Stripped Gearbox Drain Hole

robs ss · Dec 1, 2021

Well it's not entirely stripped, but VERY unhealthy.
It's on a friend's Mk3 and we have to do something about it - so thought I'd run our plans past forum members.
We considered helicoil (and the other variants) but have settled on simply going to a larger plug.
M12, as the M12 root diameter (9.85mm) is slightly larger than the major diameter of the 3/8" BSF (9.53mm) OEM plug.
He's already bought an M12 x 1.25 magnetic plug plus dowty washer.
Our plan is to, very carefully, do it in-situ.
First push some grease up the hole then slowly drill out - pulling out many times to clear swarf.
The hard part will be keeping the drill bit square to the sloping rear of the gearbox.
When drilled, flush out with kerosene.
More grease then tap the hole.
Again staying square - using a 300mm extended tap wrench.
Another flush with kerosene and we should be good to go.

Thoughts? (nice ones please!)

gortnipper · Dec 1, 2021

I repaired mine years ago with a Timesert. Used grease on the tap to capture the swarf, and used a number of greased up cotton buds to clean up in the case until they were all clean. Then flushed some oil into the case and out the hole to check for swarf.

Craig · Dec 1, 2021

The time-serts seem to be top shelf , the times I used them …. Oil drain plug on old Combat was repaired with one , never a drop

SteveMinning · Dec 1, 2021

I've used the Timesert as well with great results. Expensive but a very good solution in my opinion.

baz · Dec 1, 2021

How are you going to keep the drill square? And the tap square because if the are not the drain bolt will not seal
I don't think a bit of debris in the bottom of the gearbox will do any harm it should just stay there
It's not like having debris in a engine with oil being pumped around

BERT · Dec 1, 2021

My main concern would be whether the chips would clear properly going through grease

gortnipper · Dec 1, 2021

baz said:
How are you going to keep the drill square? And the tap square because if the are not the drain bolt will not seal
I don't think a bit of debris in the bottom of the gearbox will do any harm it should just stay there
It's not like having debris in a engine with oil being pumped around

The kit uses a drill that is square cut to mount in a tap handle. It pretty much follows the existing hole. No issue with leaks on mine. But I did my engine drain... :-o

robs ss · Dec 1, 2021

baz said:
How are you going to keep the drill square? And the tap square because if the are not the drain bolt will not seal

I've done a dummy run on my bike.
Raised the bike on scissor lift - bike vertical and centre-stand up and out of the way.
Chocked the rear wheel with timber to steady the bike fore-and-aft.
Used a small engineers square clamped to a steel rule to check where the correct line is, using the sloping rear of gearbox a reference. On my bike that line is 38mm below the tangent to the rear tyre.
Cut a plywood "template" to act as a guide for drill/tap angle - use it as a "sighting board" when drilling or tapping.

robs ss · Dec 1, 2021

I'll check with him if he wants to Timesert instead.

texasSlick · Dec 1, 2021

I say your method, post #1, and #8 are well thought out and should work out fine.
I fail to see how a Timesert offers any benefit .... you still have the chip and swarf capture and removal concern, and drilling square to the original hole problem, although in the latter regard, I think the drill will naturally align with the original hole.

I would go with your first method .... drill and tap to a larger standard size. Starting the tap square, IMO, is your major problem.

Slick

htown16 · Dec 1, 2021

l tried this on a gearbox shell out of the bike. Handheld drill. Found the aluminum alloy wants to grab the drill bit even using lube. Ended up with oversized crooked hole.

texasSlick · Dec 1, 2021

htown16 said:
l tried this on a gearbox shell out of the bike. Handheld drill. Found the aluminum alloy wants to grab the drill bit even using lube. Ended up with oversized crooked hole.

Grabbing can be mitigated by drilling with increasingly larger drill sizes rather than in one go.

Hope it works out.

Slick

robs ss · Dec 1, 2021

htown16 said:
l tried this on a gearbox shell out of the bike. Handheld drill. Found the aluminum alloy wants to grab the drill bit even using lube. Ended up with oversized crooked hole.

Yes - I have thought on this and discussed with my neighbour (he's a toolmaker by trade)
He gave me a small "stone" to blunt the leading edges of the drill - to reduce the "grab"
I'm going to test the drill on a piece of alloy plate to see if this is necessary.

robs ss · Dec 1, 2021

texasSlick said:
I say your method, post #1, and #8 are well thought out and should work out fine.
I fail to see how a Timesert offers any benefit .... you still have the chip and swarf capture and removal concern, and drilling square to the original hole problem, although in the latter regard, I think the drill will naturally align with the original hole.

