Powerglide advice - Impala Tech
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post #1 of 9 (permalink) Old 03-07-2020, 07:23 PM Thread Starter
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Powerglide advice

Hey fellas. I have a 66 impala with what I believe to be an original non refurbished powerglide. The day after I got the car home I noticed it was leaking transmission fluid. I thought it was coming from the rear seal so I had my mechanic replace the rear seal as well as a new transmission pan and seal. After replacing those it’s still leaking. He said it might be from the torque converter seal or the pump seal. It’s also leaking from a steel transmission fluid line up front.

I called a transmission shop in my area and they said they would charge about $500 to drop the tranny and replace the other seals. A shop that I used before and offers a lifetime warranty said $700-$800. The car has 127K miles that I believe are original. Am I reading that correctly? Here’s a pic. The transmission shifts just fine. Should I go the extra mile and get it rebuilt since it has 127K miles? How long are powerglides known to last?
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post #2 of 9 (permalink) Old 03-07-2020, 08:17 PM
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First thing about old cars is they use old technology. The seals in your tranny swell with tranny fluid to seal better (assumes they didn't crack when they dried out from sitting). Some of those leaks may seal themselves with use.

Can't state this often enough. Transistors and plastics didn't exist before the early sixties and the engineers where still experimenting with different types of plastic for different applications.

I have seen a lot of leaking cooling lines. It is usually due to some one over tightening the line and cracking the case where the line threads in (it is an NPT compression flair so it needs some pressure but keep King Kong and Godzilla away from tightening anything).

If those numbers on the case (date of build mostly since a base engine PowerGlide in 1966 won't have a partial VIN stamped on it) are really important to you; you can heliarc the crack closed along with most of the threads around the crack as well, to try to use a more modern O-ring to seal it up (but it is going to require time in a machine shop and lots of money).

Buy an inch pound torque wrench. You will need it on every small fastener (such as timing covers, oil pans, and valve covers as well as your transmissions oil pan). Failure to tighten the sheet metal to the correct value (found in the Chassis Service manual) will cause the edge of the pan to bow creating a leak. Tightening it more only makes the leak worse.

You have to remove the sheet metal (which mechanics call tin, though there is no tin metal involved) and flatten the lips with a hand held anvil and a sheet metal hammer just like doing body work. Only then does the paper gasket stand a chance of not leaking.

The only seal up front is ... not surprisingly called the front seal. Like the rear seal it has to be replaced if it is worn (at 127K miles it probably is). Another reason to rebuild the tranny is if it has been run without fluid (say by leaving it sit twenty years with out being "PROPERLY" filled), then you risk burning up the clutch discs and drum friction material (it is after all only made of paper, not asbestoses like a dry disc in a clutch). As the friction material is burned up from running dry you will also burn up the steels and drums my metal on metal contact. So depending upon what the mechanic doing the work finds inside you may need another rebuildable junk yard PG core to replace what was damaged (it is also a lot cheaper than trying to salvage your old case).

Finally a PowerGlide is a broken TH350. In 1969 GM introduced the TH350 as the replacement for the PG. It is the same size as the the old PG, and uses the same output splines so it is a bolt in replacement. The TH350 uses the same two gears you have now but adds a first gear for better acceleration (torque multiplication). Driving around with a PG is like taking off all the time in second gear with a TH350.

Big Dave
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post #3 of 9 (permalink) Old 03-07-2020, 09:27 PM Thread Starter
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Thanks Big Dave. I donít mind rebuilding the transmission but itís just that it runs so good now... I think I may have the transmission looked at and see if it needs a rebuild. Will the transmission have to be completely opened to find out? I thought about a TH350 but GM doesnít make them anymore and I wouldnít want to buy an aftermarket one. Iíd rather get the real deal. Is there an aftermarket brand TH350 thatís preferred by impala builders thatís better than the GM version? Also, Iíd rather buy a brand new transmission and not a rebuilt one if I went that route.

Jack
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post #4 of 9 (permalink) Old 03-07-2020, 11:12 PM
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Back when I rebuilt them I started with a junk yard core and blew it appart. i then inspected the parts as to their condition.

A TH350 (and the PowerGlide that it was derived from) are not particularly strong (as a reference to their torque rating). The TH350 has been called a 3L60 since 1990. Where the 3 refers to the three forward gears, L stands for longitudinal (rear wheel drive: vs. a T for transverse or front wheel drive), and the 60 is the torque rating; or how strong it is compared to an Allison 1000 transmission. The Alison is rated at 100 (for a ten thousand pound gross vehicle weight light truck: aka a one ton).

