Piston Slap: Deep Thoughts on Shifty Shifting

TTAC Commentator thetopdog writes:

I drive a 2006 Corvette Coupe (45k) with the 6-Speed manual. The past few months I’ve noticed that most of the time I try to make the 1-2 shift at high RPM, I get really terrible grinding coming from the tranny. It only happens at high RPM (5000+) and it doesn’t happen every time, but it happens enough for it to be a concern. If I clutch in, let the revs drop a little, then throw it into 2nd, there’s usually no problem.

I have the CAGS (1-4 skip shift) eliminator installed. I had the dealership replace the clutch packs under warranty. Apparently they screwed up because my entire clutch stopped working, leaving me stranded. After getting it flat-bedded to the dealership, I was told the entire clutch system was contaminated and the seals were destroyed. I’m not entirely sure what they did after, but they fixed it somehow (probably flushed the system and replaced the seals?). I also had a transmission leak (what a quality car, huh?) last fall that was also fixed under (extended) warranty.

I haven’t checked the fluid level in the master cylinder recently, is it something I can do myself? I’m thinking this might have to do with worn synchros, but is there another possible solution? I’ve heard that changing your clutch fluid can help (apparently GM Synchromesh fluid is good for this?). If not, is there any solution short of a brand new tranny?

One more thing, what exactly is going on when the tranny is grinding? Is the grinding itself going to lead to severe damage if I’m not careful?

Sajeev answers:

Holy loaded question, Batman! I simply cannot supply TTAC’s snarkiness with this much content. Much like your Chevy dealer (whose actions raises several read flags) consider me overwhelmed by your Corvette.

First and most importantly, yes, change the clutch fluid regularly. From my experiences with an aggressively driven Z06, the fluid needs replacement rather frequently. So do it. The clutch reservoir is adjacent to the brake master cylinder, on a metal bracket: the easy way to replace fluid is with a syringe via the reservoir. But don’t suck the reservoir dry, lest you get air in the system. Just drain it to low, refill with brake fluid, actuate the clutch to circulate…and repeat about three more times.

If fresh clutch fluid doesn’t cure it, switch to synchromesh tranny fluid. Frank Williams pinged me on this, and sent me these relevant links: Here and here and here and (drum roll) here.

And your last question: this explains how synchros work. Using that reference, the grinding sound you hear is the “gear” not engaging the “collar” smoothly. If a fluid change(s) doesn’t help, you might have a worn gear, collar, synchro or some combination of the three. Which means you need a modest transmission rebuild: there are plenty of Corvette savvy rebuilders (outside of a GM service department) eager to help you there.

Having driven a well-sorted ‘Vette, I’ve got to say, it will be well worth it. Get ready to fall in love all over again.

[Send your technical queries to mehta@ttac.com]