exhaust thread repair on smaller lathe (DIY)

[nonsense]

Thought I'd share the exhaust thread repair I just finished, as my method was a little unusual due to the size of my lathe. I went the threaded bronze insert route, but I didn't have a big enough lathe to spin the head, and have no CNC machines... so I took the cross slide off of my lathe and mounted the head there, with a boring head mounted in the lathe chuck. It would have been nice to mount the boring head directly on the head stock, but the one I have has a shank for a mill, so it went in the 3-jawed chuck (don't worry, I clamped on the non-contact middle section of the shank). It's certainly not a conventional approach, and I was questioning it's stability when I started, but it worked out very well. Definitely time consuming having to start and stop the lathe every pass to adjust the boring head, but I was also taking smaller cuts then I could have.

I made the insert out of aluminum bronze (at least I think that's what it was... it was in the scrap pile and looked/machined as I've heard aluminum bronze should - was definitely some sorta bronze), which is what Jim Comstock uses according to his website. The threads between the head and insert are a slight interference fit, as mentioned by (I think) also Jim Comstock (comnoz) in one of the posts on this forum, I forget which one.

 

[comnoz]

Nice work. Jim

 

[Stephen Hill]

Nice job! Looks like you might have a 9" South Bend lathe?
How did you mount the faceplate to the saddle?

 

[jaydee75]

Very, very good work. I have the equipment, but I don't think I have the nerve.
Hope I never need it.
I'm impressed.
Jaydee

 

[concours]

a small lathe, in the hands of a talented machinist. Great Job.

 

[Derek Wilson]

Well done - Nice work! How did you cut the threads? Did you single-point the threads in with the boring head?

 

[mean gene]

I have bored out a poor insert this way but never tried to cut threads with boring bar I'm sure the job was time consuming Looks great, are you taking orders? LOL

 

[nonsense]

Nice job! Looks like you might have a 9" South Bend lathe?
How did you mount the faceplate to the saddle?

Yes, it's a 9" South Bend. The faceplate is mounted using an adapter I put together from a couple pieces I had in my "extra lathe stuff" box:



As u can see the threaded part of the adapter screws right onto the faceplate where it would normally attach to the headstock spindle. The other part with the triangle groove fits onto the saddle where the cross slide would normally attach/pivot, and is locked in place by the same two set screws that lock the cross slide angle. If I was to make the adapter from scratch I would probably put a larger shoulder between the two parts and maybe a way to lock the faceplate thread, but it worked well enough as is. Since there is no vertical adjustment with the setup I used shims to get the head up to the correct height (centered on the spindle) and then the cross slide rotation and Y axis adjustment to finish squaring and centering.

Well done - Nice work! How did you cut the threads? Did you single-point the threads in with the boring head?

All the threads are single pointed, the external ones on the insert done the standard way and the ones in the head and on the inside of the insert done with the boring head. I didn't have any reversed tools that fit in the boring head, so I had to thread into the head (as opposed to engaging halfnuts at full depth and running the lathe backwards). I have an automatic half nut disengage mechanism on my lathe (built myself), so stopping at the right point wasn't a big problem; I just set where I want the halfnuts to disengage at the beginning, and then I just have to keep track of engaging them at the right time.

 

[MikeG]

Nice work! It must have taken a long time to get the exhaust bore squared to the boring head.

 

[HopperXL]

Great to see a classic old lathe still being used in anger, as well as the classic old bike. Very nice work.

 

[baz]

Great to see a classic old lathe still being used in anger, as well as the classic old bike. Very nice work.

That lathe looks positively modern compared to my 1880 Barnes flat belt !

 

[acotrel]

Many Japanese racing motorcycles have stubs in their exhaust ports with the exhausts held on with springs. Conservative thinking holds many people back from being creative.
With my bike, I simply made two threaded stubs which exactly match the outlet of the port, and screwed them in against Yamaha sealing rings, and held the exhausts on with springs and slip joints. There are NO STEPS where my pipes are attached to the cylinder head - and that might be important. When the motor is running, - at TDC on half of the strokes both valves are open, and gas flows across the top of the piston and into and back out of the exhaust pipe, at high frequency. Any step there is probably bad.
Kadency effects do not only happen with two-stroke motors. - 'Attention to detail' is also an aspect of quality ? The exhaust system is not there just get the gas out. It is part of cylinder filling.
I might be dumb and leave the clip loose on the fuel line, but I take a lot of care with the ports on my cylinder head. If my motor does not stop, it really goes.

 

[Stephen Hill]

Re: exhaust pipes held on stubs with springs, I don't see how that would work with the Commando. The screw-in exhaust nuts are pretty much the only thing holding the exhaust on the bike. I would guess if you replaced the hard mounted nuts with stubs and springs the exhaust would shake itself to pieces. Anybody tried this on a Commando?

 

[lcrken]

Looks like that's what Ron Wood used on this flat track bike.

Ken

 

[jaydee75]

I drool every time I see that bike. The bare essentials.

Jaydee

 

[Stephen Hill]

Cool bike, but no isolastics, and the engine is hard mounted.
This doesn't help answer the question whether screw in exhaust stubs and springs would work on a "regular" Commando.