Bu99ered thread - options?

[TheMegaMan]

I came across a bit of a problem yesterday, refitting the clutch slave cylinder. It seems the lower fixing hole in the bell housing is stripped. Very little resistance when trying to tighten the 5/16 bolt, and no sign of any metal fragments on the thread, so it's possible it was stripped a while ago and a previous owner just ignored it.

So as I see it, I have a few options:
1. The correct solution is to helicoil it, I assume. Can that be done in situ? I would really prefer not to have to remove the gearbox, but it might be difficult to get at to tap it properly where it is.
2. Fit a longer bolt. I think the hole is deep enough to take a 1 1/2" bolt instead of the 1" bolt specified. If the hope is fully tapped, I might get away with that.
3. Drill the hole out through the back of the casing and and fit a much longer bolt with a nut on the outside,

Thoughts? Any other suggestions?

I guess I'm expecting everyone to say option 1 and don't even consider anything else, but I've never fitted a helicoil before, so am a bit anxious I'll screw it up. I'm not the best person when it comes to ensuring taps are inline, particularly in an awkward position.

Thanks.

 

[Rodeo Joe]

I'd try a longer bolt first, just had to do that on my thermostat housing where one of the bolts sheared off. Tried to drill it out and the drill bit broke!!!
Managed to get a bit of it out but had gone deep enough to get a tiny bit of thread to catch at the very bottom. At least it was a lot easier to get to than the clutch slave.

 

[rob1miles]

I have a basic set of UNF taps, very handy. Get a blind / square ended tap and cut the thread right to the bottom. The tap will stay square and you can turn it with an adjustable spanner. One way to make the best of the remaining thread is to locktite a stud in.

 

[TheMegaMan]

Thanks for the replies.
I don't actually have any taps, as I've got away without needing any so far.
It's a UNC thread. And @rob1miles raises a very good point about the thread not being useable right to the bottom of the hole, so I guess I need something like https://www.ebay.co.uk/itm/Bottomin...hash=item3aaa57d3cd:m:my1wYXfMgUA-Iuw-8-xYyYg ? UNC us 18tpi for 5/16", isn't it?
I'll have a go with a longer bolt first, and 'feel' how much thread I seem to have, and whether I need to tap it out fully.
If that fails, I'll have a go with a helicoil. I think I'll need something like https://www.ebay.co.uk/itm/5-16-UNC...426254?hash=item4ae577fe4e:g:XzUAAOSwICpaAdkP or is it worth paying the extra for a 'v-coil' version? https://www.ebay.co.uk/itm/UNC-V-Co...hash=item1a3d85d9e6:m:m0dGNC0XcoJSqSM4PCpsylQ
As a matter of interest, how do these get away with a single tap, whereas I generally see three different stages of taps in a normal set? Does this increase the chance of not getting the thread straight?
I don't have a tap holder, and given the confined space I'll be in (if I avoid taking the gearbox out), would something like Amazon product help (the ratchet might be useful), or are they impossible to keep straight?
I'll consider drilling through the back of the casing as a last resort!

 

[Rodeo Joe]

Thanks for the replies.
I don't actually have any taps, as I've got away without needing any so far.
It's a UNC thread. And @rob1miles raises a very good point about the thread not being useable right to the bottom of the hole, so I guess I need something like https://www.ebay.co.uk/itm/Bottomin...hash=item3aaa57d3cd:m:my1wYXfMgUA-Iuw-8-xYyYg ? UNC us 18tpi for 5/16", isn't it?
I'll have a go with a longer bolt first, and 'feel' how much thread I seem to have, and whether I need to tap it out fully.
If that fails, I'll have a go with a helicoil. I think I'll need something like https://www.ebay.co.uk/itm/5-16-UNC...426254?hash=item4ae577fe4e:g:XzUAAOSwICpaAdkP or is it worth paying the extra for a 'v-coil' version? https://www.ebay.co.uk/itm/UNC-V-Co...hash=item1a3d85d9e6:m:m0dGNC0XcoJSqSM4PCpsylQ
As a matter of interest, how do these get away with a single tap, whereas I generally see three different stages of taps in a normal set? Does this increase the chance of not getting the thread straight?
I don't have a tap holder, and given the confined space I'll be in (if I avoid taking the gearbox out), would something like Amazon product help (the ratchet might be useful), or are they impossible to keep straight?
I'll consider drilling through the back of the casing as a last resort!