I would go with your first method .... drill and tap to a larger standard size. Starting the tap square, IMO, is your major problem.

Slick

Yep - I'll start with a tapered starting tap then, so as to not protrude too deeply, finish with a bottoming tap.
Also, the 300mm extended tap handle will, in my opinion, reduce the chance for error.
Cheers

marshg246 · Dec 1, 2021

robs ss said:
Well it's not entirely stripped, but VERY unhealthy.
It's on a friend's Mk3 and we have to do something about it - so thought I'd run our plans past forum members.
We considered helicoil (and the other variants) but have settled on simply going to a larger plug.
M12, as the M12 root diameter (9.85mm) is slightly larger than the major diameter of the 3/8" BSF (9.53mm) OEM plug.
He's already bought an M12 x 1.25 magnetic plug plus dowty washer.
Our plan is to, very carefully, do it in-situ.
First push some grease up the hole then slowly drill out - pulling out many times to clear swarf.
The hard part will be keeping the drill bit square to the sloping rear of the gearbox.
When drilled, flush out with kerosene.
More grease then tap the hole.
Again staying square - using a 300mm extended tap wrench.
Another flush with kerosene and we should be good to go.

Thoughts? (nice ones please!)

I would first try to run a greased taper tap in the hole to see if I could cut the threads without drilling. That way getting the drill straight is eliminated and a taper tap in a hole with some threads will hopefully start straight. If is advances OK, forge ahead; if not, you can still follow your plan - just be careful that the tap is actually working. BTW, IMHO, you grease the tap, don't push grease in - the chips will stick in the grease in the tap hollows. I've done this on a couple of drain plug holes in aluminum (not Norton) and it worked fine.

You may need to switch to a bottoming tap once part way in - not sure of the clearance at that point for a taper tap (don't remember).

gortnipper · Dec 1, 2021

texasSlick said:
I say your method, post #1, and #8 are well thought out and should work out fine.
I fail to see how a Timesert offers any benefit .... you still have the chip and swarf capture and removal concern, and drilling square to the original hole problem, although in the latter regard, I think the drill will naturally align with the original hole.

I would go with your first method .... drill and tap to a larger standard size. Starting the tap square, IMO, is your major problem.

Slick

I think the advantage with a Timesert is that it gives a more flush surface of the hole than a helicoil, so may be less prone to leak.

t ingermanson · Dec 1, 2021

I imagine the back of the handle is drilled for one of those pointy spring loaded tap guides, so find out where that hole would be in-situ by using a tap of the existing thread in the existing hole, using the same handle. Mark that spot in space with a strip of tape or string dangling from the bike, a square from the bench, etc, etc. It doesn't have to be that accurate, but a general 1"sq area would be better than eyeballing. Drill out the hole as square as you can, put the new tap in the handle and match the back of the handle to your mark in space.

If you really want to go bananas, figure out how to mount one of those pointy tap guides in space. You could even get a square shank drill bit and use that in the tap handle!

Or just drink a couple beers and go for it.

robs ss · Dec 2, 2021

Just had an additional brain fart - if I use a countersinking bit immediately after drilling, there will be a good "flat" around the hole that the sealing washer will mate with.
Not making excuses for poor workmanship but maybe good insurance for a good seal.

L.A.B. · Dec 2, 2021

I tapped my Mk3's gearbox drain plug standard M12 x 1.75 with the gearbox in situ some years ago (maybe before I joined accessnorton I can't remember or if I drilled first or just ran a taper tap into the stripped hole) but it hasn't leaked.

baz · Dec 2, 2021

robs ss said:
Just had an additional brain fart - if I use a countersinking bit immediately after drilling, there will be a good "flat" around the hole that the sealing washer will mate with.
Not making excuses for poor workmanship but maybe good insurance for a good seal.

Use a spot facer and you will definitely have a flat surface

Stripped Gearbox Drain Hole

robs ss

gortnipper

Craig

SteveMinning

baz

BERT

gortnipper

robs ss

robs ss

texasSlick

htown16

texasSlick

robs ss

robs ss

marshg246

gortnipper

t ingermanson

robs ss

L.A.B.

Moderator

baz

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