70, motor mounts, 6 bolts,

[Warren L.]

Wondered what the correct finish is for the motor mounts (#6258154), are they just left natural?

Have tried to find the correct 6 bolts total for the two mounts in the pile.

They are listed as 3/8-16 x 15/16. I have found 6 with a half inch shoulder with no threads. Their head markings have three lines and and S B on the head. Can you tell me what I am looking for and if these are wrong do you know where these end up? I would post a photo of this bolt if I could stick it here by attachment.

Thanks,

Warren

[Warren L.]

6 bolts,

Just found an unthreaded section on the flywheel so I think these 6 bolts with the raised shoulder are to bolt the pressure plate to the flywheel not the motor mounts to the block.

Warren

[Joe L.]

Re: 70, motor mounts, 6 bolts,

Warren-----

The 70+ motor mounts were natural steel except for the areas where the bonded rubber sort of covered the steel. The currently available GM mounts that you mentioned (GM #6258154) are the best ones available, by far. However, they are not configured exactly like the originals which were of GM #3980701. The difference is that the oblong "spacers" (through which the long bolts pass) is different. The original mounts used spacers which were a casting or forging and these were attached to the frame of the mount by spot welds. The current GM #6258154 uses spacers which are fabricated steel, have a parting line (gap where the ends meet), and are mig welded to the frame. It's very hard to see this difference, though.

The motor mount bolts (to block) were a 3/8-16 idented ("untrimmed") hex head bolt of 15/16" length (full thread) with a black phosphate finish. They had an unusual head marking of one long straight line bisecting the center with a perpendicular line intersecting the long line at the center (i.e. a cross with one leg missing). This mark indicates a bolt manufactured of GM-275-M material. On my 1969, the original bolts had an "anchor" manufacturer's headmark. Other manufacturers were likely used, too. These bolts, GM #3748038, were used in PRODUCTION from 1964 through 1973, but were never available in SERVICE.

Starting in 1974, bolt GM #331246 was used for the motor mount to block application. It is the same dimension and finish as the 64-73 bolt, but it is manufactured from GM 300-M material (SAE grade 8/ 6 radial lines) and it is a non-indented ("trimmed"), hex head bolt. It will work perfectly for the 64-73 applications, too, although it's not, strictly speaking, "correct". It's still available from GM for a current GM list of $4.01/each.

How come the 64-73 Corvettes got GM-275-M motor mount bolts whereas the 74+ got much stronger GM 300-M bolts? I don't know.

[Warren L.]

Re: 70, motor mounts, 6 bolts,

I checked my orginal mounts and they are the same number 3980701 which is on the rubber on one side.

The replacements purchased from GM in 1978 appear the same are part number 330973, they do have the parting line you mentioned.

Will search for the bolts you detailed tonight, any ideas on these other 6 if they are for the pressure plate? will e-mail a photo.

Thanks,

Warren

[John H.]

Re: 70, motor mounts, 6 bolts,

Pressure plate bolts are 3/8"-16 (coarse thread), and flywheel bolts are 7/16"-20 (fine thread).

[Chuck S.]

Re: 70, motor mounts, 6 bolts,

Warren, I have one motor mount bolt cataloged, but the other five are not recorded (I wonder where...???) That one is 3/8 NC X 1" long (it could be 15/16" as Joe described). The head stamp is "T" with a kind of rectangle. The rectangle is not straight sided but, as I remember, but has angle breaks in the sides. I could describe it better if I had it in front of me (cartouche? ).

The pressure plate bolts on my base engine (at least the one I can see) are "N" in a circle with no radial lines. The head is not indented or flanged, and the circle around the N is large, almost to the flats. I add this comment about the circle size because there are several bolts on my 70 that have a small circle around the N.

As Joe said, either of these head stamps may mean nothing to your car because fasteners could be from several sources. My 70 is a very late car.

[Joe L.]

Re: 70, motor mounts, 6 bolts,

Warren-----

The mounts that you purchased in 1978 are the GM #6258154 mounts which are still available today. GM #6258154 is the part number for a mount UNIT. It includes 1 mount of GM #330973, a through bolt, a lock nut, and some washers. The number 330973 is embossed on the rubber portion of the mount. This mount was used in PRODUCTION for 1973-82 Corvettes and it is SERVICE for 63-72 Corvettes. As far as I can tell, the 330973 mount was never available seperately in SERVICE; it was always supplied only as part of the 6258154 unit.

L1970-72 Corvettes originally used mount GM #3980701. Originally, this mount was available in SERVICE as a seperate part under that number. Later, it was included in a unit under GM #3990916 which also included the same hardware as supplied in the 6258154 unit. Thereafter, the 3990916 was discontinued and replaced by the 6258154.

The pressure plate bolts have a unique configuration. They are 3/6-16 as John mentioned. Also, they are 1" long, but they have an approximately 3/8"-1/2" unthreaded shank below the head. This is a non-standard configuration for a 1" bolt; the standard configuration would be all-thread for this length. So, if you have 6 bolts which are configured like this and are of grade 8 strength, there's a VERY good chance that they're pressure plate bolts. For me, though, it would be somewhat moot. I would never re-use a pressure plate bolt, especially a stock variety one. No one can see these bolts, anyway, and you don't want to compromise anything here. I use only ARP pressure plate bolts and ARP flywheel bolts, too.

The best e-mail address for me these days (until I get my new computer set up and, even, afterwards, for that matter) is joelucia@comcast.net. I can respond from that address (at least, I still can at the moment). My joelucia@yahoo.com address is "receive-only" until I get things squared away here.

[Joe L.]

Re: 70, motor mounts, 6 bolts,

Chuck------

Yes, GM bolts "official" length is often a "skoche" different than the measured length. I don't know why.

The "T" headmarking that you describe for the motor mount bolts is the GM-275M material ID mark. The manufacturer's ID that you mentioned sounds like the ID for Elco Fastners (now a division of Textron). Elso was once a major supplier to GM.

Bolts with a large raised circle as you described your pressure plate bolts are equivalent to SAE grade 8, although I think they have some special properties exceeding grade 8 standards. The bolts that GM has used for this application for the last 20+ years, or so, have usually had the 6 lines, though. The large raised circle headmark bolts are commonly seen for small block head bolts, exhaust manifold bolts, and some main cap bolts.