24 replies to this topic

[Ethel]

How about replacing the M20 with 1/2 bsp? You could get a brass reducer (1/4bsp) that you could drill & tap for unc without access to a lathe - or just  replace the plug with 3/8 bsp.

[Spider]

Chris, do you pin or Loctite the M20 into the sump?

 

If you have a good, flat surface for the sealing washer, I suggest switching from the copper washer to a bonded rubber/metal seal (Dowty washer).  They seal with very little torque and the squished rubber prevents the plug from backing out.

 

Hi Doug, I usually Loctite the M20 hollow bolt in place, haven't had a worry with them as yet and they are not intended to be removed. My idea of using the Loctite is not only to lock it in place but it also seals it, but of course that will only work if it's all clean! A few of the guys have fitted them with the engine in situ. The few that have been done, have copper washers only because that's was what I had to hand at the time, but I now have Fibre Washers which I fell would be better and kinder. Thanks for the tip on the Dowty though, I have used them before and they are excellent, but need to be careful that they suit the fluid being sealed and these days with everything being made in China, the writing on the packet doesn't mean much anymore.

 

I have fitted up a few of these hollow M20 bolts now, however I have done them at the point of a Gearbox overhaul, so while I have the gearbox in bits, I drill tap and face the spot in the mill, but as I mentioned, a few guys have done them in situ with success (or they are just being nice).

 

 

 

 

How about replacing the M20 with 1/2 bsp? You could get a brass reducer (1/4bsp) that you could drill & tap for unc without access to a lathe - or just  replace the plug with 3/8 bsp.

 

I guess as a temp repair, that could work, but like the repair plug from Mini Spares, you'd want to be careful that you don't make a small problem bigger.

 

Another advantage of using the hollow M20 (or what what ever) hollow bolt, tapped to 5/8" UNC is it uses the stock magnetic sump plug.

Edited by Moke Spider, 20 July 2015 - 08:29 PM.

[minimat]

Ac dodd did an article fairly recently in whichever mag he contributes to showing how to helicoil these.

[dklawson]

The HeliCoil installation process itself is not the hard part.  With the copper seal it is important that all the drilling and taping is carried out as perpendicular to the gearbox face as possible so the seal will not leak.  I have not seen AC's articles but I assume he points this out.

 

The nice thing about Spider's fix is that the modified bolt will have a flat face for the seal to work against regardless of whether the re-tapped hole in the gearbox is kept perpendicular or not.

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[Ethel]

Spider,

 

I meant use a 1/2bsp reducer bush in place of the M20 and the 3/8bsp in to that. I suppose it does read like I'm suggesting 3/8bsp could replace the unc plug into the casting.

 

1/2bsp is about a millimetre bigger than M20, so I wouldn't be 100% sure you could drill and tap the case without fouling, but it's unlikely 1/2mm more shaved off would matter.

[Spider]

Sorry Ethal, yes, that would probably work. Sorry for my reservation though as I've always found trying to get an Oil tight seal on a tapered thread, like that found with BSP stuff, not easy to achieve and in any case, they need to be done up quite tight. I do recall looking at something like thing a while back, but it's best if a seal, for oil, can be made on 'flats'. This is why the sump plug has the sealing washer and it seals under the head and on the spot facing on the gearbox case, ie, not on it's thread, although I do do this with the repair plugs, I don't rely on that for a seal.

[mini-auto]

I also currently have the stripped sump plug issue as per the other post mentioned.

 

I have bought a helicoil kit & was ready to press on with that repair but having read the M20/5/8"-UNC bolt solution I am thinking about going that route instead.

 

I served my time in an old fashioned tool room & have access to a home model engineering workshop with full turning/milling machining facilities so converting an M20 bolt isn't an issue.

 

I thought I might be able to fit the helicoil now & then M20 it later if needed but the o/d of the tap with the helicoil is .740" & the tapping size for M20 x 2.5 is 11/16" (.688") 

 

I work in industry & we use threaded inserts similar to the M20 bolt idea to repair stripped & damaged stud holes so I am drawn to that method.

 

Whilst writing the above I've ordered an M20 second & a 5/8-11UNC second.

 

 

 

 

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[mini-auto]

Well today was the day the sump plug was getting fitted.

Beautiful sunny day, car up on blocks with plenty of access, perfect, what could possibly go wrong?

I opened the hole up to the correct tapping size for M20 x 2.5, measured & confirmed with a vernier caliper.

But as hard as I tried I couldn't get my ebay special M20 used second tap to start, even with a bottle jack applying pressure, it was cutting metal but just wouldn't get a grip & take itself in, it just cut an ever deepening taper, arrgh!

I persisted but was concerned the way it was going the hole would be no good for either M20 or helicoil & I would be well stuffed, so I decided to try the tap that came with the helicoil kit I'd bought.

Thought I might start the helicoil tap & then try the M20 again but the helicoil tap went so well I tapped it right through.

Next good idea was to trial fit the helicoil, make sure it fits, remove it, clean it, then refit with a bit of threadlock, err no, once a helicoil goes in it don't come back out!!

So there you have it.

Have run the car up to temp & all is good, not quite the repair I wanted, but hey ho, another step nearer.

"The best laid-plan of mice & men", etc etc.

Car looks reasonably happy.

 

 

 

 

 

 

 

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Edited by mini-auto, 27 September 2015 - 08:32 PM.

[dklawson]

For future reference, to get a "typical" amount of thread in a tapped hole you subtract the pitch of the thread from the major diameter and that will tell you the tap drill size.  In this case, M20 - 2.5mm = 17.5mm or 0.688" (11/16").  

 

However, what may have happened is you may have been sold the incorrect tap to start a hole.  There are taper, plug, and bottoming taps.  It is almost impossible to start a tapped hole with a bottoming tap as it is ground practically flat on the end.  

 

Regardless, I'm glad you have made progress and have a working drain plug solution.

[mini-auto]

Dk with you saying .688" above I realise where I went wrong, I opened up to .622" which is the tapping drill for M18.
Doh, no wonder it didn't want to know !