Piston Slap: Deep Thoughts on Shifty Shifting

Sajeev Mehta
by Sajeev Mehta

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]

Sajeev Mehta
Sajeev Mehta

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  • ToolGuy First picture: I realize that opinions vary on the height of modern trucks, but that entry door on the building is 80 inches tall and hits just below the headlights. Does anyone really believe this is reasonable?Second picture: I do not believe that is a good parking spot to be able to access the bed storage. More specifically, how do you plan to unload topsoil with the truck parked like that? Maybe you kids are taller than me.
  • ToolGuy The other day I attempted to check the engine oil in one of my old embarrassing vehicles and I guess the red shop towel I used wasn't genuine Snap-on (lots of counterfeits floating around) plus my driveway isn't completely level and long story short, the engine seized 3 minutes later.No more used cars for me, and nothing but dealer service from here on in (the journalists were right).
  • Doughboy Wow, Merc knocks it out of the park with their naming convention… again. /s
  • Doughboy I’ve seen car bras before, but never car beards. ZZ Top would be proud.
  • Bkojote Allright, actual person who knows trucks here, the article gets it a bit wrong.First off, the Maverick is not at all comparable to a Tacoma just because they're both Hybrids. Or lemme be blunt, the butch-est non-hybrid Maverick Tremor is suitable for 2/10 difficulty trails, a Trailhunter is for about 5/10 or maybe 6/10, just about the upper end of any stock vehicle you're buying from the factory. Aside from a Sasquatch Bronco or Rubicon Jeep Wrangler you're looking at something you're towing back if you want more capability (or perhaps something you /wish/ you were towing back.)Now, where the real world difference should play out is on the trail, where a lot of low speed crawling usually saps efficiency, especially when loaded to the gills. Real world MPG from a 4Runner is about 12-13mpg, So if this loaded-with-overlander-catalog Trailhunter is still pulling in the 20's - or even 18-19, that's a massive improvement.