TA125 Information

There is lots of poor information around on the TA125 models, especially on the internet especially regarding the parts that it was fitted with from ‘street models’ from new, dont get me wrong this was the case indeed, it was very much a parts bin special from Mr Yamaha.

The TA125 was based on:

-AS3 (twin top railed frame) which was heavily modified. Also the early RD models had a similar frame, however differences in the top rails varied. The TA125 style frame had top rails running all the way up to the headstock, some later Rd125 models did not have this.

-Front forks and yokes from AS3/early RD125

-Rear hub from AS3/early Rd125

-Swinging arm from AS3/early Rd125

-Front hub set up (246251110038) and plate (3612512010). Used on the DS7/YR5 and other models.

Interchangeability of spares between RD125/AS3 models:

The AS1, AS3 and early RD125 models all have nearly identical crankcases to that of the TA125, nearly all parts are interchangeable. There were changes to these things to make the bike a ‘racer’ which are as such:

  • Gearbox ratios (see below for details)
  • Con-rods with lubrication slots cut into the big end housing (Other than that, the conrods are identical to the AS3/RD125 parts)
  • High speed coated big end bearings
  • Section coming soon… watch this space
  • Barrels (shorter than Rd125)
  • Single ring racing pistons

Gear Ratios

– TA125/GYT kit

  • 1st 31/15 – 2.066
  • 2nd 28/28- 1.55
  • 3rd 25/20- 1.25
  • 4th 23/22- 1.045
  • 5th 22/24- 0.916
  • (please note the error in the TA125 parts book mistakes the 22T gear as a 21T gear)

The G.Y.T kit

G.Y.T (Genuine Yamaha tuning) was a kit which was supplied by yamaha as a racing package to upgrade the road AS3 engine for racing. There were a few differences between the GYT parts and the TA125 parts. The main one being that the inlet port mounts were on an angle:

Screen Shot 2013-02-17 at 20.03.32

The GYT kit had the import mounted level across the cylinder rather than on an angle. Apart from this the cylinders did not differ at all.

  • Hitatchi ignition system, twin coil packs, CDI unit, stator-plate assembly and rotor
  • Cast alloy barrels with a steel liner
  • Single ring racing pistons
  • Gear ratio kit comprising of six sprockets with the ratios of:
  • Expansion pipes un-silenced, spring mounted at head and frame
  • 28mm Mikuni VM carburettors
  • Fold up footrests
  • Gear and brake lever set ups
  • 28mm clip on handlebars
  • Yamaha TZ style twin pull throttle
  • 16RPM redline tacho mounted on front bracket

Modifications, aftermarket parts 

  • Faharon water cooled set-up 
  • Barton dry clutch kit with straight cut primary gears
  • Lengthened swinging arm
  • Modern ignition as original Hitachi system was unreliable and expensive to maintain.

Yamaha Two Stroke Production Racer List

Year      Model#        Model Name      Displacement         Frame # Range

1973     400              TA125                124cc                     AS3 990101>990544

1975     400              TA125b               124cc                    AS3 991101>992000

Technical Details

Stroke: 43 x 43

4 thoughts on “TA125 Information

  1. david

    I have a ta125 that is missin expansion chambers. if u have any info on where to find a set or any one that is interested in buyin the bike please reply

  2. Pingback: 1974 Yamaha TA125 Road Racer | Bike-urious

  3. robert eckles

    Great site with a lot of info.! It’s difficult to get all the specs for these vintage bikes. I own a TA125, a GYT KIT 125 and a TZ750 and I enjoy reading anything about them. Just like to add my 2 cents and make a couple corrections in the specs here. The section on interchangeability of spares it states AS1, AS3 and RD125 cases are nearly identical to the TA125. The AS1 is not the same as the others. It has a single main transfer port from the cases to the cylinder. The others all have a larger cutaway in the cases to match up to a main transfer and a sub transfer port in the cylinder. In the GYT KIT section it states the cylinders were cast alloy with steel liners. They in fact were cast alloy with chrome bores same as the TA 125 cylinders. No steel liners. The AS1 GYT KIT cylinders had main transfer ports only. The AS3 GYT KIT cylinders had main and sub transfer ports like the TA 125. Like I said, sometimes it’s difficult to get all the correct information on these old machines. 🙂


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