The little 4T40 transmission can handle a whopping 180 foot pounds of torque by comparison (Chevy 3.8 L V6 in a vehicle under 4,000 pounds). So GM has a 40, 50, 60, 65, 80, 85, 90, and a 95 rated transmission to meet the power and weight of it's fleet. The 3L60 is similar in strength to the 4L60 (formerly called a 700R4) and can use a lot of the parts found inside the 4L65 to beef up the 3L60 to a 3L65. This is strong enough to handle a 350 SBC in a 6,00 pound pick up or van.

You will also see an "e"attached at times but that has pretty much been neglrcted as it stands for electronic or computer controlled and GM has been using TCU or ECU with a TCU built into it for the past 28 years. So most just now drop the "e"; but there is a difference in the transmissions (one is computer controlled and the other is fully hydraulic with a TV cable for shifting).

You can make a TH350 look entirely stock, but have enough aftermarket parts inside the case to allow it to handle the torque of a big block. However Chevy never intended the TH350 to be behind a big block. All Big Blocks with an automatic received a TH400: designed just to handle the torque of the new engine. Both the BBC and the TH400 were introduced in 1965.

The difference between a TH400 (now called a 3L80) is in the size and weight of the transmission. The TH400 is physically bigger than the TH350, and because it uses cast iron rather than aluminum parts inside the case it is much heavier than the TH350. But it is also a lot stronger capable of handling a 454 in a 14,000 pound RV (the RV version is called a TH475 and has very HD parts inside that racers like).

So a properly remanufactured TH350 is just as good as a stock one from the factory. Additionally you can modify it with a shift kit, or a trans-brake, or a reverse shift pattern valve body, and use case harden cryogenically treated parts that makes it much "better" than stock. But that better is subjective and cost money.

Big Dave
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post #5 of 9 (permalink) Old 03-09-2020, 06:59 AM
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Front seal leaks are notorious in the power glide transmission. I can't remember the number of transmission I dropped out of cars to have my grandfather perform the repair back in the day.

You will probably get better fuel mileage with a TH350, but if your going through all that work I would personally go to an 200R-4 overdrive if you don't have some engine putting down over 400 Ft lbs of torque. The 200R-4 is within a 1/2" of length of the powerglide so you can use the same drive shaft. You just need to modify your transmission cross-member which you would have to do for the TH350. Then you just need to hook up a Throttle Valve cable. The 200R-4 isn't as strong as a TH350, but it does give you overdrive and fuel economy at interstate speeds. Both the TH350 and the 200R-4 will require almost the same number of mods to get them to work.

Now if I was building a street and strip car a TH350 or even better a TH400 would be the transmission to go with. Unless of course you really want to fork out the cash and go with a Lanko.

Have to remember back when these cars were built interstate speed limits were 55 MPH. Now days 65 MPH is the low end and here in Michigan it is 70 MPH with a couple of roads going to 75 MPH.
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post #6 of 9 (permalink) Old 03-09-2020, 05:58 PM Thread Starter
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I decided Iím going to just have it rebuilt since I will have the car for at least 7 years and would hate to pay hundreds of dollars to have the transmission dropped and seal replaced only to have the transmission give me problems later on.
Can I have it rebuilt and keep my original core?
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post #7 of 9 (permalink) Old 03-09-2020, 06:55 PM
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Of course, unless the case is cracked. A Glide is the easiest auto to rebuild, and probably the least expensive. If you're handy, you could do it your self!


Am I the only that's tired of hearing "a Powerglide is a broken TH350"? Is a TH350 a broken 700R4? Is a six speed a broken 8 speed? Is the 8 speed a broken 10 speed? No to all. A powerglide is a reliable, easy to work on trans that is found in many many of our cars. Lots of us have no interest in replacing them. Sure, TH350, TH2004R, TH700R4, TH400, etc, swaps all bring benefits. So does fuel injection, disc brakes, better suspension, FM radio, bluetooth, XM radio, and so on and so on. Modifying is fun, I have some very non-stock cars. Keeping things original is a perfectly acceptable part of our hobby.


Jason.
Owner of:
- two one speed trans (Volt, ELR)
- one two speed powerglide (65 Impala)
- one three speed manual (65 C10)
- one three speed auto (71 Pontiac)
- one six speed manual (96 Impala)
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Two doors, four doors, wagons, and ragtops.
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post #8 of 9 (permalink) Old 03-19-2020, 12:24 AM Thread Starter
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Dropped the car off to have the tranny rebuilt. Thanks guys 🙂
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post #9 of 9 (permalink) Old 03-25-2020, 02:13 PM Thread Starter
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Picked up the car yesterday. It runs very good now 🙂
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