Stripped Threads in Flathead Exhaust/Block

[Minimichael]

Yep, it happened. My iron exhaust pipe slowly but surely chewed the threads clean out of the exhaust port in my little aluminum L-Head. I'm guessing to save the engine I'll have to either tap the port ever wider to handle the next size thread (3/4 npt?) Or maybe tap two side holes for a flanged exhaust pipe. The question remains whether there's enough material in the block for such tapping. And I'm so bummed now because I don't see a solution in sight...

I'd appreciate any advice or other words of experience you might have in this area.

It's a Briggs 6BS.

Thanks

 

[Mariah]

If you cut 1/2 the threads off the end of another pipe could you check if the remaining threads would be big enough to get a grip? If that works you might be able to reform the aluminum with antiseize compound on the area and working the pipe in and out to cold form the threads to mate up.

 

[Minimichael]

So like power tapping a squarer cut end of pipe further into the block. I hadn't thought of that. And I suppose I could even put a small one-hole flange at the base somewhere just to keep it from rattling out again.

 

[Mariah]

You want the threads to be smooth and shiny on the pipe so they push (form) the aluminum into a thread form. If you Google 'form tap' or 'form tapping' that will give you a heads up of the process.

 

[2old2care]

Does the pipe have any other support on the engine block, or is it just hanging on those threads ?
If not, that could be the cause of the problem - The weight of the steel pipe, constantly vibrating it's way loose.

 

[f4radar]

If that cutting the 1st 1/2 of pipe thread buys you enough grip you could replace that smallish lock nut with something meatier and maybe fabricate a “hold down” . You could come across from the carb bolt or someplace else and capture your pipe lock nut against the block. Kind of like an old distributor hold down.

 

[creia]

Does the pipe have any other support on the engine block, or is it just hanging on those threads ?
If not, that could be the cause of the problem - The weight of the steel pipe, constantly vibrating it's way loose.

What they said^^^^. A big old pipe like that should have had a header brace. Install one after you repair or you will destroy it again.
Michael

 

[Minimichael]

Thanks to all for this input. I did have it cradled in a steel strap hanging under the seat, but not especially mount-sturdy. Live and learn.

I like the hold down tab idea, as well as the cold forming. It doesn't have enough material for tapping deep bolts for a proper flange, so I'm also thinking I should go to something lighter altogether but I really like putting the exhaust farther back than a simple hotdog muffler can.

I suppose it may be a good time to pull this one off the bike for a good cleaning and rebuild assessment, too. It's a rope start (not recoil) and I've enjoyed the attention in this regard. It makes the kids laugh and not want to ride dad's old bonanza in the first place. They don't call it "the ankle biter" for nothin

 

[2old2care]

rope start (not recoil)

I have a British Seagull outboard, that I use from time to time - The rope start AND exposed flywheel are similar conversation starters.

 

[f4radar]

Where I grew up the rich people had wood handles on their start ropes. The rest of us just had a knot on both ends and also used it as a belt.

 

[toomanytoys]

Sorta unrelated story but with pull ropes.

when I was around 8 dad had an old huffy tractor. The electric start broke or he was too cheap to buy a battery so he took the cover off. Put a pull starter cup on it. Cut a notch and we just wrapped a piece of clothes line around it to start.

well he told me to cut the grass when he was at work. I wrapped it up nicely. Yanked. Fired right up. But the knot didn’t come out of the notch. That rope is flying around full throttle smashing off the hood, steering wheel, etc. I’m in full out panic mode. Dad is gonna beat my ass! I’m trying to duck the rope to get to the off switch when as luck would have it the rope flew around caught the steering wheel just right and snapped the rope. Old Briggs didn’t miss a beat. Just kept slinging the shorter piece of rope around.

i hurried up and cut the grass, made a new pull rope and parked it back in the shed. I don’t believe I ever told him about that.

Definitely a core memory there. I remember that clear as day!

 

[Minimichael]

Where I grew up the rich people had wood handles on their start ropes. The rest of us just had a knot on both ends and also used it as a belt.

Mine is fashioned out of a broken hammer handle I took away from the dogs. Considering the belt idea now...