To be honest if it was me I'd probably have drilled straight through in the first place.......not necessarily because I wanted to but generally I can be heavy handed so would have happened by default . Could just bolt it on then, function over form has always been something I've gone by (aka might not fly but will flap about a bit).

 

[lynall]

The longer bolt might be to long and if the thread feels dicky winding it in and you think it might not tighten nicely, remove the bolt, cut the head off and fit the remaining part as a stud with some loctite, let it set, fit slave job done.
Fine threads in alloy are always doomed ot fail.

 

[TheMegaMan]

To be honest if it was me I'd probably have drilled straight through in the first place.......not necessarily because I wanted to but generally I can be heavy handed so would have happened by default . Could just bolt it on then, function over form has always been something I've gone by (aka might not fly but will flap about a bit).

I'm slightly concerned that the hole will come out too close to the casting for the body of the cavity for a not to fit properly. I'd really rather try and avoid that solution if I can, but it is a last resort...

 

[tottot]

As per Lynall's post but use chemical metal to fix the stud.

 

[TheMegaMan]

As per Lynall's post but use chemical metal to fix the stud.

Yeah, If there is a thread there but not very much of it, I'll give that a try. I have some JB Weld somewhere, I think, and I've got some longer 5/16 UNC bolts on order so that should be doable. Thanks for the suggestion.

 

[flat]

Could fit a timesert insert

 

[TheMegaMan]

Could fit a timesert insert

Hadn't heard of those. Are they significantly better than helicoils? They look more expensive...and indeed, just found a price...£155 quid for a single-size kit. Wow.

 

[flat]

Hadn't heard of those. Are they significantly better than helicoils? They look more expensive...and indeed, just found a price...£155 quid for a single-size kit. Wow.

Yeah they are not cheap, significantly superior though

 

[Blackburn]

Can you get at the location if you take the wing off? better than taking the gearbox out.
I would helicoil if you can get access.

 

[TheMegaMan]

Can you get at the location if you take the wing off? better than taking the gearbox out.
I would helicoil if you can get access.

I've got got any of the bodywork back on yet, so it's possible to get access from the front, back and underneath! The problem I was seeing was that a T-bar tap holder wouldn't have clearance due to the engine/flywheel, so unless the little ratchet-style holder will get in there, there won't be any room to actually run a tap.

 

[Blackburn]

When fitting my slave cylinder I can fit the bolts with a 1/4" extended drive can you not use something similiar for the tap?
Bolt holes are angle slightly toward the chassis
ie not // to engine.

 

[rob1miles]

If you go down the "tap is as far as possible" path then one trick is to screw a longer bolt in as far as it will go, then measure how much is left. Cut off exactly that then add a washer, you will be getting the most out of the remaining thread. Re taps and keeping straight there's all sorts of tricks, wedge into a 1/4 drive extension the wrong way round, use a spanner (my favourite), use a drill chuck (on its own), you can even get confined space tap wrenches:

 

[TheMegaMan]

Good suggestions. Ah, so the taps have a 1/4" fitting, do they? That's useful to know. An extension would certainly help me get it square, and put the T-bar out where there's a bit more space, I think. And thanks for that photo @Blackburn - that really shows that it's way off parallel with the engine. Hopefully I'd have noticed, but useful to be fully aware in advance.

 

[Birdbrain]

I’ve got exactly same problem with my series 3 lightweight. The first few threads on lower hole stripped. If you screw the set screw in without the slave in place there’s enough internal thread so I’ve gone for the 1.5” option that I’m fitting tomorrow!

Also be very aware that the parts manual says the correct size is 5/16 “ UNC. It’s not, they’re 5/16 UNF

 

[TheMegaMan]

It's my lower one that's stripped. too. Although both screws that I took out were definitely UNC (18 tpi) and the top one has gone in just fine. Curious that yours would be UNF.
One thing I noted was that the LR Workshop site says the screws are 5/8" instead of 5/16", but the thread type was correct.

 

[Blackburn]

Most threads in aluminium are coarse to stop them stripping out.
Often a fine nut on one end of stud and coarse thread on other transfer case cover as one example.