1968 automatic "bump"
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
1968 automatic "bump"
Anybody know what's up here?
Shifting from Park to Drive or Reverse, I experience a hard "Bump" I'm told that it's "The nature of the beast" by a Corvette Mechanic that I trust. I'm bringing the car in soon to check this out, but the U Joints are fine, the Snubber is as well....
Any ideas would be appreciated
BD
Shifting from Park to Drive or Reverse, I experience a hard "Bump" I'm told that it's "The nature of the beast" by a Corvette Mechanic that I trust. I'm bringing the car in soon to check this out, but the U Joints are fine, the Snubber is as well....
Any ideas would be appreciated
BD
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
This is a definite "Bump!" Like the RPM's are high when shifting into gear. I guess I'll see when I bring her in.
I'm also told that GM had a memo that went to all dealers, told them to NOT try to fix...That the car will experience some "Lash" in the drivetrain components over time...U Joints, Half Shafts...etc
Any help would be appreciated...
BD
Last edited by DESI6327; Apr 24, 2012 at 01:01 AM.
Le Mans Master
Joined: Jan 2006
Posts: 6,489
Likes: 134
From: Butler Pa
Lash? That's called worn out parts, and the memo to not fix it I'm sure is BS. Who told you that?
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
It was posted on another forum...Not the actual "Memo" I'm glad you think it's BS..I got the same feeling. If I can remember which one it was on, I'll post it. The more I read about Snubber Cushions, the more I think this is the culprit.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
7 Bolt-On Upgrades From Extreme Online Store to Level Up Your C6 Corvette
Pouria Savadkouei
How Likely Are These Five 2027 Corvette Rumors to Be True?
Brett Foote
9 Best Corvettes You Can Buy for Half Price (& 1 You Should NEVER Buy!)
Joe Kucinski
8 Very Best Corvettes of Amelia Island 2026
Joe Kucinski
Top 10 WORST Corvette Engineering Failures of All Time!
Joe Kucinski
10 Records the C8 Corvette Generation Has SMASHED (& 1 Glaring Failure)
Joe Kucinski
7 Wildest Corvette Concepts Ever Made
Brett Foote
Instructor
Joined: Jan 2012
Posts: 201
Likes: 0
From: some hole in the ground town- camp verde:) arizona
thm350s have an anticlunk spring. designed to cushion the shift into gear from park... this does not cushion the shift from 1st to 2nd or 2nd to 3rd etc. as that is the job of the accumulator.
thm400s don't have this spring
thm400s don't have this spring
Instructor
Joined: Nov 2011
Posts: 183
Likes: 0
probley the internal lip seals leaking due to age they get brittle and let transmission fluid by pass making a slight delay then the increased psi to aplly the clutches makes them apply rouughly also worn friction plates in the tranny will make a clunk when its engauged into gear
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
It's a 427/390
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
The engine is a 427/390. As I mentioned in another post, everything else in the tranny seems to work fine. Sits at a light with no problem, accelerates through the gears with no "Bump" at all. Nice and smooth. Just going from P to D or R
We shall see.
Thanks for all the info
CF Community Team
Joined: Oct 2004
Posts: 19,322
Likes: 5,223
From: Cape Cod, Mass.
2025 C6 of the Year Finalist - Unmodified
2025 C8 Z06/7/E-Ray of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2021 C8 of the Year Finalist Unmodified
2020 Corvette of the Year Finalist (performance mods)
2019 C1 of Year Winner (performance mods)
2017 Corvette of the Year Finalist
2016 C2 of Year
2015 C3 of Year Finalist
The engine is a 427/390. As I mentioned in another post, everything else in the tranny seems to work fine. Sits at a light with no problem, accelerates through the gears with no "Bump" at all. Nice and smooth. Just going from P to D or R
We shall see.
Thanks for all the info
We shall see.
Thanks for all the info
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
Good question....don't know. Bought the car last week, and it's on it's way here now via transport. Like I mentioned, my mechanic thinks it's probably the Snubber. It's going in as soon as it gets here. Everything else about the car is great....
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
My 427/390 gives me a little bump shifting especially into reverse but I think it's just the nature of the beast. If your snubber is OK and you keep your idle down you still get a little bit of a bump but not a clunk. Your idle should be around 650 RPM's in gear and around 800 RPM's in neutral and park.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Its not the transmission, as it should not make a substantial RPM change going between park and reverse or the others....the converter regulates that...a hydraulic cam {stock} should be in the 500 RPM range in and out of gear at idle.......If a hot or hotter cam is in the engine then a stall converter will cure the noise....if the above replys happen to be incorrect
Thread Starter
Racer
Joined: Apr 2012
Posts: 340
Likes: 37
From: Plymouth, Ma
Its not the transmission, as it should not make a substantial RPM change going between park and reverse or the others....the converter regulates that...a hydraulic cam {stock} should be in the 500 RPM range in and out of gear at idle.......If a hot or hotter cam is in the engine then a stall converter will cure the noise....if the above replys happen to be incorrect
According to the receipts for the engine rebuild, a non stock cam was replaced by a stock one. The engine doesn't act like it has anything radical in it, however, when we dropped the idle down, the engine sure labored.
SNUBBER.....that's what I'm thinkin'!