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-   -   Stripped glow plug threads. ouch... (http://www.rcuniverse.com/forum/glow-engines-114/8386350-stripped-glow-plug-threads-ouch.html)

zaboaa 01-21-2009 11:18 PM

Stripped glow plug threads. ouch...
Fairly new Saito 125, the glow plug threads stripped rather easy. Of course after screwing with it it's now a smooth hole. Is there anything that can be done besides spending $119.00 for a new cylinder head?


w8ye 01-21-2009 11:20 PM

RE: Stripped glow plug threads. ouch...
Send it to Clarence Lee and he will install a heli coil. Send just the head $20 post paid

C.F. Lee
10112 Woodward Ave.
Sunland, CA. 91040
818 352 3766

Piet Le Roux 01-22-2009 02:00 AM

RE: Stripped glow plug threads. ouch...
I did the same to my OS91FX and would like to know the best way to fix it. My normal way of fixing striped thread is to drill it bigger, tap new thread and then make a bolt to screw into it, using retaining fluid to secure it then drill a new hole and tap it to the original thread. What it the thread size that is used for glow plugs? Is heli coil available for any thread size and would it be as good as taped thread?

XJet 01-22-2009 02:32 AM

RE: Stripped glow plug threads. ouch...
You can helicoil the head but as w8ye suggested, it's better and cheaper to send it to Clarence.

Just a tip if you use a GlowDevil non-OS 4-stroke plug you'll have less chance of stripping the threads because they're threaded right to the bottom so there is more thread in contact with the head. (OS and most other plugs have a significant "unthreaded" portion near the tip of the plug so have fewer threads engaged with the head.

Ultimately the golden rule for any engine is only insert the plug with your fingers not the plug wrench and make sure it's turning freely with finger pressure before screwing it home. Even then, only use the minimum amount of torque necessary to stop it coming lose. And check the plug for tightness every now and then. If the plug works lose in a Saito then the fretting that follows will quite often wreck the threads.

w8ye 01-22-2009 02:37 AM

RE: Stripped glow plug threads. ouch...
The thread size is 1/4" X 32 tpi N.S.F.

You can special order a Heli Coil set that is 1/4" NFS or 32 tpi for around $85 and fix your own plug threads from McMaster-Carr

Some engines have a cast in brass insert for the plugs but the Saito 125 does not

The threads on a OS are sort of rough also

rcdude7 01-22-2009 06:13 AM

RE: Stripped glow plug threads. ouch...


ORIGINAL: zaboaa

Fairly new Saito 125, the glow plug threads stripped rather easy. Of course after screwing with it it's now a smooth hole. Is there anything that can be done besides spending $119.00 for a new cylinder head?


Your best bet is to send it to Clarence Lee, he is the master:D

Piet Le Roux 01-22-2009 06:38 AM

RE: Stripped glow plug threads. ouch...
Thanks for all the info and recommendations but with the cost of shipping from South Africa and back its not worth it. I think I will screw in a F- Plug without the brass washer and secured it with retaining compound. It should last until my new head arrives.

w8ye 01-22-2009 12:16 PM

RE: Stripped glow plug threads. ouch...
Although I have not seen one to be sure, people have tried to lead me to believe that the new Saito 125 cylinders have larger fins than the old ones.

ace4-40 01-22-2009 03:21 PM

RE: Stripped glow plug threads. ouch...
The Saito does not have a separate head. You have to replace the whole cylinder. It is still much cheaper to send it to C. F. Lee as another member stated. Even with shipping it would be cheaper. Shipping something that small is not that expensive. Even to S Africa. Email or write to Clarence for price. I just checked. It would cost Lee $10.00 first class postage to SA or $13.00 Priority in a small flat rate box. Add that to the $20 repair fee and you are still well under the cost of a new cylinder or the helicoil kit.

