ST300 forward clutch

NoYahoo

Member
I'm working on an ST300 transmission. I have found the clutch piston cushions almost impossible to find. I purchased Viton material of the proper thickness and laser cut new cushions. I added about 1/16th of an inch to the height thinking there was probably some shrinkage/wear/compression on the old cushion. After I reassembled the forward clutch, I inserted the shaft and tried to turn the clutch hub. I couldn't turn it by hand. I wondered if anyone has any experience with knowing how tight this thing should be with new clutches and steels. I didn't think 1/16 of an inch would be enough to apply pressure on the clutches. Is this kind of tension normal due to the new clutch drive plates? I was able to purchase a NOS forward clutch piston and cushion recently and may have to exchange to verify if no one has experience with this issue. appreciate any help I can get. Thx
 
Well, with no responses, I moved on. Fortunately I found a NOS forward clutch piston complete with the cushion. Maybe the last one on the planet. I replaced the cushion I had made with the NOS cushion and could turn the inserted shaft afterwards. So I trimmed the 1/16th of an inch from my self made cushion and it worked identically. I guess these cushions maintain their shape even over long periods of time. The old ones were somewhat brittle though which is why I wanted to replace them initially.
 
Guys rebuilding their transmissions sometimes change the clutch pack size and have to watch the end clearance on the clutch packs. The typical desired figure is about .050". The rule of thumb is .010 clearance for each friction disc.

Couldn't find any posts about the ST300 clearances. There were plenty on the TH400 and Powerglide which followed the above example for typical clutches having 5 friction plates. Buick engineers dipped into the Powerglide parts bin here and there for ST300 items such as the forward clutch driven plates. So adding .062 to the cushion would be enough preload on the discs to apply the clutch. The clearance could be measured at the start to check for excessive cushion wear.

It helps to have the GM part nos when searching for the piston and cushion. The '66-69 ST300 used the cushion on the forward clutch. So did the Pontiacs and Oldsmobiles of these years using their versions of the ST300.

The pistons came in 3 thicknesses depending on clutch pack size. 6-cylinder cars had 4 drive/5 driven plates and used piston w/cushion part no 1394283 (formerly 1378662 and 1371510). V8 cars except GS had 5/6 plates and used piston w/cushion no 1394284 (formerly 1378663 and 1371511). GS cars had 6/7 plates and used piston w/cushion no 1394697 (formerly 1378664 and 1371512). There are examples of the first two types up for sale now on eBay.

The GS piston w/cushion was only used in '66 and may be getting scarce. If the supply dries up, one of the thicker types could be machined down to make one for a GS.

Some years the cushion was bonded to the piston and was not replaceable. The shop manuals go back and forth during these years on which pistons had the bonded cushions. The part no for the cushion
is 1386300 for the non-bonded piston.
 
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