What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Doublers, cross members, and busted transmissions

BadDog

BadDog

SOL
Staff member
Super Moderator
Author
Joined
Feb 24, 2001
Posts
8,003
Reaction score
232
Location
Phoenix, AZ
Background:

Some of you know about the fiasco I've had with busted transmissions in the last few years, not to mention the bad luck on the last few replacements that never even made it to a trail. In the past, I've had the rear bolts stripped and heli-coiled on one, and broken 2 other housings along with a bell housing (in the classic location). Part of it is my fault relating to the square shafts, bent/broken springs, one trip without my skid (big mistake) and getting too throttle happy. Finally got a good one (I hope) and don't want to break the housing again so I'm making some changes.

First up, I have always ran a cross member only on the doubler adapter, which put it about 6" behind the 203 adapter. That kept it clear of the front shaft, and I figured it wasn't that far out of line with the factory 203, long adapter 205, and other cases, so that may contribute as well. And I guess GM thought that front adapter alone was good enough for the 203 with all the weight hanging off the back, seemed to me this was better than that.(?) Stephen and others think it also needs a front cross member for the 203 adapter, but then you have drive shaft clearance issues. Frankly, I think the cross member was not an issue, and every catastrophic break I had was due to bent/broken springs (allowing rear axle to move forward fast when the front spring arch dog-legged) or from direct impact on the 205 case.

The plan:

Part 1:

My current plan is to build a sort of cradle that bolts solid to the doubler adapter and the 203 adapter. Then it will have a riser (sort of like a wall) that goes up the front of the 203 adapter and bolts to the th350 rear 2WD mount surface. So, that would tie all 3 mounting points into the cradle to help eliminate much of the load on those 4 little bolts, which are all that hold the adapter to that aluminum case.

Then there is the matter of tying ot the cross member with bushings. One option for bushings is to use stock case mount bushings flipped over so the "C" is on bottom. Then the cradle mounts set in the "C" with the blocks on top. Or, poly spring bushings could be fitted close to the side of the case, with the bolt parallel to the frame rails. Either way, it would be midway between the doubler adapter and the 203 adapter.

These mounts would support the cradle above a cross member built from 1.5 x 0.188 square tube. This, along with poly motor mounts, should flex well enough without being over stiff as they are (IMO) when located out by the frame rails. Considering the people who do get away with the wide spaced bushings, this should be fine.

Part 2:

You know those OE struts from a "K" or "HD" converter inspection cover up to the engine mounts? And the ones put on the FWD side of many 205s, 208/241s, and others that ran up to the bell housing bolts. Well, I'm thinking it would work better to run from the top bolts of the 203 adapter up to the 10 and 2 o-clock bell housing bolts. Maybe with a coupler nut or turn buckle to allow a bit of preload tension (or at least exact adjustment). That would keep the heavy 205 from trying to wag like a vertical tail, flexing the transmission housing till it breaks the rear off, and should take some load off the bell housing as well where it is so frequently broken near the top.

Doesn't look like there is much way to do the same on the bottom, though it would be nice. Maybe I could find a way to run one or both up from my cradle to the bottom bolts? <shrug> Seems like the top would be most helpful anyway since the loads would be tension, and the cradle will help with the corresponding compressive loads on the bottom when you drop hard and hit something with the frame or something.

What do you guys think?

What about that cradle tied to all 3 points? It's going to have to be pretty precise to keep from putting too much pre-load on the trans housing. I have no mill or other precise machine equipment, so I figure it will need shimming (like a starter) at the least, or otherwise some built in adjustment of some sort. Or, maybe just a little preload and a 2WD poly mount? I don't think I want to go that route, thoughts?
 
Well, it really didn't. Full size converters are really too big. I eventually got it to fit with countersinking the pump/retaining bolts 0.100 more and switching to thin hardened washers to get the heads flush (still left over 3/8" of aluminum below them) AND adding an 0.092 shim (sort of like a mid-plate). Then, after getting it in there with the converter *barely* clearing, it leaked from around the pump bolts AND I found out that the housing I was working on had hairline cracks everywhere. Basically, junk. I also found out found out from a local trans builder that does race trans that he uses sealant on the bolts in spite of the directions to the contrary to keep them from leaking.

