Hi team I am after specs on a 74 125F
Engine #487#####
I have been offered the engine and I want to know what the gear ratios are before I buy it.
If they can be interchanged into a CT bottom end.
Talking with Mark T we believe this model was kick start only? Does that mean 6v?
It is also a 4 bolt head.
Thanks!

I'm guessing a bit but i'd guess not a straight swap so far as shafts & all go. I had an AG100 G from 1980 but shape is old school. Then i had a trans out of a DT100 of that's still current model but the shap of your DT125 motor & had to swap gears on shafts to mix & match & ended up with a good close combo but had to gear it to the moon with sprockets or was too low in top. All worked well, & still have the selector drum & selectors as wouldn't swap over. Did still have neutral at the bottom as Ag's do but that was ok.
Think you said CT but did you mean AG175 or are you wanting better ratios for a CT as i thought they were pretty good as they were & probably same ratios as your DT125F.

MARK said you would know/cherp in!

Yes sorry for the AG. Anther local has fitted a ct trans to his AG no worries. Just wasn't too certain about this model.
Guess there is only one way to find out...!
A ct top end will still bolt Upto this DT bottom correct? It's still 4 bolt.
Cheers!

I've never swapped a top end from the later motor onto the early one but have always thought they'd swap straight over on any of the 125/175 through to the end of the 4 Long studs that hold the cylinder & head on as one. same stroke different bore same base gasket i think. So far as swapping gears on shafts goes there's a small gear pressed on to the end of the main shaft that i hope goes back on as a tight press fit [mine did] & have to be carefull pressing it right up as needs running clearance. With the dd one i've done i just got it close--where you know one more press movement will be too much so i just apply press pressure trying to judge how much, then bang the press somewhere close to the action to shock it a slight movement. Had those pressed on gears let go on early V90 scooters.

For the long stud 125/175, (Enduro and Motocross models) the only cylinder/head that won't swap is the 1976 YZ175. (Yamaha spread the studs to install a larger reed valve.)

To use oil injection with some model/year cylinders, you might have to add a fitting to the carb for the oil line like they started doing in 1976.

On the transmission, shaft spacing and dimensions are identical with the possible exception of the clutch pushrod diameter. I think they went to a smaller pushrod when they went to the internal mechanism.

You'd have to use the AG shift drum and maybe forks... and possibly do something with the clutch pushrod... having a special pushrod made or boring the mainshaft hole to match the earlier pushrod size (if different).

With possibly a little work to the clutch pushrod, the trans will fit an earlier model.

I don't know about the ratios as we didn't have that model in the USA... knowing Yamaha it's probably the same as a CT3 or 74-75 DT175.

P.S. I think you mentioned the AG had a lower primary ratio? (Clutch/crank gear). While changing the secondary ratio (chain and sprockets) won't affect the "gaps" between the gears, keeping that lower primary gear ratio will actually make the CT trans effectively slightly "closer ratio".

Forty odd years ago , my younger brother had a CT3 . At some stage we had to do a gearbox
repair on it . My Yamaha shop that helped me out racing took care of this .
I recollect the spares man saying that the gearbox in that CT , seemed to have DT 175 A parts
in it . So based on that , and what Mark has already stated , you probably could swap things out .
I will have spec sheets for DT 125 FA and CT3 in my garage . If you're still interested , I'll go out
there later , and send through the details .

It's been a while and I seem to be able to remember some things better than others... my source of info is one of my land speed racing projects...

Lance's CT 175 effort seemed to be struggling with gearing... gear it tall enough and it couldn't pull 5th... gear it down and to pull the shift to 5th and it would run out of RPM and not go fast enough. MX close ratio trans helped. From that I figured I needed to find better gearing.

What I have pretty much finished is installing a close ratio six speed transmission in CT1 cases... I'm using parts from various 1969 to 1981 125's and 175's. No modifications to the cases were needed. This was because Yamaha kept the same basic layout and shaft spacing... I'm even using a 1980 DT175 crankshaft in the CT1 so I can use the taller primary ratio and YZ clutch the later models had.

P.S. I'd like to see the DT125FA specs!

Ok guys , I have the Oceania specs for a number of 125 / 175 models

CT2
Pri. Drive 65/21
1. 38/15
2. 34/19
3. 30/23
4. 26/26
5. 23/30

AT2-J
Pri. Drive 74/19
1. 34/12
2. 30/16
3. 25/19
4. 24/32
5. 22/23

DT 125 FA
Pri. Drive 74/19
1. 34/12
2. 30/16
3. 26/19
4. 34/22
5. 22/23

DT 175 A
Pri.Drive 74/19
1. 35/11
2. 30/15
3. 26/19
4. 23/23
5. 20/25

74 AG 175
Pri.Drive 77/17
1. 35/11
2. 30/15
3. 26/19
4. 21/24
5. 18/27

Interested to note . I checked Oceania DT 125 FA ratios against US DT 125 A and they differ .
I wouldn't have expected that , Hope this is of some use .

Some of those numbers look like they might be "Yamaha typos"... like I'd bet the 37/15 primary ratios are secondary ratios?