Thread repair in cylinder head, yes or no - RC Groups
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Nov 09, 2017, 01:26 PM
Hellcat 113
Question

Thread repair in cylinder head, yes or no


I recently got a very used O.S. 46 SF engine. The glow plug threads are totally gone. I cannot find a replacement head. Are the thread coils (helicoils) worth the trouble, or should I just get another engine?
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Nov 09, 2017, 02:10 PM
Registered User
On a 2 stroke, with no valves in the way, it is pretty easy to press in an insert, and drill and tap it . It will need to be in a lathe to clean up nice and flush, but not necessarily if the right length is used in the first place. I would prepare the hole first, and make the brass plug after to get a .001" press fit. I guess it may be better to have the thread or at least the hole in first.
Nov 09, 2017, 02:10 PM
the great Gassif´er
I have now done several helicoil replacements.
If you go about it a bit careful, yes, it is absolutely worth the trouble and the head will be just as good as new. Better even, since it is extremely unlikely that you will strip a helicoil.

But you need to be a bit careful, it is easy to end up with a thread that won't let the plug seal properly.

I got the best results when I re-ground the landing surface of the plug in a lathe (using a Dremel attached to the toolbit holder) after rethreading.

AFAIK the SF heads are round, so chucking it in the lathe (if you have one) is not a problem.

But even without that, it is possible to achieve good results.
Nov 09, 2017, 02:19 PM
Methanol, The Cool Fuel !
Gary Cee's Avatar
I'll do it for you, free of charge, just take care of the shipping.
BTW, I spot-face the mating surface with a piloted counterbore tool. This squares it up for a perfect seal.
Nov 09, 2017, 03:03 PM
Registered User
Yes it's a worthwhile engine to fix but , honestly , if you have to ask about thread repair ,just send it to Gary and take him up on his offer it will be easier quicker and better in the long run .
Nov 09, 2017, 07:01 PM
Hope to get out of life alive
kenh3497's Avatar
Quote:
Originally Posted by Gary Cee
I'll do it for you, free of charge, just take care of the shipping.
BTW, I spot-face the mating surface with a piloted counterbore tool. This squares it up for a perfect seal.
There is an offer that would be hard to refuse.

Quote:
Originally Posted by Geoff Potter
Yes it's a worthwhile engine to fix but , honestly , if you have to ask about thread repair ,just send it to Gary and take him up on his offer it will be easier quicker and better in the long run .

Best advise so far in this thread.

Ken
Nov 10, 2017, 07:14 AM
Hellcat 113

thread repair


Thank you all for the help. I have already found a machinist willing to put in the coil. I will update you on the result.
Nov 10, 2017, 08:29 AM
Registered User
Cougar429's Avatar
I never personally liked Heli-Coils for this app. The temp and physical stresses tend to wear the contact points between the steel wire used in the coil and the base alloy. Eventually loosens them up with the risk of plug insert threading the coil down into the chamber.

We did use them in aerospace, but the military app was not designed to be taken apart often, if at all, and also liquid cooled.

Was introduced to TimeSerts back in the 90's when we were being flogged with trying to repair the result of the manufacturers latest campaign. Try to imagine what pulling a plug was like after their "100 thousand miles before your first tune-up". That and bad QC often had the alloy threads come out with the plug removal, (Some, like the GM Quad 4 were a guarantee to require repair).

TimeSerts are an internally and externally threaded sleeve that locks in during installation and they do offer them for 1/4-32 in different lengths. I have looked into grabbing one for my own use, but the installation kit was where the most expense is incurred and unless doing these often one or two not worth the over $100 invested.
Nov 10, 2017, 10:03 AM
Registered User
The PIPE's Avatar

One could do FIVE mills with a Time-Sert kit for 'em...


Dear Cougar429:

The PIPE Here...I'd seen the Time-Sert thread repair system mentioned elsewhere at RCG, but I don't QUITE remember exactly where that was...

...anyway, as I know my OS FS-90 rear-camshaft four-stroker was from an era when the aluminum alloys in OS's mills was apparently prone to glowplug threads stripping without any warning, I'd have someone...BEFORE I ever fire up that beloved old mill of mine ever again (it DOES have a full replacement set of stainless crankshaft bearings (2) and regular steel camshaft bearings (3) professionally installed in it) professionally upgrade those glowplug threads in its cylinder head with a Time-Sert 1/4x32 thread repair insert placed in there, to be certain it can NEVER strip out at any time.

The link to the "inch-size" Time-Serts is HERE...click on the 1/4x32 thread item, and a small window pops up about getting five Time-Serts in the kit, hence the ability to do upwards of five engines with one kit.

Yours Sincerely,

The PIPE....!!


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