# Cut the First Metric Threads on my Heavy 10L



## MrWhoopee (Nov 10, 2019)

I bought an old McKissic Mighty Mac chipper shredder off CL for $200 the other day. The motor on it was a well-worn 6.5 hp Greyhound (Lifan) from HF. It had some minor issues but started and ran well. While using it I noticed that gas leaked from the filler cap due to vibration. I tried replacing the poorly improvised gasket, with something better, but the leak continued. I then realized that the cap was cracked. After an hour or two searching online, I realized that I was going to have to make one.

About a year ago I assembled an 80/63 transposing gear set, but had never gone any further with it. This was the obvious opportunity. The filler measured 55.6 mm, so obviously a 56 and the threads were a 2.5. I set about trying to figure out what other gears I might need, but was getting more confused as I went. Then I read that the transposing gears effectively converted the 8 tpi leadscrew into a 2.5mm, and the light went on. I mounted the transposing gear on the banjo, leaving the stock stud and screw gears. Then I set the QCGB to 8 tpi and made a test pass on the piece of gray PVC I had selected. Voila, 2.5 mm!

I set about making the blank, then started threading. I made a few false starts trying to thread from inside out, including cutting left hand threads. I finally settled for the slowest back-geared speed, threading in the conventional direction and stopping the lathe before I hit bottom. All I had to go by where the major diameter of the filler neck and the minor diameter of the old cap, and none of it really made any sense. I ended up having to remove the part from the chuck twice to go out and test it, then put it back and pick up the thread again. The threads are less than perfect but more than adequate.




It's little things like this that made me want to learn machine work in the beginning. It's only taken me 40+ years to get here.

I realized that an added advantage to the 80/63 transposing gear is that you can leave it mounted, simply reversing the screw gear and re-positioning the transposing gear on the banjo makes the transition from imperial to metric.


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## benmychree (Nov 10, 2019)

Is there a vent to allow air into the tank as it empties?


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## MrWhoopee (Nov 10, 2019)

benmychree said:


> Is there a vent to allow air into the tank as it empties?



Yes, the tank and crankcase are connected to some sort of breather. It may not be functioning properly however. After running for 30 minutes or so, I unscrewed the cap and heard hissing (good seal). I'll be looking into it.


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## kb58 (Nov 10, 2019)

The funny thing would be if after all that work, it leaked worse...


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## MrWhoopee (Nov 10, 2019)

kb58 said:


> The funny thing would be if after all that work, it leaked worse...



Yeah, REAL FUNNY! I've spent more time on things that didn't work, it was mostly an excuse to play with my machines. The fact that I pulled it off was just gravy. And it got my head into the metric threading thing again.


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## Canuck75 (Nov 20, 2019)

Great to be able to cut your own accurate metric threads. I too made a 63/80 transposing gear set as well as the other gears I needed. Gleaning information from our friends on this site I made a custom chart for my particular gearing setup and the QCGB selections for any required metric thread. I thought it might be of value to you so here it is.


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## MrWhoopee (Nov 20, 2019)

I kind of wish it had dawned on me that I already had an 80 tooth gear before I bought one. According to this chart,
I only needed 63, 36 and 32 tooth gears to be able to cut all standard pitches between .2 and 5





			https://i47.photobucket.com/albums/f165/drewlp/metricthreadingchart.jpg
		

(with thanks to Waterlogged on PM)


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## SLK001 (Nov 21, 2019)

Canuck75 said:


> ...I made a custom chart for my particular gearing setup and the QCGB selections for any required metric thread. I thought it might be of value to you so here it is.



You should add a column called ACCURACY of the final pitch, like 99.7, 98.6, etc.  This info will be needed for longer screws.


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## Janderso (Nov 21, 2019)

MrWhoopee said:


> It's little things like this that made me want to learn machine work in the beginning. It's only taken me 40+ years to get here.


Those words I can relate to.
The fact that you got those 2.5mm threads in the end made it all worth it.
You are a better man than I sir.
I don't think I would have figured it out.
I bought a lathe that cuts metric and imperial. I do miss the old 13" South Bend though.
I enjoyed reading your story.


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## MrWhoopee (Dec 7, 2019)

I finally purchased the 32 and 36 tooth gears. Both required modification (smaller bore and keyway). The 36 is done, not pretty but functional. Waiting for a 3/16 hss tool bit so I can broach the 32. I don't have any need at this point, I just wanted to have the complete set.


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