scottonthefen

scottonthefen

Well-Known Member
I recently went to change my overdrive oil for the first time on my 2A (it's been in bits since I bought it, I'm not neglectful :) )

The drain plug was loose and gunked up with some black silicone sealant. The overdrive was dry. The large bolt in the picture is this one. I can wiggle it into the casing by hand, it does not tighten.

I ordered the smaller shiny one by part number RTC7177 taken from the Fairey manual, but that doesn't fit at all. As far as my little finger tip will go into the casing there is no thread at all for it to catch.

Could someone with this kind of overdrive confirm what their drain plug looks like? Cheers.
 

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From memory I think mine does look like the brass one. Is it possible that the thread was stipped and someone put that bolt in with a nut on the inside? You could still drain the oil by sucking it out the top if that was the case.
 
Right, I'm back to this now.. :)

I've never used a helicoil before. When I buy a recoil kit, like this one: https://www.ebay.co.uk/itm/9-16-x12...176793?hash=item3fc0f45199:g:ugsAAOSwhspb-b8P - is 9/16 UNC in this case the size of the thread I'm tapping for the new thread insert or the size of thread the insert will accept? Say the Fairey Overdrive drain plug is 9/16 UNC, do I need a 9/16 UNC recoil kit, or a bigger one? I expect a recoil kit to specify two sizes, the outer and inner thread of the inserts..but they only seem to specify one size and that seems to be the size of the tap that comes with it.. o_O

Is there an advantage to using a recoil kit rather than just tapping a suitable metric thread and using another bolt? (I already have some metric taps.. but recoil kits look to be about £50)
 
The helicoil kit is a tap with a bigger dia but the same pitch, it cuts a new bigger thread. The helicoil then winds in and returns the thread to the orginal size. They are a good repair, widely used on aircraft. The helicoil is screwed in with a tool and a tag that you break off once its in. Another option if you have it on the bench is take it to a welders and get it welded up then redrill and tap.
 
If the housing is currently tapped 9/16 UNC but loose then. A 5/8 UNF might work as the major dia for the UNC is 14.288mm and tapping drill for 5/8 unf is 14.5mm but if you have a vernier calliper worth checking hole size





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Even better, cheers! It's not the original drain plug, someone's had a go at fixing it with something bigger already and now that's stripped. I'll see what size that is tomorrow and look up the next size tap drill to see what size I need in a new plug.
 
I would take a closer look, its odd for these to strip, check there is not some fault with the casting or that its all got too big for the orginal boss and now the threads are short. In the end it may be better to weld it over and drain via suction.
 
The good news is my O/D looks in good shape. I'm no expert but it looks rebuilt not long ago. I can't find any play in anything and the teeth on everything look good, no pitting or spiked edges at all. Seals look good too. So it is worth the effort to sort the drain plug.
The bad news is there's no thread left at all on the drain. The large bolt that was in it is completely loose, I can push it in and out easily. I think they were just hoping the black silicone would hold it in there..
The opening is almost 15mm at the top. I think the 14.05 tapping drill, to go for 5/8 UNC, would not completely clear out the old mess. Whether the newly tapped thread on top of that would actually be ok or if I'd have some gaps I'm not sure. To be sure it would be a 17mm tapping drill bit to go for a 3/4 UNC plug. If I go for 5/8 UNC and it doesn't work out, have I then blown my chance for a 3/4"? I love Land Rovers. :)
 
Some photos.. and I've realised since my last post the 17mm tapping drill for 3/4 UNC would clear out the 5/8 completely so there's no reason not to try 5/8 first.

Perhaps I should get a 14mm drill bit and see how well it clears things out. If it looks good I can order the 5/8 drain plug and tap, if not I can drill out to 17 and go from there. Is the tapping drill size essentially the "minor diameter", or will some metal be removed from the whole surface during tapping?

In the back of my mind I'm still wondering about helicoils. The only reason I can think of using those instead here would be if I wanted the drain plug to be a specific size and was not happy to simply go bigger.. ?
 

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What do people think to me drilling and tapping this with the O/D upside down and flushing through after? Worst case if I missed a shaving it would be a lot softer than anything it encountered inside? I don't know how involved stripping the case is yet.
 
Just realised I've left the clutch sleeve in the transfer box. Back to the garage to check the splines on that.. :D (edit: outside splines look good, nice smooth tops on them, none broken I can see. mainshaft nut only finger tight so I'll need the tool for that..)
 
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Plenty of metal left on hole if you want to go bigger , you can try with the 14mm but I don’t think you will end up with a clean hole.
Are you using a pillar drill to help get it square, drill slow.
I would do it upside down and see if you can pack some rag in the bottom to stop swarf moving around make sure you don’t drill into rag
I would fill the flutes with grease on the drill and the tap and clean it a couple of times as you go to collect the dwarf
I agree odd bit of ally shouldn’t cause a problem hopefuly none but you can swill it with a bit of paraffin might even get your little pinky in the hole!
you can still do the 3/4 if needed if 5/8 doesn’t work in the UNC
Glad the OD looks good proof of the pudding is when it runs
 
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