78 Doesn’t like going into Reverse (often grinds)
Thread Starter
Instructor
Joined: Mar 2020
Posts: 167
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From: Virginia
78 Doesn’t like going into Reverse (often grinds)
I have a 78 Corvette, recently rebuilt the engine, 400sbc. I also put a new clutch kit in it when I rebuilt it. The only problem I’m having, and I can’t remember if it did it before is; when shifting into reverse, it doesn’t always want to go into gear. Sometimes with the clutch fully depressed it will just grind, like it isn’t engaged at all. When this happens, either pressing and releasing the clutching a couple times improves it or shifting into first and then back to reverse fixes it and it is a smoother shift. Any ideas? The rest of the gears are generally fine, now and then it is harder to go into gear but nothing I would say is abnormal.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Did you test your pressure plate for full disengagement at the travel distance allowed by the Corvette clutch linkage? A lot of pressure plates are not suitable for use in a Corvette due to the travel distance required to achieve full disengagement, regardless of how you adjust the linkage. See my detailed article here:
https://www.corvetteforum.com/forums...vel-range.html
Lars
https://www.corvetteforum.com/forums...vel-range.html
Lars
Burning Brakes
Joined: Sep 2020
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From: Chapin SC 29036
It could also be that with the clutch engaged the input shaft of the transmission is spinning. When you press the clutch the shaft keeps spinning for a while. Reverse does not have syncros on it and it you try reverse too quickly it may grind. I always put it in first to stop the rotation and then go to reverse. An extra step, but you don't hear that grinding sound and your tranny might last a little longer.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
There's actually a GM spec for how long the input shaft should keep spinning after you have depressed the clutch.If the pressure plate is in-spec (as defined in my article noted above) and the linkage is correctly adjusted, a transmission that is up to normal operating temperature should go into reverse without grinding with the clutch fully depressed for 9 seconds. if you cannot put it into reverse without significant grinding after 9 seconds, you have an out-of-spec pressure plate, or you have not adjusted the clutch correctly. You can "Band-Aid" the problem by pushing the tranny into 1st or 2nd gear first, and then quickly shifting into reverse, but that doesn't make it "right."
Lars
Lars
Thread Starter
Instructor
Joined: Mar 2020
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From: Virginia
So I didn’t test it, I read your post and if testing it requires the hydraulic press I don’t have one of those. I did use the McLeod Super StreetPro Clutch Kit 75224. I couldn’t find where is says the disengagement distance.
It does definitely goes to “normal” when I shift back into 1st then reverse. So maybe that is just the issue.
It does definitely goes to “normal” when I shift back into 1st then reverse. So maybe that is just the issue.
Thread Starter
Instructor
Joined: Mar 2020
Posts: 167
Likes: 42
From: Virginia
There's actually a GM spec for how long the input shaft should keep spinning after you have depressed the clutch.If the pressure plate is in-spec (as defined in my article noted above) and the linkage is correctly adjusted, a transmission that is up to normal operating temperature should go into reverse without grinding with the clutch fully depressed for 9 seconds. if you cannot put it into reverse without significant grinding after 9 seconds, you have an out-of-spec pressure plate, or you have not adjusted the clutch correctly. You can "Band-Aid" the problem by pushing the tranny into 1st or 2nd gear first, and then quickly shifting into reverse, but that doesn't make it "right."
Lars
Lars
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
It means it's working normally and in-spec. Nothing wrong with it. No adjustment needed. To prevent the grinding, just push the shifter into either 1st or 2nd before you go into reverse, if you don't want to wait the 9 seconds. As Robertea noted, reverse has no synchros, so it will grind if the input shaft is still spinning slightly. It takes 9 seconds for it to slow down and stop spinning.Completely normal.
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Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Thanks for confirming my 9-second spec! I knew I remembered that from my tech memory... who says I'm going senile yet..? And people think I just pull this chit out of my ****...
Lars
Lars
Le Mans Master
Joined: Aug 2005
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From: Temple Terrace Florida
When you replaced your clutch, did you change the throw out bearing? If so, are you sure you installed the correct one? GM made one for passenger cars and one for Corvettes. The passenger car bearing will cause the exact symptoms you describe. I learned this the hard way on my 68 after NAPA sold me the wrong one. Jerry
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
The wrong TO bearing changes the linkage geometry and can screw things up in terms of adjustability, but it does not change the total travel distance of the Corvette linkage system. Chevrolet has used only two sizes of throwout bearings for decades - 1-7/8" for passenger cars and trucks (flat-finger clutches, part number 908092), and 1-1/4" for Corvettes (bent-finger clutches).
You can e-mail me for a complete copy of my paper, "How to Replace, Check, & Set Up Corvette Clutch Systems" for a detailed description of the different throwout bearing heights, clutch fork ball stud heights, clutch fork designs, and clutch linkage geometry.
Lars
V8FastCars@msn.com
You can e-mail me for a complete copy of my paper, "How to Replace, Check, & Set Up Corvette Clutch Systems" for a detailed description of the different throwout bearing heights, clutch fork ball stud heights, clutch fork designs, and clutch linkage geometry.
Lars
V8FastCars@msn.com
Last edited by lars; Nov 3, 2024 at 10:41 PM.