Lay shaft inspection and replacement

[Mark Strang]

I've got TC7801 under restoration. The transmission is on the bench with the bellhousing and rear mounting plate removed. An inspection from the top shows that all of the gears appear to be good with minimal wear. The forks appear to be in good condition as are any other parts that I can observe. The operation is smooth and quiet while turning it by hand. I have no idea as to the history of this car or the condition it was in before I bought it. It was a stalled project. Is there any point in removing the layshaft to check the roller bearing surfaces? I don't have a dummy shaft to install while I would be removing the existing shaft. Seems to me that I can recall that TC boxes don't have a history of layshaft failures/wear. I've done several TD tranny's and in each case the layshaft was worn. Any guidance would be appreciated. Also, if it is recommended to remove the shaft, does anyone have a suggestion for what to use for the shaft?

Thanks,

Mark

[Tom Lange, MGT Repair]

Mark - you are correct; TC boxes are much more robust than TD/TF boxes (that's why TD racers installed TC boxes). I have had only one high-mileage TC box needing a new layshaft.

That being said, needle bearings do wear, and sitting with a micrometer and measuring diameter and length is really the only way to detect wear in the TC box. And for that, the box has to come apart, which is not a big deal as long as you don 't dig too deep and disassemble everything. I really only pay attention to the needle bearings, ball bearings, and thrust washer end float; everything else has always been spot on. I have been known to cut an old broom handle to use as a dummy shaft, but usually just use lots of grease to hold the needles in place, and either turn the box upside down or pull up on a wire to bring the cluster gear in position to insert the layshaft. One last thing - since originally some of the needle bearings came as caged bearings, I have seen the cages come apart and need new needles.

In summary, I would give your box an 79% chance of being fine, but it's not hard to tear it down to check the bearings. And won't you really feel better?

Tom Lange
MGT Repair

[Mark Strang]

Thanks Tom. I guess it comes down to the 21% that I don't know about. I've disassemble several TD boxes in the past and installed new bearings , etc. I guess one more wouldn't hurt. My feeling is that since the TC layshaft is 50% larger in diameter than the TD shaft, it doesn't see nearly as much wear as the TD. I've never run across needle bearings that showed any signs of wear on the TD boxes even though the layshaft was starting to wear. My guess is that the hardness of the needles is much harder than the shaft.

[Mark Strang]

Tom,
I used your method with a hardwood dowel machined to slightly under .750 inches. Then i greased up the dowel and drove out the layshaft. All the needles and washers stayed in place with the dowel inserted. I then examined the layshaft and it looked perfect in the areas where the needles run. I then pushed the dowel out with the layshaft and installed the locking bolt. I think I'm going to leave everything alone. There is evidence based on a locking tab that has been reused that this box has been apart in the past. Why, I'll never know. Given this evidence and the condition of the layshaft, I've made the decision (perhaps bad) that all necessary bearings were replaced if the mechanic was reasonably good. Given the general condition of the gearbox compared to the TD's that I worked on, this unit is a good one needing very little more work. I suppose I'll find out in about a year plus or so when I get the car back together. I just finished the chassis and am now working on the engine/gearbox portion of the project. Thanks for your help. Mark

[Tom Lange, MGT Repair]

I would do the same thing.

Tom Lange
MGT Repair

[Kenmgtc]

Did you measure the end float? It can be a cause of poor ability to stay in gear and easy to check without taking everything apart. The thrust washers are expensive though!

[Eric Worpe]

Tom makes an important observation about caged needle rollers. Early TCs and possibly all TBs used caged layshaft and first motion shaft roller bearings known as Hyatt bearings. These eventually break up as the small nipples that locate the individual rollers in the cage wear away. This can be catastrophic due to scoring of the bearing surfaces.

[frenchblatter]

I replaced all bearings and thrust washers. I didn't have a dummy layshaft so used heavy grease. No real problems.