70-82 Front Motor Mounts

[Joe L.]

A while back a member e-mailed me with a question regarding the configuration of the locking style motor mounts used from 1970-1982. Although I didn't have the time then, I thought I'd now provide the information on this board in case anyone else is interested.

As many probably know, in 1970 GM changed front motor mounts on a variety of its vehicles, including Corvettes, to a locking style, "safety" mount. Supposedly, cars equipped with the earlier, non-locking type mounts had sometimes experienced safety related problems when a motor mount failed and allowed the engine to rotate excessively. The new mounts featured a "tab" which would restrain engine rotational movement in the event of seperation/breakage of the vulcanized rubber portion of the mount. In 1971, the earlier, non-locking style mounts were removed from service and replaced with the locking mounts to service all applications. After that time, the non-locking style mounts could no longer be obtained through GM Service Parts, but were still available through aftermarket channels.

During the 1970 through 1982 period only two different part numbers for front motor mounts were used. For 1970, 1971, and 1972, the PRODUCTION mount was GM #3980701. For 1973 through 1982, the PRODUCTION mount was GM #330973.

The SERVICE engine mounts were the same as PRODUCTION mounts, but, to fully understand the situation for service, I'll need to provide some additional information. As I've mentioned in the past, PRODUCTION part numbers are the part number for individual parts. KITS are never used in PRODUCTION. In SERVICE, GM may supply the PRODUCTION part in a KIT which includes several other components associated with installation of the part. The part number for the KIT is different than the part number for any of the components of the kit.

For 1970, PRODUCTION motor mount GM #3980701 was available as a single unit under the PRODUCTION part number. Some of these units were supplied with the motor mount bolt and prevailing torque nut fastened to the mount, but, for whatever reason, this was not considered a "kit". The number 3980701 was molded into the rubber portion of the mount. The mount was not supplied in a box.

For 1971, GM part # 3980701 was replaced with part #3990916. This was EXACTLY the same motor mount, including having the 3980701 part number embossing, but it came in a box as a KIT. The kit included the mount, bolt, nut, washers, and an instruction sheet which described how to modify Corvette lower ignition shields when using this mount to replace the 69 and earlier, non-locking style mounts. Part number 3990916 continued to service all 63-72 Corvette applications until February, 1977 when it was discontinued and replaced by GM #6258154, the 73+ mount KIT.

Part #6258154, which became available in 1973 to service 73+ Corvettes, was a mount KIT which included a PRODUCTION GM #330973 mount, bolt, nut, washers, and instruction sheet as described above. The part number 330973 was embossed on the side of the rubber portion of the mount, in the same position as was done on the earlier mount. GM part number 6258154 continues in SERVICE to this day to service all 63-82 Corvette applications.

Although PRODUCTION mount GM#3980701(including SERVICE mount KIT GM#3990916)and PRODUCTION mount GM #330973(including SERVICE mount KIT GM#6258154)are functionally equivalent, there are two important configuration differences. First, the earlier mount uses forged steel, spot welded oval-shaped reinforcement/spacers for the mounting through-bolt holes. The later mount uses a fabricated steel reinforcement/spacer which is mig-welded to the steel core of the mount. These details are difficult to detect with the mount installed in the car. However, the second difference is easily detectable. On the earlier mount the locking "tab is flat, with beveled ends which are turned down slightly. The later mount uses a tab in which the forward edge is "rolled down" to provide reinforcement. To the informed observer, this difference can be detected at a glance.

Incidentally, every aftermarket or "reproduction" mount that I've seen is TOTALLY INCORRECT with respect to configuration and construction. Many of the modern-day available versions are foreign-made and use thinner steel. Both original mounts use steel which is 11/64" thick; "reproductions" are often 9/64" thick.

[bob g.]

Re: How About '69 Motor Mounts

In 1969 Chevrolet had a recall for defective motor mounts. The fix for this was a bracket added to the right front exhaust manifold and a braided steel cable lashed between the bracket and the upper control arm. Does anyone know if Corvette was part of this recall? Examples of this can be seen in junkyards that still have 1969 Chevrolet passenger cars, but I don't know the extent of the recall.

[Joe L.]

Re: How About '69 Motor Mounts

Bob----

I remember the recall you speak of, but I don't remember the exact details of it. However, I can tell you this much. I purchased my 69 Corvette new in 1969 and I still have it. My car was a very late build, with a body build date of September 14, 1969. My car definitely was factory equipped with the non-locking style front engine mounts. My car did not have any type of restraint cable, which was the method used in the recall to "fix" the cars. Also, I never received any recall notice, nor was my car so-repaired without my knowledge on a service visit. I first replaced the original engine mounts about 1973 with the GM service replacement engine mounts of the locking style. The replaced mounts appeared to be the "plain, old" V-8 engine mounts and no restraint cable was installed.

The 69 and older Corvette engine mounts were used in other applications. However, this particular mount,although very similar to other Chevrolet V-8 engine mounts, was used primarily in only 62-67 big block passenger cars. Perhaps, for whatever reason, this mount was not subject to the recall.

[Kenneth G.]

Re: How About '69 Motor Mounts

Joe ,

Was just curious as to why you had to replace your motor mounts in 73 . On my 69 when i was putting the motor back onto the chasis i used N.O.S. non-locking motor mounts to keep originality. I am wondering about the safety issue now and if this was the right move. Any comments would be appreciated.

Kenny-#31871 (new kid on the block)

[Gary Schisler]

Re: How About '69 Motor Mounts

Although I have never broken a motor mount, I decided to replace my original non-locking mounts with the new style locking or safety mounts when I rebuilt the engine. My 66 is a driver, although it is correct is virtually every detail. Sometimes you do what is best for your car and not for the judging manual. If I ever decide to do the judging thing with this car, it would take me a long weekend to replace things like the motor mounts and a few other odds and ends.

[Gene M.]

Re: How About '69 Motor Mounts

Joe, Will the guys using the safety motor mounts have problems correctly fitting in the lower side of the block ignition shielding? It seems to me that the shield interferes at the 3rd mounting screw if I remember correctly.

[Joe L.]

Re: How About '69 Motor Mounts

Gene----

Yes, if the locking style mount is installed on a 69 or earlier small block the lower "V" shaped plug wire shields have to be modified. Instructions for the modification are included with the GM service mount kit. An alternative would be to use 70+ lower shields. These are configured to fit properly with the locking style mounts.

Big blocks, of course, did not use the lower shields so no modification is neccessary when installing the locking mounts on a big block car.

[Joe L.]

Re: How About '69 Motor Mounts

Kenny----

During the first 7 years or so of its life, I used to drive my 69 on a daily basis. By 1973, my car had accumulated about 80,000 miles and I decided that it was time to replace the engine mounts. As was my general policy, I would only use GM parts to repair my car and, by that time, the only engine mounting available from GM was the locking style mount. Although I was always concerned with originality, I would make a change if I thought that it was for the betterment of the car(like stainless steel brakes, for example). I figured that these mounts must represent an improvemnt and a GM-approved one at that. I figured that GM wouldn't have started using the new-style and more EXPENSIVE mounts on their cars if the old style worked just as well.

I am surprised that you were able to locate any NOS GM non-locking motor mounts. These were discontinued in 1971 and I would think that these would have been quickly exhausted or purged from dealer stocks. Are you sure that someone didn't sell you "reproduction" or aftermarket replacement mounts? These are very common and easy to get, but the quality is not up to the GM mounts. In conclusion, let me put my feelings this way: If I were able to obtain some NOS GM non-locking style motor mounts would I use them on my car? No. I'd buy them and just put them into my parts collection.