The first part that will put the GMC in a museum is the transmission. There is only one we can use. We were blessed with having Tornado & Eldorado car share our gear grinder but that’s it and the last one made was in 1978. Front wheel drive transmissions after that date were too light duty to last and the fact that no one really supports that weaker one…….our numbers are limited to the TH425 Hydra-matic.
Because of all that and the fact of their age though, we are loosing transmissions and the expertise to repair them. We are loosing then because some of the major components in the transmission are no longer available. When a transmission fails and damages ket internal parts that’s it. Another coor must be found to build up. Also, because there was only one designed to fit and again the age we are also loosing the expertise to put them together the way we need them to pull 12,000 pounds.
Just like engine rebuilders, there are 2 rebuilder business plans…a “production” builder is focused on volume spitting transmissions out as fast as they can. Replacing the defective part, clean it up and put a gasket kit in is about all you get from this type of shop. It’s not they are bed mechanics but they repair transmissions on a budget…….reusing good parts, slapping it on the butt and leter fly.
The other builder takes more time…time is money so it’s pretty easy to tell a production shop and a performance shop. A performance shop knows their rebuilds will be stressed and must have the best attention to the details as possible. As many new parts as can be used, it’s not about finding the bad part and replacing it rather everything is considered. Specs sometimes are changed to beef up the build. The key is the attention to detail.
Here is the whole thing…..differential, torque converter and transmission. It’s said it’s similar to a Hydra-Matic 400 with a few differences…..like an outboard governor, no drive shaft, a constant velocity chain that turns the power from the back to the front
and independent drive shafts connected to the diff. . It’s getting increasingly harder to find good cores to replace some internal parts as I said before and with fewer techs knowing what to do with this thing….THAT is our problem!
Until recently we have been blessed with a GMC enthusiast who had taken upon himself to keep our transmission fires going offering the community properly built performance transmissions at a production builder cost and we all should give his all the thanks and adulation and for the most part the community did. Supporting him supported ourselves because the part could be had. Actually we got lazy because he took care of us. But after building over 500 transmissions himself it was not fair taking so much time from his real life. Knowing all good things one day must end……..now the GMC community is there.
We are back to searching for a production builder who will take the time to do transmissions the way we need them. But just like trying to get a Delmonico steak and Burger King….. the 2 styles of building simply do not interchange. If a builder sells a transmission for less than he paid for to build it his check book will tell him how many he can build. And if you are looking for a performance built transmission at the process of a production one…brother someone will not be happy in the long run.
So….this is now our problem to solve, how can we get a performance transmission built by folks who actually KNOW what they are doing ….at a cheap price? That’s like breaking the rule:
Quality, price, speed…pick 2……….. that is not reasonable to expect all 3….no one can survive and do it. So….. if hovering over a production guy to pay more attention is out of the question, how about working with a performance transmission builder to treat our transmission builds as they need to be……a robust brazed fin, bearing style low stall converter, attention to detail in the build, no plastic bushings….yada, yada, yada. I mean we MUST have as strong a transmission as we can find….no short cuts and all that!
Everyone, us as well as other dealers and GMC owners need a quality transmission. As you know, being focused on all aspects of a motorhome restoration we have to have the best of parts to put into our projects. I mean we do sell parts but some things we have professionals in their field to build component parts with their knowledge……. this is how you get the best of the best and as far as transmissions, the beat goes on and we need a source for more quality transmission builds.
So here is what I cam up with for our restorations and for that matter anyone else who needs a seriously built grinder box…………it happens that one of the number one hot rod performance transmission builders is here in Florida. Monster Transmission is in Brooksville @ 2 hours drive from us and @ 5 miles from our Honda generator guru Pinellas Power Products.
Look them up on line…… www.monstertransmission.com They are a pretty serious transmission company They advertise on Hot Rod Magazine and are hooked up with about anyone who what’s a snappy tranny for their rocket. Here is a truck project from Grease Monkey Garage getting a new set of gears These are the guys…That’s Thor there in the middle who produces the baddest torque converter on the planet
I’m telling you guys, these people are serious! I’ve made several trips to discuss the perameters of a transmission that would work for our 12,000 pound hot rods…they were very receptive so I gave them the challenge to come up with what we needs.
