Repairing KT100 exhaust stud threads

Pete_Muller

Moderator
I thought I'd post a few photos of how I fix KT100 exhaust stud threads (using the same inserts I make for repairing cylinder head stud threads).

A couple of photos of the front and back side of a simple fixture I made (exhaust face is inclined 15 degrees on a KT100).


1.JPG



2.jpeg
 
For those that might have interest in the inserts I use....

I originally made some on my own probably around 35 years ago -- the first time I worked on an engine that had pulled a head stud on the exhaust side of a KT100 cylinder. My original design (which has never changed) was/is: .600" long, 7/16 x 14 OD thread, and of course M8 x 1.25mm ID thread.

The reason for the the 7/16 thread is three-fold:

1) It keeps the OD of the insert as small as possible/practical, which prevents it from cutting too much into the cast-in-place liner on a KT100,

2) It allows the use of a ⅜" end mill to "tap drill" for the 7/16 x 14. A 4-flute end mill is very stiff compared to a drill, and since the existing hole will at least be partially butchered from being stripped, the end miill will cut much straighter than a drill, and it also will deflect very little if it ends up touching the cast iron.

3) The 3/8" tap drill (or end mill) will *just* clean up a stripped M8 heli-coil. So if the cylinder had been fixed once that way, but the heli-coil didn't last long (not unusual), then the ⅜" diameter tap drill (end mill) will just clean up the major diameter of that stripped heli-coil thread.

Not long after I started making those inserts (on my engine lathe), I had a buddy with a small gang-tooled bar-feed CNC lathe make them for me. I think my first order back then was 1000 inserts. They were single-pointed on both the external and internal threads. I wanted the 7/16 x 14 to be right on the high-limit of pitch diameter (so they would fit quite tight on a standard tapped thread), and also wanted a very good fit for the studs so that was cut just a touch tight. I've never changed that approach.

I've had a web page online for close to 25 years that shows how to install the inserts in the top of the cylinder.
It can be found here: http://muller.net/tech/howto/insert/index.html

If you have a Bridgeport (or even a drill press), that can be done quite accurately by following those directions.

Over the years, i've installed those inserts in not only the top of the KT100 cylinder and the exhaust stud holes, but also in the base of a variety of engine cases.

insert.jpeg
 
Nice. I wish the dummy who sold me a used KT100 would have used those instead of the crap he used on the exhaust area and came off, other that being so oversized that even needed to work the header and of course messed the cylinder to the point once it came off I had to use another cylinder.
 
Yep.... almost impossible to find an off-the-shelf M8 insert that isn't either ½-13 or M12 on the OD. On the cylinder studs, it interferes not only with the head gasket recess, but cuts into the cast iron liner a good bit. And as you saw: on the exhaust stud repair, it ends up cutting into the gasket recess. :(

PM
 
For what it's worth, McMaster-Carr now sells an M8 insert (steel) that is 7/16 x 14 on the OD (same as the one I make). It's a bit shorter, at .400" long (vs. mine at .600") but would still be usable.

The part number is: 90259A230

Here is a screen capture of the CAD model of the McMaster part.

PM


90259A230.jpg
 
Just out of curiousity Pete what would be better for the kt application? Brass or steel in an aluminum head? Or wpuld they both do the job just fine? Makes me wish I practiced more with cnc in highschool metal shop. But I was best in class with the bridgeport and on a lathe lol.
 
I've always preferred brass in this situation, which is why I made/make them this way. CTE (coefficient of thermal expansion) of steel is about half that of aluminum (so a steel insert in aluminum will get "looser" is the parts get hotter), whereas brass is much closer to aluminum in CTE. Bottom line: there is less chance of a brass insert becoming "loose" (due to heat) in aluminum.

All you need is a Bridgeport to do any insert installations (whether exhaust face, cylinder/head stud holes, or bottom of cases). I've done the exhaust face inserts in my CNC mill (thread-milled the 7/16 thread for the inserts), but could just as easily have been done manually.

Fire away if you have questions.

Pete

PS: I have inserts if you need some.
 
The thermal expansion would be the base for my question. I know we use a steel insert in the animal heads but I was a bit curious as to why you used brass. Pretty cool. Learn something new everyday.
 
Also, should you ever need to get it back out through destructive means, the brass is much easier to deal with than a steel insert...with less danger to the surrounding AL. I have wondered about milling a hole and using that "durafix" or "alumafix" brazing-type rod to fill it, then remill and tap OEM size. Not sure if it needs a rough surface to make any kind of mechanical bond, or if molecular-level bonding occurs, or if it relies on porosity.

 
Ted,

An interesting approach, though I'd personally be uncomfortable putting a torch to a KT100 cylinder I'm trying to repair. :oops:. That could also be an issue since the cast iron liner intrudes slightly into that space -- typically about ¼ of the hole (once it's drilled to the tap drill size for an insert) ends up being cast iron, so not sure any sort of welded aluminum repair would bond in that area.

When it comes to cases, I've seen numerous examples where people attempted to repair something with a TIG welder, and the cases warped so bad they were unusable (deformed so much, they could not be machined back to a usable dimension).

fwiw, I have never once in 30 years had one of the brass inserts strip or fail in any way. :)

Pete
 
Rainman and everyone else -- thank you, I really like helping people/racers.

I far prefer explaining *why* something works a certain way rather than just providing answers (as many of you have seen by my often lengthy answers). Bob's is such a great hangout for that, and I have definitely enjoyed learning about LTO racing here (and continue to learn things all the time).

PM
 
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