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Stripped plug thread

how do you fix it?

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shades15/07/2008 15:02:00
66 forum posts

I have just come across a glow engine on which someone (definately not me by the way), has been a bit over zealous with the old plug spanner, and stripped the thread in head of his engine. The engine in question is in fact an HPI 21 truck engine, but we'll try not to hold that against him. The replacement engine is well over £100.00 and he has no money. Still at school in fact.

Normally of course you would just drill it out and tap up a size, but of corse a glow plug is a standard size. Is there a way of fixing this?


Ultymate15/07/2008 15:25:00
1703 forum posts
62 photos
His choices are heli-coiling it (if you know someone with the kit) or replacing the cylinder head (not the whole engine)
birdy15/07/2008 17:43:00
1423 forum posts
110 photos
You could try some mole renches.
shades15/07/2008 18:54:00
66 forum posts

Yes, I have heard of the heli-coiling thing, but didn't know whether it was possible to use this on such a small scale. I have already tried to find a new head but to no avail. Thanks.

I don't get how mole wrenches are going to fix a stripped thread, Birdy, or am I missing something.


Myron Beaumont15/07/2008 19:01:00
5797 forum posts
51 photos
You could try Devcon, failing finding such a small heli -coil .We have used it on cracked cylinder walls on large diesels (oil rig) with success .I'm talking about the  stuff used by the Navy not the stuff I've seen in your local DIY shop .Know any sailors ?
Simon Chaddock16/07/2008 01:05:00
5777 forum posts
3055 photos

Hi Shades

Helicoils are made in small sizes but you will also need the appropriate special tap to cut the new thread. It is larger diameter but the same thread pitch as the original. The helicoil is then scewed in with a special tool and it locks itself in place. The glow plug is screwed into the helicoil.

Neat and effective but I don't know if a helicoil & tap is made for a standard glow plug thread.

You could try a motor cycle repair shop. Stripping threads in aluminium bike engines is quite common so they usually have helicoil kits. 

Martin Harris16/07/2008 16:54:00
9502 forum posts
256 photos

The thread is 1/4 UNF but if you can't get it helicoiled you might be able to make a threaded bush from brass (similar expansion rate to aluminium, I believe, which is the reasoning behind ABC engines) if there's sufficient material around the thread to make it practical and lock it by staking with a centre punch.

 Getting it gas-tight will be the only problem but perhaps making it slightly oversized / tapered would help.  Insert it with a shouldered bolt or a bolt and locknut once you have the length / fit about right...

 Not much to lose by trying...

birdy16/07/2008 17:23:00
1423 forum posts
110 photos
Sorry, I was tweeting up the wrong tree.
birdy16/07/2008 17:24:00
1423 forum posts
110 photos
Sorry about the rubish joke.
Brian Parker16/07/2008 18:36:00
538 forum posts

 Are Glow Plug  threads  not 1 /4 X 32 TPI ?

david vine17/07/2008 09:56:00
6 forum posts

Hi shades

If this were my engine, I would drill and tap the hole in the head out to something like 10mm depending on how much meat around the stripped thread.

As suggested make a replacement   bush with a thin flange  and with 10mm thread on o/d and  new plug threaded hole ,then put bush in the freezer for a couple of hours after which carefully heat  up head in oven.

Screw bush into head quickly , use old plug to drive it in tight down to flange. Nearly forgot use one of the high temperature Loctites / flange sealants under the flange.

Try it, you wont be worse off than you are now!!!!!!!!!!!!!!!

Dave V

Mark Lubbock17/07/2008 11:06:00
313 forum posts
13 photos

Hi Brian is correct glow plugs are 1/4 x32tpi & is an old ME(Model Engineering) thread. I think it also approximates to a 1/4 UNEF not UNF.

You would be hard pressed to find a Helicoil in this thread, so the bush method described by David would be the best if you can't find a new head. All you need to do is find (say) a 3/8 or M8 brass bolt, drill & tap a plug thread down the middle, dril & tap the head to suit & trim the bolt to the correct depth & thin the bolt head to a thin flange-easy with a lathe, but possible with careful filing. If you or a mate has access to a lathe, a new head would be a simple job.

jaktan17/07/2008 11:27:00
34 forum posts
9 photos

do as I do and that is to put the head in a lathe bore and tap it to 10mm useing a first tap and dont take the tap right through and then run a 10mm die down a piece of brass rod heat up the head and loctite the rod and screw in as tight as possible and leave to cool cut of the rod as close as possible to the head place the head back in the lathe face off the rod and centre it and bore it 7/32 machine it flush and tap it out 1/4 32 tpi Iv' done that many times for the boys in the club and i aint had any complaints yet ive even done that to a cox 049 but not with a brass insert

Allan Jordan17/07/2008 18:19:00
495 forum posts
 Hi Shades, I'm with the " make and fit a bush brigade" Question is.. do you have access to the machine tools to carry it out ? I have no idea where you are located so why not send me a PM..perhaps I can help.
Matt17/07/2008 23:41:00
67 forum posts

Why don't you give these guys a call. I've never delt with the firm but I have used helicoils on aircraft...if its good enough for a Tornado it should be good enough for a model engine

shades18/07/2008 17:06:00
66 forum posts

"Oh, you guys"

Many many thanks for all your help and suggestions. Knowing the actual thread size and type is really usefull and gives  a good starting point. I don't actually own a lathe, but that doesn't mean I can't do it. Keep the suggestions coming.

Thanks again. Shades.

Watch you don't fall from that tree, Birdy. You're supposed to crash planes into trees, not perch in them.

birdy23/07/2008 12:48:00
1423 forum posts
110 photos
Chris Channon23/07/2008 21:55:00
57 forum posts
1 photos

Give Dave Turner a shout on the RCMF forum, he might be able to help.


Chris Channon08/08/2008 20:53:00
57 forum posts
1 photos

Did you contact Dave ? did you get is sorted?

Chris C

Garry Pollard09/08/2008 13:44:00
1091 forum posts

Doesnt Seem interested Chris. Another case of possible rudeness as has been discused in past by Timbo. On the other hand he might not come on the forum again or his computer could have blown up , who knows


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