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Stripped ES2 engine drain plug

Submitted by richard_salt on Mon, 19/06/2023
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The 3/8" plug underneath the ES2 1961 has stripped. It is, in my opinion, not correct to use such a fine thread in aluminium. 

I'm thinking of simply tapping it to 7/16" BSF.

Does anyone have any advice on this?

Is the material thick enough to do this?  Should I rather use a helicoil or insert?

The engine is in the bike so whatever I do is going to be difficult.

Submitted by ian_soady on Mon, 19/06/2023

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fine threads in aluminium aren't a good idea - I remember on my Enfield trials machine struggling with 1/4" Cycle threads into the head.

If you could lie the bike down on its side it would make access easier - I'd go with a helicoil myself.

Submitted by richard_salt on Mon, 19/06/2023

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..... but my experience with helicoils is that they are fine in blind holes, but in open end holes the tang usually gets forced out before it breaks, thus taking the last coils with it.

I think an insert is better.

Does anyone know the thickness of the aluminium at the bottom of the ES2 engine where the drain plug is located?

Submitted by michael_rettie on Mon, 19/06/2023

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...that you had better be certain that the tap runs dead straight or you will not get a very good seal. Not that one more leak is a big deal.

Submitted by ian_macdougall on Mon, 19/06/2023

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My 19S was repaired many years ago using a steel "top hat" insert. Simply going up a size runs the risk of the thread stripping again in the future. Sorry I can't remember the thickness of the case in that location. An added complication is that the drain hole emerges into a very shallow chamber which will significantly limit the distance a tap can be inserted.

Submitted by richard_salt on Tue, 20/06/2023

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So I'll need a finishing tap with almost no taper. I'll cover the tap with grease to trap all cuttings. I'll use a Dowty washer in case the tap goes in crooked but I have found that taps usually follow the hole - it's the drilled hole which is usually crooked. 

My problem with an insert is that it may be longer than the case is thick.

Reading in the literature about threads, it is advised not to use a fine thread in aluminium. Apparently a coarser thread is stronger. As there isn't much force needed from the stud, and with the Dowty washer, the coarser thread should be less likely to strip in future.

I think I'll go the tapping to 7/16" BSF route.

If all goes wrong I'll have to strip the engine and weld it up for a new drilling - make the case thicker in that area too.

Submitted by david_cooper on Tue, 20/06/2023

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The fine thread would have been chosen because it is less likely to come loose when tightened to a low torque.  Since it is under essentially no load in service, the original fine thread is perfectly logical.  It is of course well lubricated at all times, so should always be easy to fit and remove.

Submitted by neil_wyatt on Tue, 20/06/2023

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The threads stripped on my 1959 ES2 sump in 2016..

The cure to date and still fine, was:

1) With no more than about half a tank (Of E0 of course) lay bike on side, on cushions.

2) Remove the sump plug. (Empty so when upright enough oil to wash out swarf)) and measure the plug length.

3) (No room for a magnet (As Ian Mac Suggests) Cut to the length of a Commando sump drain plug. TPI available. 

4)Tap as required.

5) Partially refit Commando plug. Undo plug and see the swarf with the remainder of oil drop into clean plastic tray.. Clean threads etc and refit using a soft washer. Do not over tighten. The final part with bike upright. What ever WORKS for you.

PS: Not enough length for a course thread.therefore Commando plug easily available.

Submitted by ian_goodhall on Wed, 21/06/2023

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A previous owner re-tapped mine and I'm pretty sure it is UNC as an AF spanner fits it and it isn't a fine thread. I was worried about either overtightening and stripping it or it coming loose so I drilled it for lockwire and just nip it up and wire it. Has been fine and doesn't leak.

Submitted by richard_salt on Sat, 24/06/2023

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I have tapped mine 7/16" UNF (I don't have a finishing tap in BSF!)

I will use a Dowty washer and wire it in place.

Submitted by james_lynch on Wed, 05/07/2023

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put a brass insert in, then 5/16 bolt, problem solved, this is what I did, worked a treat

 



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