the exhaust seat width and Phil Irving quoted 3/32-1/8” (0.094-0.125”) as a reasonable requirement (depending on valve size), so there isn’t a lot of scope for narrowing OHV exhaust seats and SV seats are probably best left alone.

The inlet valve has a much easier life as it gets cooled by the incoming mixture, so more liberties can be taken and narrowing the valve seat will have beneficial effects in terms of low-lift flow. This time there is only about 1 atmosphere pressure difference (at best, unless you’ve done extensive tuning) to force the air through the valve so anything which reduces the passage restriction is worthwhile. BSA specified the same 1/8” inlet valve seat width as for the exhaust, but P.E. wrote that the inlet seat can be narrowed right down 1/32” (0.031”) to good effect, preferably by increasing the port throat diameter. Again the hotter running of a SV engine may suggest a little more caution.

Whilst being vaguely aware of “interference” seat angles I haven’t read up on the subject. I guess the idea is to have an almost point contact between valve and seat (in effect reducing the seat width to near zero). Excellent for low-lift flow, no doubt, but I question how many miles you’ll get from your motor without the hammering effect of valve closure meaning that you have to take it apart and re-cut the valve seats. Perhaps with modern materials it works well, but with cast iron and 60-year old steel?? I wouldn’t go there!

ChrisJ

Perhaps I should have been clearer about BSA and 1/8" valve seats - that's the spec for an M20 and I haven't checked regarding B31. ChrisJ

I agree with Chris's comments regarding the exhaust seats in both engine types..they are best left standard for the reasons given. Narrow inlet seats with the surrounding areas cut back are primarily used to improve gas flow as the valve lifts off the seat initially. Every improvement to gas flow is beneficial but there are implications with regard to a higher 'maintenance' requirement.
In isolation such alterations would provide such small benefits as to not really be worth consideration. However, as part of a tuning package where the aim is to maximise efficiency in all areas every modification, however small has to be considered.
In the case of the M20 a greater gain would be achieved by fitting a bigger valve and reshaping the area under the head, which restricts flow at all stages of valve opening....Ian

Thanks for all the info! So exhaust and inlet valve seat widths for M20 is 1/8". Any anyone know what is standard for a B31??
I'm not looking to build a race machine or experiment, just looking for the standard values.
Cheers
Pete

email (option): petercomley@web.de