Repairing damaged thread

[John McCullaParticipant @johnmcculla]

Last night I was changing the engine oil on my Skoda Octavia. As I screwed the drain out, it was tight the whole way. ‘Not a good sign’, I thought. And I was right. The bung had some of the aluminium threads from the sump stuck between it’s threads, and looking at the threads in the sump it was clear that they were damaged, presumably due to the last person over tightening it.

Now this was at 9pm last night, and I needed the car going again, so I picked out all the loose threads that I could until the bung at least would thread in by hand. I then nipped it up as much as I dared which wasn’t very much at all as it almost immediately had that feel of a bolt about to strip the threads. Whilst I’m a little ashamed to admit it, I then daubed some CT1 (Construction Adhesive/Sealant) around the bung in the hope that it would stop the bung vibrating loose and falling out whilst driving.

Now obviously this needs repaired properly. Originally I was going to use a Time Sert as we use them in work and I think they’re very good, however to buy the proper kit (M14x1.5) is, according to this link anyway, £325. Now I could probably buy it for less through work off Wurth direct, but it’s still going to be expensive. Sealey do a kit which is much more reasonably priced, where you get an M15x1.5 tap and some bungs to suit, and you simply tap the hole again to the bigger size.

Now here’s my question, because both the original thread and the new thread are the same thread pitch, can you simply start the M15 taper tap in what’s left of the M14 thread and tap it? There certainly appear to be tutorials on YouTube showing that. However Google tells me the correct drill size for an M15x1.5 tap is 13.5mm, so does this mean the new threads would already be weak?

I would appreciate your wisdom!

[Andrew CrowParticipant @andrewcrow91475]

As the core size for the original 14 x 1.5 thread was 12.5mm and assuming the thread wasn’t completely stripped you should still finish up with better than what you have now. Just make sure you put plenty of tapping paste or grease on the tap to catch the swarf.

[KenLParticipant @kenl]

Could you open the hole to 15mm, tap it 3/8 BSP and use a tapered iron stop end wrapped with PTFE tape as your new sump plug?

[noel shelleyParticipant @noelshelley55608]

Ken L has beat me to it ! BUT use parrallel thread and a bonded seal ! Tapers in alloy can split the casting, a common fault in hydraulic fittings and castings. If your near me N Norfolk I can lend you a set of taps, and give you a bonded seal, may even have a plug as well.  Noel.

[mgnbukParticipant @mgnbuk]

Or you could go a bit bigger if there is enough metal on the drain plug boss. M16 x 1.5 is a standard electrical equipment thread so taps are readliy available & so are drain plugs in that thread. Tapping size is 14.5mm.

Nigel B.

[Pete RimmerParticipant @peterimmer30576]

M14 Helicoil/recoil will do it, I’ve done several. Mind you, if you can get a M15 tap and a M15 plug then that would work just fine as a chaser too.

[Neil AParticipant @neila]

If you intend using a Helicoil don’t fit a Dowty type seal, the bonded rubber falls inside the hole of the thread. Use a plain soft aluminium washer instead that will sit outside the hole and seal on the face.

The M15 x 1.5 tap might be alright, all depends on how much depth of the original thread is left. I notice that Sealy also make a M17 x 1.5 drain plug set. Difficult decision to make. Don’t be in too much of a hurry to decide.

Neil

[John McCullaParticipant @johnmcculla]

Thanks gents,

Some great ideas there and lots to think about.

Thanks for your kind offer Noel, but I think I’m a bit too far away here in Northern Ireland.

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