TH 375 Questions

My 70 455 was advertised as a th400 but its a th375. Its probably fine for my street build 455 as far as strength but has two problems.

It has some leakage when it sits up. Its been pegged to a leak in the shift arm seal. I'm advised the th375 has to be sealed from the inside. Drop the pan and valve body about $200-300 to do it. Sounds reasonable.

The transmission has a whine most noticeable in first gear (sounds like a truck with worn gears) also in third at 25mph just coasting along. The whine isn't loud enough to notice at speed and its also louder when the transmission is hot like a long drive. Advised its the pump. Tried a can of transmission treatment several thousand ago. Can the pump be replaced at the same service as the seal?


The transmission is performing well, just the small nagging leak and the whine, but shifts well and no slipping.


Thanks in advance --Chuck

 

This is incorrect. The TH375 manual shaft seals from the outside just like a TH400. The pan may have to be dropped to remove the shaft so the seal can be removed, but I don't think it requires removal of the valve body.

Not so simple. Servicing the pump requires removal of the transmission. Before you go to this expense and trouble, just service the fluid & filter. A clogged filter can cause the whine as well.

The TH375 is essentially a TH400 with a TH350 output shaft, and sports fewer plates in the clutch packs. Internally it's the same other than this. Sounds like it's time to find a new transmission shop...

Devon

 

i have a th375 and its identical to turbo 400 as far as i can tell. and what devon said, change filter and use 2 orings on filter.

 

OK I'll recheck. My general repair shop tried to seal the arm, he has the tool to pop it out but no go. He consulted his transmission buddy for the fact that its an inside repair. But the inside repair makes no sense based on the similarity between 400 and 375?? Why would GM make it so hard to repair the 375 vs the 400?

I'll be sure to doublecheck the filter.


Thanks Guys


Chuck

 

I don't understand how a TH375 got behind a big block, maybe its a replacement? How can
you tell its a TH375, cast into the tail piece? Vacuum modulator still on the passenger side,
I'm told there was a TH375B based on TH350, modulator at rear?

I have plenty of problems with TH400 leaks at their age. Speedo gear, when the plastic
gets worn. My shift shafts leak because the outside has rusted, right up to the seal. The
rust then ruins the seal. My solution is put in a shaft with no rust. Fortunately that is
possible without trans removal (just barely). Bend that retaining nail to clear the front cover;
there is just barely enough clearance to slip a cotter pin back later. I am running out of good
shafts. Some machinist needs to do a restoration on the outer surface. Maybe braze on a
little brass, then finish to original dia (and now rust proof)? Bruce Roe

 

My guess is that it could have made an easy swap into what may have started out as a small block car. Using the TH375 would mean a bolt-in regarding driveshaft length and yoke spline design.

Devon

 

The TH375 is a th400 with a th350 output shaft. Unless someone swapped the guts into a BOP case it should retain a Chev bell bolt pattern with a adaptor. The only time I remember seeing these units come into the trans shop they were in light duty vans and were from the 80's if I recall correctly?

 

It sounds like the BOP version is out there in limited quantities, found in Catalinas and Delta 88's behind their respective Pontiac and Olds engines from what I've read. There's mention of their use here http://www.442.com/oldsfaq/oftrn.htm including case tag codes before GM started using the corporate engine strategy.

Personally I've not come across a BOP TH-375 yet, the chevy cases seem to make up most of the ones I've seen out there, but there's at least some evidence backing up that this could be a BOP. We'll have to wait and see if we get more info.

Devon

 

You guys are correct on the application of the th375. I don't remember seeing them up here in anything other then the light duty vans and maybe some pick ups but a lot :beer has gone under the bridge since then :laugh: What we would see is the th400, th350 and th350c's and it was common to see the th350's with a unibell (chev/BOP. I wonder if most people ordered the th400 over the th375. I was correct that they are a th400 with th350 output shaft right?

 

Yup. My understanding is that they were used to reduce complexity at the plants, so that the driveline and crossmember position was more commonized regardless of engine choice. I guess even with fewer clutches than a TH400 they were probably more durable in a heavy car than a TH350.

Devon

 

I have never seen the 375 behind anything other then a small block so I would think those buicks listed with these transmissions were all SBB's.

 

It would be interesting to know if the 375 was used on passenger cars for a heavy duty option, like a towing package. o No:

Devon

 

The biggest engine I've seen them used with in my searching was the 400 Pontiac. Wouldn't be surprised if it was used behind the 400 sbc either.

Devon

 

Back in the day I swapped an st300 out for a th375 in my '68 skylark. I believe it came out of a mid-70's Pontiac Catalina with a 400 in it.


G