XJet 01-22-2009 04:28 PM

RE: Stripped glow plug threads. ouch...
If you know someone with a half-decent workshop, they ought to be able to re-tap the head and machine up a brass/bronze insert that will thread in in a similar fashion to a helicoil. I've done this before with mufflers that have had the threads stripped out of them. Works great and only takes an hour or so to do.

ace4-40 01-22-2009 06:44 PM

RE: Stripped glow plug threads. ouch...
You can't use a brass insert. The plug hole is too close the the valve seat and will damage the seat if a bigger hole is drilled for an insert. Clarence Lee tried to do this on a Saito 91 I sent him. The seat was damaged when he tried to repair the cylinder and the cylinder had to be replaced. The seat is just the aluminum head and not brass like the original Saitos. Soft and easy to damage.

rick555 07-02-2009 02:20 PM

RE: Stripped glow plug threads. ouch...
Just for your information there is a TIME-SERT thread repair kit
for the 1/4-32 spark plug kit p/n 0142SP
The kit is about $89.00 and would probably be useful for RC engine rebuilders.

Rick :D

gadix 01-16-2012 04:39 AM

RE: Stripped glow plug threads. ouch...
1 Attachment(s)

had the same problem, due to bad insert of the plug, i have strap the thread tapping on the saito.

couldn't fix with heli-coil so i had to add a base insert of M9 and thread it to 1/4 32 for the Glow plug.

today it was done by the CNC/Milling guy, glued with High Temp Locktite

it out for dry for a few days then i will put it all back to place and do cam valves settings with a filler gauge to see that all is good.

see pics :)


earlwb 01-16-2012 06:14 AM

RE: Stripped glow plug threads. ouch...
One has to be careful and examine the glow plug hole carefully before they screw in a glow plug. The hole may not be threaded all the way through. OS started the practice with their 4 stroke glow plugs by making them, sort of, 3/4 length, in between a long and a short length glowplug. Plus they machine the tip of the glow plug to be smaller in diameter too. Thus if you use a regular long glow plug it will but up against the shoulder on the inside of the glow plug hole in the head of the engine. Many modellers have stripped out their engine's glow plug holes because of it.

For those engines where the valves are too close, you have to machine a shallower hole and not thread it all the way through and install a shorter insert and or thread it for the glow plug.

OS has been doing this on their 4 stroke engines for a long time too. Here is a example on a old OS open rocker .60 engine. You can see the shoulder down inside at the bottom of the glow plug hole here in the pic.

OS, FOX and some other glow plug manufacturers make special glow plugs to fit into these glow plug holes. You need to use this type of glow plug in these engines with that kind of a hole in them.

jessiej 01-16-2012 04:19 PM

RE: Stripped glow plug threads. ouch...
The previously mentioned Mr. Lee is the answer of choice for most. Excellent work, fair price.


Ernie Misner 01-16-2012 11:07 PM

RE: Stripped glow plug threads. ouch...
Another way to avoid stripping the threads is to get a 1/4 - 32 die and ALWAYS run a new glow plug through the die before installing in your engine. Why? As Clarence Lee pointed out in an article, the glow plug threads are simply rolled and not precision cut.. So when you install a new plug there will be tight spots in the threads...... then if you have a hot or lean run those tight spots are spot welded together and guess what happens when you go to unscrew the plug? Right, aluminum comes out with it. So get that die (mine was $9.50 at the local screw products store but did have to wait 2 weeks for it to come in...) and ALWAYS run your plugs through it before installing. I had a whole bunch of plugs, new and used, and when I ran then through the die, every single one of them needed cleaning up, no kidding. Tower has that die I believe bit for a little more money.

Ernie Misner

eatherbreather 02-08-2012 12:15 PM

RE: Stripped glow plug threads. ouch...
I've repaired a few engines using inserts like heli-coil with excellent results.  I have noticed that Saito engines do seem to be more prone to glow thread damage due I think to the lack of brass insert but also because of the location and depth of the plug in the engine. I think because of this it is far too tempting for some people to simply put the plug into their socket/tube spanner, place this over the glow plug hole and start cranking. Resistance is met and this is assumed to be the plug tightening. A final nip see the the thread destroyed. I've seen this done to r/c glow powered model cars and helis too (no I haven't done this myself).  Probably anything where the plug is recessed is at increased risk of this.

Here's what I like to do - simply use a 4" - 6" length of silicone or rubber type fuel tubing that with be a firm slip fit over the post on the top of the glow plug and then "screw" the plug in with this. There is little chance of dropping the plug into grass, cowlings or anywhere else and no chance of ever stripping the thread. When the plug is fully seated, then and only then do I get out my plug socket/spanner/whathaveyou and tighten the plug properly. :D

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