Monosoupape threads sizes and errata.

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:


Well-Known Member
Jan 28, 2020
Reaction score
Regarding the 1/5 scale, 49cc Chenery monosoupape, Elliot asked me what BA sizes were used in the engine. My drawings have faded to the point of complete illegibility- My Dad tried to send me his, but that didn't work out, but I have my engine parts to look at, so I can say with certainty that 6,8 and 10BA are used. 0BA appears on the engine mounting plate, but aside from the flank angle, it is identical to M6x1 which many people will already have. If not, if you do need to buy taps and dies, M6 is a size you are likely to use again after the engine is built! 0BA is a bit of an oddity. There are several 32 tpi threads to be screwcut, down to 7/16". That will be part of the extended British Model Engineer thread series, so a tap will be available; for making one nut, I would suggest buying a single second tap for that. (That's a "bottoming" tap, for our American friends.) Internal screwcutting at that diameter is a bit of a fiddle.

A Brief Rant on BA threads. Pinched off the Swiss, after some paltry amendments to root and crest radii, the british association thread is METRIC, but annotated in INCHES to avoid offending the sensibilities of its target audience. Its use was (and is) widespread but it was never given British Standard status. Starting at 0BA, each subsequent thread is 90% of the diameter, with 90% of the pitch of the previous one. That's why it looks so "correct" on scale models. That's why many people don't understand why the diameters are so "weird". That's why some refer to it as "Bastard Arseholes" thread.

It is, however, unique among screw threads because it is the only series with a clear mathematical progression.

Errata... Speaking to my Dad, I was reminded of an error on the drawings which I had better share. The engine mounting plate (the bit which the dummy magnetos are attached to) has a tapered bore to mate with the crank, and set it at the correct distance from everything else. I have two of them. My recollection is that the given dimension on my prints was WRONG, and that resulted in a scrap part, sliding too far on. My Dad's copies are different drafts; a correction may have been made; but watch out!

Les' modus operandi appears to have been to model a prototype, tinker with the design until he was satisfied with its running, then get someone else ("Ken") to draw it up; there was bound to be an error or two, but that's the one I remember. 'Cos I had to make the damn engine mounting plate twice.


Note on deepening tapers- set the topslide, use the cross slide to start the taper, then lock it off and leave it alone. If you want the mating part (the crankshaft in this case) to go in 20 thou further, advance the carriage 20 thou. Sounds like a no-brainer, I know, but I thought I'd best mention it. I've worked with under graduates, and know the folly of underestimating the ingenuity of complete fools.

Now, as an aside, I'm trying to get castings to Elliot in the US, but our postal service is a shambles right now. First it was mail strikes, now a cyber attack. I'm thinking FedEx. I don't think he has easy access to this group. If you're reading this mate, please get in touch to confirm pricing.


Latest posts