So, the bell is sitting in my shop for now. If I had not minded buying a smaller diameter converter, and sealed the bolts as the tranny guy suggested, it might have been ok. But when the case was spider-webbed, I just got disgusted with it all…
 
What no fabricators and closet engineers left here? Anyone got a thought on this?
 
via PM we discussed this a while back... it seemed at that time (and even more now) that you had a reasonable plan. I'm not even close to being an engineer, but I don't see why the craddle + struts wouldn't work. Of course, if you are really worried that those 4 bolts aren't going to be able to handle the load, you could always upgrade to a TH400 and have six. :D :thumb: Thats what I'm doing.

AFA my plan, I drew up a simple cradle system using leafspring type bushings just outside the adapter and doubler feet. Seemed like it would work great... then I realized a driveshaft would be going through there somewhere. lol! Definitely gonna require some more thought. Once I get my drivetrain sitting in my shop, all assembled and I can take some meaningfull measurements, I plan on knocking out a simple AutoCAD model showing my plan so you real engineer-types can critic it...

j
 
Times a wasting, I'm going out to start on it in a few minutes (or so I keep telling myself). ;)

With what I'm considering, the driveshaft will be well clear of the bushings.

And the th400 I have, but it's longer, heavier, and has more parasitic loss. Plus I already have the other goodies for the th350 like the +3 pan and an HD "center support", shift kit, custom governor, and so on.

And "you real engineer types" certainly doesn't apply to me. My honorary degree is CEE ("Close Enough Engineering) credit to Matt
 
Well, since time's a wasting and I've got my own stuff I need to get to here's my thinking:

Seems that the th350 case is the problem. It's weak on both ends and has heavy stuff bolted to each end.

So build a t/c-doubler cradle of some sort and tie it to the engine via struts. Bridge across the weak points.
I would not use poly bushings, I would use rubber for it's superior flex. Something's gotta give somewhere and right now it seems like that's the trans case.

Much beyond those general thots and I'll need to look at some pics.
 
Thanks, that's my general line of thinking. If I don't change my mind, I'll be using the stock rubber "C's and blocks", only flipped upside down. But if I decide to go with poly spring bushings, I've got those sitting out there too. Well, off to the shop I go...
 
my thoughton the flex issue was this. I made .5" poly pucks with .25" sleeves(for bolt guides in the poly). Placed those between my adapters and crossmember mount plates. Then I built my crossmember(only done one so far and not happy with it, not for the bushing/mounting issue though), and ran it to the frame where I tied it to the frame with YJ poly bushings. I figured that sure, poly doesn't give as much, but twice the poly? My tranny is also a TH350. However if that damn thing busts on me I'll likely put in a SM465 in a heartbeat. I tied to the frame at 4 points. for driveshaft clearance I ran up along side the 203 adapter and across at the top of the frame rail. Now granted my frame is fully boxed back passed the first mounting point from the factory and I boxed it back another 20" so far.
 
Came in for a drink, but I think I've got it worked out with a few revisions, we'll see...

I thought about that up and over in the front, but it's a PIA, hence, the cradle. Double the poly flex points was on my mind too, and I sketched a few designs, but just more complexity than needed was my final decision on those designs. I think the stock rubber will do what I need. <shrug>

One problem I see at the moment is that the front 203 adapter feet are not "square", so since I don't have a mill, I'll have to get creative to get an angled surface for the front mount of the cradle. Main cradle will be 2 x 1/4 angle with a piece of 1.5 x 1/8 strap diagonal across the inside to triangulate along with a perpendicular triangle gusset at the ends of the inner strap to seal it off. This is for the loads generated by the mount outrigger feet that will try to bend in the top of the angle iron sides. Should be a strong as 2 x 1/4 square tube, maybe stronger, for the loads it will see.

