Loose primary case filler plug on semi-unit engine

[KiwiGF]

Ref prior comment, because I made the cap in two pieces I did not have to thread up to a shoulder, the whole 6mm length of a 1 1/2” disc was threaded with the piece held on a mandrel (2 nuts and a long bolt)

[RichardL]

Good point Kiwi. In retrospect, making the cap in one piece would  be tricky. The shape of a typical symmetrically ground bit will cause it to hit the flange about half-way along the thread length. Need a bit ground on its left side and with a tiny 55 deg. point, I believe.

Richard L.

Edit: I'm leaving this post in place to remind myself to read previous posts first. RD's explanation is very good and comprehensive, though, I'm not sure I understand the bit he described (nor if I've described the correct grind/type.)

[berger]

RD fella i am no turner but brother is. i took a job back to him that needed extra thread and he cursed me about getting things right first time. he was explaining to me how difficult it would be to set the job up properly again and get the undercut and also saying about having to stop the cut. he did manage it but it was a learning curve for me about something i didn't understand.

[groily]

Looking at your pix on 'small vintage lathes' of the stock being used for this item GB, I did wonder how easy it would be to hold, thread and finish the cap - largely due to this 'up to the shoulder' thing, and the short length of thread required. It certainly isn't easy.
RD's professional advice is clearly the best way to do this, 'inside out', if you have a reversing switch? (Can't remember!)

If you DON'T, would you be able to mount a cranked handle or something onto the outer end of the headstock spindle on an ML4? If you could, I would be tempted to do the threading - in towards the chuck - with very light cuts, turning the thing manually. 
You could then at least stop before destroying the workpiece, and also leave the lead-screw engaged while reversing. Repeated cuts at the same setting should allow a reasonable finish if you wind steadily, adjust the compound slide as suggested - and make that narrow rebate adjacent the shoulder first.

A few years ago I had to make a radiator cap for my LE Velocette. (Unpressurised, just a female screw-on job.)
So, a bit of round stock needing an internal thread up a blind hole about an inch and a quarter bore, a bit longer than what we're talking here, but some of the same sorts of issues. 
Because I was scared of making a total mess of something I couldn't see very well, I bored a rebate to major thread diameter plus a few thou up at the blind end and cut the thread in light stages by hand, turning the chuck in the usual direction. Could have gone the inside-out route under power as I do have reverse, but time being no object, it worked for me  . . . (and as I don't have a dial indicator - something I should have done something about a very long time ago! - I always keep the lead-screw engaged.)

Just a thought, maybe? Only a chunk of ali to lose, after all!

[Greybeard]

I'm still digesting all of your comments. Thank you guys! 👍
I will be practicing on some scrap before I start on the real deal.

Here is my workpiece for the primary case plug, (resting on a pen) . I've left the right hand section in the picture with the intention of using it to hold the work in the chuck while threading. I'll chop it off when I finish. Alternatively, the largest diameter is 9mm wide, (at the moment) so I think I could use that to hold the work squarely in the chuck. You may see that I've cut a gutter before the shoulder, (as advised by BlondiHacks). That may turn out to be a mistake if the gutter wastes useful thread in the primary case. I guess I'll have a fibre washer on there though.

The idea of turning the chuck by hand sounds good.

[terryg]

Agree wholeheartedly with Groily’s advice to manually rotate the chuck - to the left and on the return.   It’s only ally, it’s only 20 tpi, it’s only a few threads!
You’re not going into production, I assume, so this is a good way to introduce yourself to thread cutting on the lathe.
Plenty of time in the future to cut under power - which can be challenging and a bit scary.

[Greybeard]

Groily,
I downloaded that thread chart document, thank you.

Looking at BSC threads around the size I need, life gets complicated! I'll take the old plug out and have a measure to decide what size it really is.

[terryg]

“ I'll take the old plug out and have a measure to decide what size it really is.”

That’s the way!  You’re making a part to fit, not to a spec.

When you finally fit your creation you’re going to feel quite pleased with yourself, I think.

[Rex]

Another point to consider is that you need to make the cap slightly over-size to compensate for the worn chaincase threads, and then you're into the question of "how much oversize?"  although I suppose you could try it in the case while it's still mounted in the lathe until you get a good fit.
Make it STD and you may as well just buy one off-the-shelf.

[Greybeard]

Maybe not Rex. I went out to the bike and took out the plug in the primary case and the one in the oil tank as they are the same size thread. I measured the thread in the primary case at 20tpi. Measuring the oil tank cap it is also 20tpi. I screwed that cap into the primary case and it got a really good grip on the threads. I then had a good look at the plug I was having trouble with. I believe I bought it as a replacement for the original which may have fallen out during a ride, I cannot remember. Anyway, the thread on the 'modern' cap has been distorted, (from overtightening to cure leakage) but appears to be 19 tpi and also has a 30 thou smaller OD.

I could buy another replacement and hope it's accurate but I still like the idea of making my own plug.

[RichardL]

GB,

I don't remember if you said you have thread gauges, so, forgive me if I'm being redundant.  I'm showing two pictures of my drive cover cap. The after-market cap for my oil tank is actually 1.495", versus what's shown here. Also,  the gauge fits better in the threads than what's shown here due to juggling parts and phone. Edit: That thread gauge says "20".

Richard L.

[Greybeard]

RL,
Similar to my two caps;  the original has an OD on the thread of 1.48";  the replacement, modern cap is 1.45". My brain tells me that's a difference of 30 thou.

20tpi seems to be correct.

[Greybeard]

Plug done! . The boss on the top in that picture was removed when all the machining was done.
https://youtu.be/wh_P5gvMs6M

[Greybeard]

I suppose you could try it in the case while it's still mounted in the lathe until you get a good fit.

Almost!

[Greybeard]

Hey chaps, thank you very much for all the help with this project 👍

I am immensely proud of that plug and every time I go to the bike it will give me another little kick of pleasure when I think that I made it myself! 😁