Reverse Gear Engagement

Sunday I went for a non-ethanol gas run which is a total 40 mile run at sustained higher speeds, 60-80. It was a hot day, 90 degrees. Car was running mostly at 90C on my gauge, up to 110 when I left the station after filling but in relative short order came back to 90C.

When I got home, I went to back into the garage and the car did not want to go into reverse. I had no trouble in any of the forward gears. It would grind to try get into reverse and not the slow type like I’ll get in a couple of my other cars where with a little coaxing will slip in. There was no way I was going to force this situation. I will add that things were really hot and cooking driving at speed on a hot day. The transmission tunnel was certainly warm, the chrome part of the stick quite hot. The engine temp was climbing fiddling around to park and by the time I turned the car off it was belching coolant.

Two days later, started up like it was never turned off (coolant topped off) and slipped into reverse numerous times (cold) with nary a sound or resistance. I recently did change the tranny oil to Royal Purple 75 W 90. I also went to a smaller master clutch cylinder to get a lighter pedal, but up until the hot, hot ride, never had an issue with any gear after the conversion. I did pump the clutch a couple of times and bury it into the floor to try get into reverse to no avail when the issue presented. I was able to get into reverse and engage normally with the clutch if I turned the car off, put in reverse, and restarted with the clutch in which I had to do three times to turn the car around and back in.

Any thoughts on the issue?

If your temp gauge is accurate, you REALLY need to do some work on your cooling system. It should not be running near that hot. The ONLY time mine has ever gone over 100C was sitting in stop and go traffic in a 110+F ambient. 90C is rare - extreme conditions or a brief spike after a short shut-down after a hard run.

On the clutch, are you sure the freeplay is set properly? Have you flushed and bled the clutch recently? If there is moisture in the fluid, clutch arm travel will be reduced when the fluid is hot, preventing the clutch from releasing fully. That is likely your problem.

I am working on the cooling system. I installed a relay on the Coolcat fan and motor setup and now have to verify that the otter is kicking in as it should. I also need to verify the temp gauge readings. You seem to be an outlier with low driving temps, many say 90 is quite normal on a hot day.

The entire clutch sequence has been recently redone (last summer/fall). New master, new slave, new flex hose, new fluid, numerous bleeds. All was/is working great, no shifting issues. Freeplay is properly set verified by measurement at the slave rod and visual inspection of the TO.

If the clutch is not releasing fully when hot, why did it work “normally” when I turned the car off, put it in reverse, started (wouldn’t the car jump?) and then had some travel with the clutch before engaging and was able to release with the clutch to change to a forward gear? It also worked fine in all the forward gears.

Thanks for the reply.

Hi John,
The only thing that will have reverse gear grate when trying to engage, when the engine is running, is for the Main Shaft to be rotating; this should only happen if its being driven by the back wheels (car moving), or if the Main Shaft is stationary (car not moving), the First Motion Shaft is rotating. The later should only happen if you were to try to engage Reverse too quickly after disengaging the clutch (trying to select reverse before the First Motion Shaft has stopped rotating), or the clutch is dragging (not disengaging completely). If the First Motion Shaft is not a free running fit in the Flywheel Bush, this can also cause the First Motion Shaft to continue to rotate to varying degrees, notwithstanding that the clutch is completely disengaged.


And this might be an issue in a very hot situation with expanded metals vs. a cool or normal situation?

Hi John,
I think the most likely is something related to the clutch dragging. But the Flywheel Bush can’t be ruled out; not much motion of the First Motion Shaft and in turn, the Lay Cluster, is needed for reverse to grate. The Synchro Hubs can cope with a small amount of Lay Cluster rotation, but not Reverse Gear.

If the clutch disengagement is marginal, heat can have an affect.


This is what my OTS does on a hot day and I have a Coolcat fan and an aluminium radiator.

I see this as quite normal.

I wonder whether this is the cause of your problem.

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This sounds like a solid theory to me. After an extended run on a hot day, you’re likely to get heat-soaking effects that could be difficult to produce in any other conditions. I would vote for the pilot bushing grabbing, and keeping the 1st motion shaft spinning.

At work, we once wanted to know how long it took a particular turbine disk to be fully heat-soaked to its very center. The FEA model predicted over 2-hours of running b*lls-to-the-wall. Nobody believed it could take that long until it was proven out during an instrumented test.

It certainly could be or a contributing factor. Also, installed the clutch MC with a spacer so my clutch engagement is closer to the floor (that’s where I like it). That would imply the risk of less release of the clutch without additional throw of the pedal. Up until this hot, hot situation all was great. I’ll see how it behaves with more time and if hot, hot is the only issue, well, I’ll make sure I don’t need to back up!

I like the sound of it too. Hard to say which it is definitively, could be some of both.

Hi John,
I would exhaust every possible source of the issue being related to Master and Slave cylinders before picking on the Flywheel Bush, as the later involves removing the engine/gearbox.


Agree, but if it only occurs under fairly atypical circumstances, and there’s a work-around (start it in gear), then a grabby pilot bushing isn’t the end of the world.

But, yes, fully explore potential clutch issues first. However, from what we’ve been told, those sound unlikely.

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I will note that other than the other day when things were smokin’ hot, I’ve never had any grunch or issue getting into any gear, including reverse after my clutch master/slave replacement. After this issue, two days later when I moved the car from one garage at the house to another (car didn’t warm up), I put it in reverse 3 times with not so much as a peep and it slid right in.

Hi John,
One thing is for absolute curtain and that is, either the Main Shaft and hence the Reverse Gear, or the First Motion Shaft and hence the Reverse Idler Gear is rotating for Reverse Gear to grate when attempting to engage.

Very hot, less viscous oil will sometimes cause issues when shifting between forward gear when the car is in motion, but when selecting Reverse Gear, hot, low viscosity oil should not have any affect.

It could be that when the oil is cold/cool, there is enough drag to stop the First Motion and Lay Gear Cluster from rotating when the Clutch is disengaged, but this would only be an issue if for some reason the First Motion Shaft is still being driven slightly.

If the car is at an absolute stop when the attempt to select reverse is made, the Main Shaft would be stationary and therefore, the only thing that will cause reverse to grate is for the First Motion Shaft to still be rotating. Focus on seeing what may be causing that to happen.


I think the first test I’ll have to do is see how it’s acting under normal operating conditions, warm car. Thanks.

I think the way I would start investigating this is take the car for a similar length drive, but at night.

If you can do similar mileage in cooler conditions, but not have your reverse engagement issue, then you’ve added credibility to the idea that something got unusually hot that day.

Whether it’s the pilot bushing or the clutch fluid still remains to be determined, but at least you’d have a better feel for whether it’s primarily temperature related.

There will be an opportunity to.

Hi John,I think the fact that you have your car set up to make the clutch bite near the floor leaves you no latitude in extreme heat?
Regards Gerry 62 Ots

Update. Took a shorter drive the other cooler day and when I returned, same thing, grinding at reverse. Put the car on the lift and checked the free play at the TO. I’d say 1/4", I probably set it generously when I replaced the MC, flex hose, slave and fluid, or, it settled in a bit. Tightened it up to about the 1/16" spec, verified free play at the fork and gap at the TO to the pressure plate through the inspection hole.

Took it out today, drove about 20 minutes, warmer today, chrome stick warm along with tunnel. Car running at 75 like the other day above. Got home, slipped into R without a peep. I’ll call it a success after more time, but seems the little extra throw with the tightened free play gave me the movement I needed to disengage when hot. Added benefit, the clutch pedal feels a bit more “there.”