12 MM EXHAUST STUD!?? - is this an acceptable repair?

[cooperxs650]

Hello,

1st time builder and encountering one adventure after the other.

I am in the process of tearing down a 78 special. I was taking the exhaust headers off, and, 3 of the studs were 10x1.25, while 1 stud was 12x1.25. The 12MM stud looked realatively new compared to the others, which are rust encrusted.

It looks like the PO must of broken the original stud and replaced it with a 12MM, then, drilled out the mating exhaust flange. The edge distance on the stud to the cylinder looks good as does the edge distance on the flange.

So, I have a couple of questions that I'm hoping some more experienced builders can answer.

1. Is this a common and accpeted repair method?
2. Is there a 12 x 10 stud available, so, I would have a 12 mm hole and a 10mm stud, and the flange would not need to be drilled and the nut would be the same as the other 3?
3. Is it worth changing, given the risk of really screwing things up, or, should I forget about it and move on?

Thanks

Cooperxs650

 

[jd750ace]

I can't tell you where to find one for metric sizes, but you need a Twinsert. It will size you back down to the proper stud size. I'm sure if it has edge distance, it will go fine, but if you want to change up the header you'll be screwed later. Also, brush non-flavored milk of magnesia on the exhaust nuts prior to installation to prevent siezing. Works on airplanes, works on Chevy pickups, works on lawn mowers, works on my bikes.

 

[HooliganCycles]

Are you planning on taking the motor apart at all? If so i would just remove the stud weld up the hole and drill and tap it to the correct size, if your not i dont see any reason why the different sized stud would matter at all

 

[cooperxs650]

Right now, I'm not planning on a rebuild for the motor. I did a compression test and got 153 / 150 psi, which, I think is pretty good. The motor ran well (appart from some backfire) and there was no smoke, white or blue, so, I'm thinking it doesn't need a rebuild. May be next winter it will be a project, but, right now, I'm cleaning up the frame, rewiring, PMA, fixing brakes etc.

 

[1974jh5]

I'd leave it be for right now if you plan to reuse the exhaust flange. At some point, you may want to either Twinsert it or look for a 'stepped' exhaust stud, those have one large and one small thread. VW aircooled motors have that problem all the time and there are 10 to 8 mm studs just for that.

IIRC Dorman has a whole bunch of them listed too, see if the local parts house will let you go through the buyers' guide to find a 12 to 10.

A quick Google: this site shows their P/N 044-02X as a 12 to 10 stepdown stud.
http://www.sor.com/sor/products2.tam?xax=16093&category.ctx=044-02&page#.ctx=044

 

[cooperxs650]

Well, I think I will leave if for now. If I do a rebuild in the future, I'll worry about it then.

I like the "step down stud" solution. I figured these things were available (I know they have them for aircraft applications, with Keensert locks or Rosan Ring Lock locks), but, I couldn't find a simple step down for an exhaust stud. I'll search a little longer.

Thanks

 

[cooperxs650]

A quick Google: this site shows their P/N 044-02X as a 12 to 10 stepdown stud.
http://www.sor.com/sor/products2.tam?xax=16093&category.ctx=044-02&page#.ctx=044[/QUOTE]

I checked that P/N with the distributor, and, unfortunately, its 37mm vs. the 43mm stud that is stock. After the gasket, flange and lock washer, there isn't much left for the acorn nut to engage on if you take away 6mm. I doubt it would work.

I'll keep searching and post my result!

 

[cooperxs650]

I got a step down stud made.