Piston Slap: Shifty linkages and overweight knobs?


Chris from Canada writes:

I own a 1958 MGA (1500 cc), and the transmission jumps out of second and fourth gear when I accelerate. I thought this could be a problem with the tranny, so to make the car more useful on the freeways of today I bought and installed a new five-speed transmission, supplied by a UK Company. (I did this prior to the Japanese conversion kits being available.) Well, low and behold, I still have the same problem! Because it’s a five-speed, I don’t have to hold the gear shift lever all the time now; I just get the car into fifth gear and I’m OK for the long haul. Any idea what could be causing this? (I thought maybe the chassis was twisted, but I’ve done some measurements and it appears OK—mind you the measurements were with a tape measure and not super accurate.)

Sajeev answers:

There’s no doubt the five-speed conversion is worth it, not to mention it might speed up our diagnosis: Either the shift linkage needs work, or the transmission mounts are worn. I think I found your conversion kit, and if so, the Ford five-speed swap includes a new crossmember. New crossmembers suggests new bushings, so the issue is likely between the shift knob and the shift linkage.

Consider the linkage: Odds are you have the T-9 gearbox, commonplace on UK-spec Fords of that era. These units are, over time, known for sloppy shifting compared to their T-5 counterpart (used on the Sierra Cosworth). Having shifted a used T-9 in a Merkur XR4Ti and a used T-5 in a Thunderbird Turbo Coupe, I immediately knew (among other factors) which one I wanted for my Ford Sierra project car. 

T-5 swap on my 1983 Ford Sierra, not pretty but it gets the job done. Sajeev Mehta

But I digress …

The T-9’s problem is that most of the linkage is inside the transmission, and unless you sourced a fully rebuilt unit to install with the kit, you might have internal issues commonplace with used gearboxes. But let’s table all that diagnostic work, unless I got this wrong and there’s another transmission present with external linkages. (Inspect all rods and bushings for wiggle and consider getting a rebuild kit, if available.)

Instead look at the shifter: Depending on the vendor’s work to make a “modified gear lever assembly,” there still might be a sloppy bearing (bearings?) that pops the shifter out of second and fourth under engine load. Remove the shifter and check for play in the bearing before it actuates (the transmission below it).

Aftermarket knob concerns: If your shift knob is significantly heavier than factory, high engine loads can easily pop the transmission out of gear. Back in 2013, I did a Rallycross where my 2011 Ford Ranger popped out of second gear during a time trial. It happened at full throttle, on a bumpy dirt road that was clearly too much for the shift linkage. The problem was cured by removing the solid 8-ball (right) for a hollow one (left). Remove the knob and see if it helps … the things you learn while racing! 

If the shifter is rock solid, if the linkages cannot be seen/adjusted, and the shifter pops free even without a shift knob, the biggest concern is the condition of the linkages within the gearbox. Hard to know without seeing everything in person, so I will hope you just need to switch your shift knob. What say you, Hagerty Community?

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    A brilliant bit of physics pointing out that shift-ball weight can not only change tactile feel but alter the performance/longevity relationship. Hadn’t thought of that before.

    Sidenote: this new forum system blocks you “slow down, you have posted too recently” if you try to comment on a second post in a thread. I did this in the recent Adrian Clarke article that has lots of comments (but not trending… sigh) and it not only blocked me but poofed what I had typed.

    This version of the forums will stifle conversation as far as I am concerned. The media portal is a let-down thus far (compared to the previous community site).

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