I desperately need some help from somebody that knows automatic transmissions inside and out. Last year I changed my ST400s oil and filter in my 67 GS400. The tranny has the new broiler pan style trans filter. In the process of installing a new filter, I inadvertently overtorqued the filter retaining bolt and pulled some threads in the valve body. ou: This summer, I dropped my trannys oil pan to determine the extent of the carnage and helicoil the damaged threads in the valve body. The retaining bolt for the oil filter is a 5/16-18 bolt. It is 5/8in long from the end of the bolt to where the shoulder of the bolt bottoms out against the outside of the valve body. However, at the shoulder the bolt necks down to the bolt's minor diameter for approx 1/8in. Please see the attached PICs. Why was the bolt designed like this? I was going to install a standard .781in long helicoil, and then got to thinking that I may be creating more problems than I would be solving. That is, is the bolt necked down like it is for a reason? Does an oil passage in the valve body intersect the bolt for example, and does the necked down section allow oil to flow around the bolt? If so, the helicoil that I planned on installing would drastically impede the oil flow through that passageway. I kinda doubt that this is the case, as I have to believe the shoulder of the bolt would not seal very well against the valve body, and any oil going around the bolt would also leak past the bolts shoulder. But I need to ask the question, why was this bolt designed this way? o No: I cannot see up inside the filter retaining bolt hole in the valve body very well. My car is up on jackstands, between being low to the ground and my fighting my bifocals I cannot see any passageway that intersects the bolt hole. But that doesnt mean it isnt there. It means that I just cant see it. Thats why I am asking for help. HELP!!!!! I just do not understand why a regular shoulder bolt, without the undercut, was not used to retain the trans filter. It would have been a cheaper bolt and considering the volumes it would have saved GM a bunch. Additionally, I probably would not have pulled the threads in the valve body like I did if a standard shoulder bolt was used. Thanks,
The bolt has no function other than to hold the filter in place. The shoulder stops the bolt from gripping the filter tightly so there's no side load on the filter-to-tube o-ring. The fact that the bolt has a "neck down" behind the threads is probably just a characteristic of how the threads were rolled in the manufacturing process. The bolt hole should just be a blind hole. Devon
That bolt was most likely made in the Hydramatic plant in Ypsilanti. Any board members out there know about this? I don't think it's significantabout the rolled thread but I think the undercut is there to allow the filter to center freely on the taper under the head of the bolt. o No:
Thanks for the information. :TU: I started thinking about the repair (thinking is usually my downfall, I don't do it very often because it's so hard) and I thought that there may be an ulterior motive for the undercut. I only get one shot now at repairing this 42year old transmission, I already had one awsh*t. I would rather ask a really stupid question such as this one, than ruin a irreplaceable (numbers matching) transmission. The bolt has a shoulder (just above the undercut) that bottoms out against the valve body. The shoulder is where the steel and rubber washer go along for the ride :3gears: , and the filter gets trapped between the washers and the valve body. Thanks again.
