Prophylactics & small holes (Guys, guys, take it easy!) >sump plate threads

[RichardL]

I'm about to drill and tap a new sump plate hole where my crankcase has been welded-up due to a broken-out thread boss. I noticed that the PO of this barn-find A7 had 1/4-20 SAE (American inch) bolts in the other three holes. Checking a 1/4-20 Whitworth screw in these holes and in an SAE nut, the fit seems clearly suitable for use at the sump plate. However, I am wondering if I should prophylatically take out all those threads in favor of BSW helicolis, which I have, thus avoiding the potential need whilst the engine is in the frame. My A10 has three 6mm coils and one original Whitworth hole.

Any clean thoughts?

Photos attached show sump plate in place for hole location, and a backside view showing the amateur machining of the case alignment groove, of which I'm kinda proud.

Richard L.

Title edited to show topic subject :O)

[Rex]

Personally I'd Helicoil the Whit threads while I had the chance, maybe for no other reason than I like to keep the threads consistent.

[muskrat]

G'day Richard.
The sump should have studs not bolts so the threads into alloy don't get "worked" too often. 1/4"W and 1/4"UNC are both 20 tpi but differ in the angle of the threads 55 to 60 degrees. So in reality either can be used.
Cheers

[a10 gf]

Yes, studs.

For the A10, someday make 3x GT Special Turbo MK2 studs? (6mm crankside, 1/4"W nutside :O)

[Swarfcut]

While you have easy access and existing stock it makes sense to helicoil the weaker threads if necessary, and tap the new hole to 1/4 Whit.  The original studs are 1/4 Whit into the alloy, 1/4" Cycle on the retaining nuts.

 Sticking with the original threads means they all match, one spanner does all with no chance of mixing threads. Modern replacement studs fit just fine.....the less the threads in the crankcase are worked, the less likely trouble ahead with an awkward to fix pulled thread.  Avoid using bolts, as continued use in and out will wear the relatively soft alloy thread. Assemble with Loctite on the studs and nuts, sleep easy.

 Yes, that precision machining gets a well deserved accolade from me. Also thanks for a not often seen view of the pick up pipe and strainer in all its glory. You wonder why the original design of the sump plate never included a drain plug/access hole to free that pesky ball valve.

Swarfy.

[RichardL]

Done. I went ahead with the SAE 1/4-20 thread in the new hole, for lack of a BSW tap on hand simultaneously with the time availble to go to the DIY-for-hire machine shop. In any case, as I write this, not sure I could get a BSW bottoming tap. (EDIT: Yes, I see bottoming tap readily available. Oh well.) Before making this decision I tested the existing BSW threads with 6 ft-lbs and found no weakness, so I figured, if it ain't broke, don't fix it. I just can't see a big downside to taking the most expedient path to an assembled engine/motorcycle when the compromise appears to be so trivial. (EDIT: I guess I won't go down in the annals of great original-everything restorers.)

Now, on the topic of studs, is there a set out there for use with the flange thickness of an SRM sump plate?For nuts, I'm thinking, same as those for rocker box covers. One very small downside with studs, perhaps, is some added trouble cleaning gasket sealer off the surface each time the plate is pulled.

Richard L.

[muskrat]

G'day Richard.
I made my own studs (whitworth bolts with plain shank and head cut off then threaded 1/4" cyc) tappet cover studs might be long enough. I also use tappet cover nuts for ease of get to.
I use a non hardening gasket goo https://tinyurl.com/wpl7xhg most of the time the gasket can be re-used and the surface cleaned with carby clean.
Cheers

[morris]

Now, on the topic of studs, is there a set out there for use with the flange thickness of an SRM sump plate?For nuts, I'm thinking, same as those for rocker box covers. One very small downside with studs, perhaps, is some added trouble cleaning gasket sealer off the surface each time the plate is pulled.

Richard L.

De Groot has them; https://degroot-bsa.nl/product/sump-plate-studs-nuts-a-b-c-m-model-long-4-en/
Could be costly though to get them posted to the USA?

[Greybeard]

I'm pretty sure I changed my studs for rocker box studs after I fitted the thicker alli sump with drain plug. I wish I had put rocker box nuts on as well, as Musky suggested. I'll order some now.