Unique Transmission

[joe7]

This showed up on my workbench for a rebuild. We have a Mag BH with some type large cutout for the starter and a very long pivot for the clutch. Also a mag gear case with some numbers. Maybe someone knows something about this. The front bearing retainer doesn't have a seal but uses a reveres Archimedes scroll. Not a trans one would standup on end if its full of oil. The ally tail housing is out getting a new bush and a rear seal fitted. Its the single bolt lug type.
Next up is the gear train. Very odd indeed!
1st gear ratio is 31 "T" on G & 18 "T" on CS = 2.29. Appears to be from a "rocket box" (DBE pg F3) . Not done yet. The gear ID has been machined to accept an early GB type bush that has the ID machined to fit the MS and the OD machined about 2/3 to fit a single roller set with the remaining 1/3 at standard.
2nd gear ratio is 28"T" on G & 22 "T" on CS = 1.69 unmodified.
Syncro hub, from a "later full syncro ring teeth" type GB, is in good shape but rings need to be replaced. The shift fork is in excellent shape.
3rd gets interesting. The gear has been welded onto a OEM gear. Its a straight cut gear with 26 "T" on G & 27 "T" on CS. Need some help on the ratio for this one.
4th/1st motion shaft is a straight cut gear that also has been welded on to an OEM gear part. 23 "T" on G, 1st motion shaft & 30 "T" on the CS. Ratio?
The syncro hub and rings are the segmented teeth type from a "later" GB and are in as new condition but the shift fork will need to be replaced.
The counter shaft appears to also be from a "rocket box" with 1st & 2nd standard. Similar to the 3rd gear a straight cut gear was welded onto the CS. Also 4th is a straight cut gear welded onto the CS.

[promotor]

Very interesting, thanks for posting.

The scroll type clutch bearing carrier is from an early Anglia 105e with crash first gear. Some later Anglia crash gearboxes had oil seals so used a 113e clutch bearing carrier.

Seems strange to use the weaker early full-tooth synchro rings on 1st and 2nd as those were always the two rings that suffer the most with those heavy gears!

I believe proper Bullet gearboxes (which were 3 rail 'boxes) had a 21 tooth input shaft not an 18 one as per Dave Bean's info. If you use his tooth counts for the Bullet gearbox you get ratios of 2.679:1, 1.980:1, 1.493:1 & 1:1.
However, I agree with his info that Bullet ratios are 2.29:1, 1.69:1, 1.280:1 & 1:1 when using a 21 tooth input shaft in place of an 18 tooth one.

Proper Rocket gearboxes were a special close ratio version of the Type E gearbox (a single rail 'box) with helical gears but ratios of 2.54:1, 1.67:1, 1.26:1, 1:1. They had a 20 tooth helical gear on the input shaft.

Based on your tooth counts I get 3rd gear ratio as 1.256:1 and 4th is 1:1 having put the tooth counts into my gearbox ratio calculator. I have 1st as 2.246:1 and 2nd of 1.660. I've double checked them so I'm happy with the results.

It looks like you've got an original helical Bullet cluster on 1st and 2nd gears as well as the accompanying mainshaft gears, and replacement straight cut gears to keep it somewhat near to Bullet ratios. Perhaps the gearbox stripped gears on 3rd and 4th and replacements were sourced from other 3 rail straight cut gearkits.

What size is the internal bore of the input shaft - does it take a normal 3-rail pilot bearing?

[joe7]

Promotor thanks for the info on the gear ratios. Yes, the input shaft takes a normal 3-rail pilot bearing. Any idea how the builder cut off the OEM gears to weld on the straight cut gears? I imagine getting them on perfectly straight and properly spaced was quite a challenge.

[prezoom]

I remember hearing about welded gear sets back in 1965. I believe it was in relation to the close ratio gearbox in my S7. Never needed to have the gearbox looked after during my ownership, but the Triumph Tr10 rear axles were another story.

[promotor]

Promotor thanks for the info on the gear ratios. Yes, the input shaft takes a normal 3-rail pilot bearing. Any idea how the builder cut off the OEM gears to weld on the straight cut gears? I imagine getting them on perfectly straight and properly spaced was quite a challenge.

No problem. I suppose the original gears would have been worked on in a lathe, I can't think of any other way to keep it round / true.
It's certainly a skilled job to get everything true and lined up.

I would guess that the needle rollers at the front of the gearbox would now suffer a little bit more due to the separation effect of the straight-cut gears. I don't think it'd be a quick process (if they do wear at all) and its just a general musing on the possible cause-and-effect of modifying things.

It'd be interesting to know how the gearbox behaves in use regarding noise as well as wear of parts.