977L Transmission Piston Ring End Gap

[jezow2010]

We are rebuilding a transmission on a 977L and there are a number of rings that go into the pistons that seal the fluid. The original rings taken out of the transmission have a gap of 0.013" and the new rings we bought (CTP) have a gap of 0.025" on the ID ring and 0.043" on the OD rings.

Does anyone have a spec for these? There has to be something published somewhere...

The rings in question are 5M6132, 5M6141 and 7M1177.

To size the rings I ordered OEM CAT rings, one of each, standard size, and the gap was similar to the original rings. Thinking maybe the discrepancy could be in the CTP rings.

 

[pittsburgh cat man]

The ring gaps are in service manuel under power train specs. I would try calling your cat dealer and ask for technacal they should be able to tell you

 

[Nige]

The ring gaps are in service manuel under power train specs.

I would have thought that also, but they are not. The only reference I can find to them is a Reuse & Salvage Guideline SEBF8006-01 entitled "Clutch Housings and Clutch Pistons For Transmissions And Torque Dividers". Unfortunately that document is not freely available.

There is a SMAG SEBD1721 from 1977 with the following text (the bold is mine) -

More oversize cast iron seal rings are now available for clutch housings and clutch pistons. With the use of oversize rings, clutch housings and clutch pistons can be salvaged (repaired) if their bores are machined larger. A revision to Guideline For Reusable Parts, "Clutch Housings and Clutch Pistons For Transmissions And Torque Dividers" gives a complete list of all the oversize rings. The form number for the revision is SEBF8006-01 and this is a replacement for the former SEBF8006. The important addition to the revision is part numbers and dimensions for .030" (0.76 mm) larger rings for clutch pistons. Formerly, only .015" (0.38 mm) larger rings were available for the clutch pistons.

 

[jezow2010]

All we have is this, no ring gaps here ☹️

 

[jezow2010]

basically the trans has end gaps everywhere with what was in it... 0.014" all the way up to 1/16".

Not sure what would happen if we ran with a target gap of 0.035" and it was too much but I know what would happen if the gap was too small...

 

[pittsburgh cat man]

I know I've seen that witten is it possibly in the assembly instructions

 

[jezow2010]

Didn’t see anything there either. You saw it for a 977?

 

[jezow2010]

We are going to wing it and go 0.015”-0.035” on our gap spec. 12.5” diameter or so. How does that make everyone feel…?

 

[pittsburgh cat man]

Sounds about right

 

[jezow2010]

If it is too small or too large what do you think could go wrong

 

[pittsburgh cat man]

It’s been a long time since I did transmissions but those numbers sound right to me I think you will be fine

 

[Mobiltech]

It doesn’t surprise me that the Ctp rings aren’t sized right. I’ve had ctp planetary shafts that were oversized and the planetary gears became hard to turn when new needle bearings were installed. Ended up ordering cat shafts.
As long as the modulation valve can deal with the extra leakage it will work. Might be a slower shift.

 

[Nige]

I managed to dig up some information. Target end gap for those cast-iron piston rings is between 0.005" minimum and 0.020" maximum.

 

[jezow2010]

yeah I am not surprised either... it was just hard to point fingers when we didn't even know a spec.

Now that we have the spec to work to he can mix and match rings and get it right... I think he's working to about 0.013"

 

[Shimmy1]

All we have is this, no ring gaps here ☹️

How did you type that emoji???

 

[Shimmy1]

.

 

[jezow2010]

on my iphone lol

 

[Shimmy1]

on my iphone lol

I see how it is @CM1995

 

[Shimmy1]

My Droid can type them, but they don't show up.

 

[jezow2010]

ok besides the emoji talk...

One more question, the parts book (pic attached) shows two different piston types. first, third, second and reverse are the same while forward is different and has the different ring, noted.

In reality, both the forward AND reverse pistons are different on our trans. We have the rings for the forward piston but do not have a PN for the odd ring on the reverse piston. Does anyone have a different parts diagram or is there an update that was made? Machine SN is 11k07398