Piston Slap: Probing the insides of a manual gearbox?


Patrick writes:

Good day, I own a 1989 Ford Probe, 5-speed manual, 2.2 I-4 engine, rust free Rocky Mountain car, 158,000 miles, excellent interior and very nice exterior. Runs pretty good, and I only drive it about 1500 miles a year. The latest development is occurring in the gear box. When shifting in all of the forward gears, there is a “crunch” (for lack of a better word) as the stick goes from gear to gear. It may originate in the “H” crossing of the pattern.

The gears themselves continue to operate smoothly. Yes, I am aware that the mechanicals are Mazda MX6. Is it clutch? Is it transmission? Is it a lost or broken nut somewhere in the pattern? I did a little YouTubing and Googling on line but have not found any discussion of this particular problem or a solution.

Sajeev answers:

You came really, really close to finding the issue on your own, Patrick. So here’s what I think you missed: synchronizers.

Synchronizers (or synchros) are the things that ensure smooth gear changes without the need for double clutching. Over time, issues like abuse (by jamming gears at full throttle) or an out of spec clutch (i.e. dragging clutch) can kill synchros. Since this is 30+ year old vehicle, presumably with an original powertrain, there’s a good chance the gearbox is a little worse for wear.

Maybe you only need a new clutch and pressure plate, but I am not gonna go out on that limb. There’s a real chance you need to pull out the gearbox and pay for a good transmission rebuilder to go through the whole thing, replacing worn parts as necessary. No matter who you choose, this won’t be cheap: Find the most reputable shop in town to do the work.

Sometimes, finding a good used transmission (transaxle, in this case) with low miles is a better idea, especially if the quality of local rebuilders is suspect. They are usually a lot cheaper, which is a dealmaker for some budgets. Just for you, I went to Car-Part and found a Mazda MX-6 transaxle with 45,000 miles for a measly $400. And I bet the junkyard in question would happily row through the gears as a kind of rudimentary assurance to verify is is indeed a working gearbox.

No matter which route you choose (rebuilt or used) you need a new clutch. Maybe you can install a new clutch by itself to solve the problem, but if not, you’re gonna just end up spending hundreds in labor to go back in there and fix the problem a second time. Only you can decide if that’s worth the risk/money/effort.

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