Disaster strikes again..

[bitsobrits]

And when you finally have the problem resolved, use ARP stainless studs for the corrosion prone applications like the thermostat housing and exhaust studs. Then no worries ever again.

[alan.barker]

Maybe you can use the thermostat housing held in place by the good screw as a drilling guide
Once you have a center started in the broken screw shank go down to smaller drill dia less than tapping size for 5/16" unc. That means less than 6.6mm dia which is the tapping drill dia.
Alan

[JonB]

Alan, I thought about that but as it is aluminium isn't there a danger it will be damaged? The drill bit is going to push up against the side of it.

[2cams70]

All I can say is think very carefully. Time taken stuffing around with drilling by hand versus time taken to remove the head. The risk and the value of the head.
I was in a similar situation and the cost to have a broken bolt removed professionally with absolutely no damage done to the existing threads was $20. It was a no brainer.

[JonB]

Well I was doing OK with a cobolt drill bit. Broke through the underside of the bolt then it snapped off.

Now trying a clamp.

Temporary test with a new gasket, no silicone.

Engine at 85 degrees C and it is holding.... fan is on... no leaks.. this is a welding clamp with a certain amount of flexibility in the jaws. Something made of angle iron and threaded rods would be stronger. I may just try it. Will be ok if it doesn't let go on the road. (Carry a gallon of water in the boot?)

[alan.barker]

Nice thinking,
Jesus you don't have much luck.
Alan

[Elanman99]

Well I was doing OK with a cobolt drill bit. Broke through the underside of the bolt then it snapped off.

Now trying a clamp.

Engine at 85 degrees C and it is holding.... fan is on... no leaks.. this is a welding clamp with a certain amount of flexibility in the jaws. Something made of angle iron and threaded rods would be stronger. I may just try it. Will be ok if it doesn't let go on the road. (Carry a gallon of water in the boot?)

Because of it overall length that clamp shown is going to be under a lot of stress as the engine vibrates and 'rocks' on it mounts. It might be worth trying to tap what is left of the steel stud to take a smaller diameter temporary fixing. As your drill (what diameter?) has broken through the end of the stud you might be able to drift it deeper into teh small gap in the original hole.

I would be reluctant to remove the head until I had tried all other options. With a little engineering It would be possible to make a jig that uses the one good fixing and has a large diameter spigot that engages with the machined recess that the thermostat locates in. Once you have a solid base a motorised drill of some sort could be bolted to it, maybe with on a vertical post so it like a pillar drill.

There are chemical methods of stud removal that will work with steel in ali, it would be slow though. What we really need is a portable spark eroder.

[JonB]

The welding clamp is just to test the idea. I wouldn't drive it like that, but I think something with angle iron and long bolts might do the trick for now.

[2cams70]

I don't know if it's just me but I've never had much success with cobalt drills. I once needed to drill quite a thick piece of stainless steel and both HSS and cobolt drills were useless. What really worked well however was a carbide tipped masonary drill. The tip needed to be ground sharp to suit metal but after doing that it worked like a charm and sliced through it. It was much cheaper than a proper carbide drill and I think even better. Because only the brazed in tip is carbide the rest of the drill isn't brittle and therefore less likely to break.

[JonB]

I?ve just perpetrated what must be the most egregious bodge in the history of the twin cam engine. Or not, you decide.

I made a clamp for the thermostat housing. It fits over the whole casting on the corner where the dead bolt is. A bolt with a second bolt head welded to it passes through the clamp. There is a captive nut underneath. The bolts engage with the thermostat cover hole and press it down onto the cylinder head when you tighten the top bolt. Err... hard to explain but here are some pictures.

As with the welding clamp experiment, it seems to be holding.

[Bigbaldybloke]

As long as it holds only you and us will ever know unless you open the bonnet!
Should be gone till you work out a permanent solution

[JonB]

I took it for a test run this afternoon. Very hot. Somehow, the sun just gets in and roasts you. PO did say I'd cook in the Summer, but I thought he meant "...because the heater won't turn off". Well I fixed that, but it made little difference. I suppose if it hadn't been out on the drive all afternoon in the sun things would've been better. Ha! But I digress.

It being the middle of a "heatwave" (in the U.K. that's one day of sun followed by torrential rain) the thermostat went up to 95C which is the highest I've seen it. My blobbily welded bodge held! So I'm good to take daughter #2 to the prom at the end of June, then it's "head off" time and off to an engineering shop to get it sorted properly.

Thanks as ever to all contributors for the good advice!

[69S4]

These "creative engineering solutions" have a habit of lasting longer than you think if they work and cause no further problems. I remember reading about someone who had the head gasket blow on his old side valve motorcycle while he was out on the road somewhere. As a get home bodge he put a screw jack between the cylinder head and the top frame rail and used it to push the head down. It was still there decades later ...

Not that your idea will have to last that long Jon

[MarkDa]

And it stops that crack in the 'head' opening up any more - Two jobs with one bodge!
Glad the car is running and you're out and about.

[USA64]

I would say it's not "the most egregious bodge in the history of the twin cam engine" until you sell it on without a proper fix.