Let me introduce you to Monster GMC#1
This is a balls to the wall badass transmission specifically built for us. The low stall converter is a piece of jewelry Not only perfectly brazed inner fins but also outer. The center bushing as even performance converters use is also a bearing Every detail is put into these “THOR” converters. The transmission gets the same detail under the watchful eye of their head guy Tagged with an inspection report on each step. Boxed I loaded up our first Monster in the truck for a 2 hour drive back to Orlando Oh boy…..this should be good! Traffic sucked as usual down here and I lucked out dodging rain though it was wrapped in plastic….I wanted to hand the box on the wall! And our baby is now home ready to go in Monday into our next delivery and in just enough time. I hate rushing around…we had sent another tranny over for building 2 months ago..they found it had too much internal damage so we’ve been peddling overtime to get this baby here! OK…so I can hear the question…How much! Well remember we were lulled into thinking a production priced transmission is all we need. I’m sorry to tell you that build is no longer possible but with Monster I feel the quality that we need will be here still. Cost is $2695…comes with a 3 year prorate warranty….shipping from and to a Estus terminal is $100 each way. Go to their web site and read their full warranty sheet. They will also offer the “white glove treatment” that will even rent you a car while the transmission is fixed! labor, shipping and everything is covered if you want it!
Call them direct, ask for Gary (1-800-708-0087) when you call, he knows exactly what you want! Don’t call me, they sell their work directly to you as they do to me. See it’s not the price it’s the attention to detail and quality you should be happy to pay for…and I am!
I MUST have the best transmissions I can find and it sure looks like these guys have a good part of the market. Now, just as before…we need to support anyone who builds something custom for our GMC Motorhome. I am satisfied we are on the right track having a performance builder like this to support my restorations…and maybe you should too. We have to all help eachother…we get by with a little help from our friends and I feel we have a new GMC friend in Monster Transmissions…..check them out.
Andrew Hay says
i just read the pg about manny trovao’s diesel swap http://gmcws.org/blog/manny-trovao-announced-his-plan-to-provide-a-drop-in-diesel-engine-for-our-classic-gmcs/ and a little bell rang in my head… the gm 4L80/85 is afaik an o/d electronic th400, and gm just tossed mfg over to amg for hummer support since they don’t use it any more. could we get amg to make a fwd case for it? retain the th425 transfer chain and diff…
it is electronic, but gm performance had a standalone controller – it even triggered the o/d in every gear for 6sp! and they had a wide-ratio main planetary which coincidentally evened the gear splits out.
two hitches – i’m guessing that adding the o/d made the tranny longer than the th400/3L80, which would mean either moving the diff fwd or the engine back. and with a genuine o/d a lower final drive would probably be needed – even lower than the rare early 3.21. [anyone know where the original gears came from? iirc in the ’60s olds had their own unique rear axle, with ratios including 3.21…]
i’m also *assuming* that the lockup converter would be triggered the same way the switch-pitch converter is, and someone’s already done that on the th425.
Andrew Hay says
just ran across manny troavo’s diesel swap and your th425 problems pg
and a little bell went off in my head…
* the th425 is a th400 variant
* afaik the 4L80/85 is an electronic th400/3L80 + o/d
* gm has just tossed 4L80/85 mfg to amg for hummer support, since they
no longer have anything using it
could amg be talked into making a fwd case for the 4L80/85? yes it’s
electronic, but when it was new gm performance had a standalone tcm for
it that would even engage the o/d in every gear for 6sp, and a
wide-ratio main planetary that coincidentally evened the gear splits
i can see at least one problem, which could be a show-stopper:
i imagine that the addition of an o/d has lengthened the th400. for
the motorhome, that would translate into moving the diff fwd, the
engine back, or both. you’d know better than i whether either can be
done. i also imagine that even with a diesel the o/d would want a
3.55-ish final drive, and that would have to be cut. i vaguely recall
that the original th425 gears related to a unique-to-olds rear axle now
long out of production.
btw, i wouldn’t rate the 425 at 500 ft-lbs, unless you’re talking *SAE
GROSS* judging by mopar’s tf727/32rh and 4xrh families, i’d put it
more like 400 *SAE NET* – but if amg can be roped into this, they’ll