Then the offset up to the trans mount is also going to be a bit of a pain. Still thinking on a couple of ways to do that. Not even sure how much it will help with the top struts in place. The struts should reduces the stresses that strip out those bolts, and the forward cradle mount is really just helping support vertical loads. Those 4 bolts in shear are more than up to the task. And the vertical support for the case itself is not really helping that much since it's so close to the back anyway. My last tail housing was busted right through the mounting boss across the bottom...
 
Russ i don't have a doubler so if i'm off excuse me. I run the factory HD cover on the K case with the rods that attach to the cover and the plates behind the motor mounts. The only problem with this idea is the bolt holes being aluminum so i tap the threads all the way thru and use a carbide bur to make enough room for a locknut on the trans side.

Also depending on where the 205 sits with a doubler is there any way to incorporate a 69-72 sidemount to the frame with the rubber bushings gm used on those early models?


DW
 
Problem is, I don't have a "K" case, and they are getting darned hard to find. The last one I found had a broken bell housing when I got it... :crazy: Which led to what I now call "The Ultra Bell fiasco". Anyway, I want to find a way to use the more common cases so I don't spend weeks looking for a decent "K" case or pay through the nose at a yard (assuming they have one, I've had trouble there too). Last yard I was at had several "K" 700s and some 4L60s (maybe a 4L80? Not sure), but no th350s at all in "K" case.

Also, I think the brace on top (which wouldn't work with a stock floor I don't think) will be more effective than the ones from that cover anyway.

As for the old style side mount, I've considered it, but several problems occur to me. For one, that's where I mounted the pivot for my transfer bar on my tripple sticks. Second, I worry that it's actually going to be the limit for tortional loads between the engine and output, which means more stress on the transmission. Third, GM stopped using them, I wonder if there was a problem? Which feeds into worry #2.

Also, I think since I don't want it to be the limit on torque rotation of the drive train, even assuming it wouldn't be a problem, it seems it would only be of value when you slam down hard and don't want the inertial load acting as a torque input, or possibly the same thing but in the opposite direction for a direct hit on pass side tcase or drive shaft. With my skid in place, direct impact shouldn't be a problem, and my front shaft is barely even marked, so all together, I don't think it would be worth the effort to redo the shifters and try to deal with (or figure out if it's a problem) the potential for it being the drive train torque limiting point.
 
Update: I've got most of the cradle done, but still haven't even started the cross member. Cross member should be easy, and all I lack on the cradle is the front trans mount riser (that may become a phase 2 upgrade) and slotting the "wings" for the ovals the stock mounts ride in, and welding them on. I ran into 2 problems.

First, the front adapter (the 203 adapter) foot is not perpendicular to the assembly and rear adapter. If you lay a straight edge flat on the doubler adapter foot, it will hit the forward edge of the 203 adapter with about a 0.120 gap at the rear. With a 1.5" pad, a little trig gives us approx 5* of angle. There is also the PTO cover plate and lower bolt, along with the "bowl" of the range box that will interfere with anything trying to go straight from the rear to front mount.

So you can't just lay a piece of angle across there and drill some holes. I had to make a tapered pad for the front mount, and cut a relief for the bolt, offset to clear the PTO cover, and of course, drill the holes to fit.

Enter the second problem. I had the harder of the 2 sides done except for drilling the holes. But when I stacked up the measurements to find the center points, I forgot to add half the bolt. Stupid, stupid STUPID! I'll blame it on distractions. Of course, I didn't catch the problem till I had drilled the hole and realized it didn't look right. At this point there is way to much work in it to scrap it, so I cut a piece of 1/2 rod, 1/2" long (to go through the pad), and welded it in place, then ground it down and redrilled.

Hopefully I'll finish early next week and have some pictures.
 
I'd have a comment of some kind if I saw some pictures of what you're talkin about. I'm a visual kinda guy...all the words just make me yawn and get bored. :D
 
Hey, sorry to hear about the ultra bell fiasco.
If you have any reason to part with it, please let me know.
I'd like to see what I could do with it...

I can't give you any advice on doubler crossmembers.
I can give you a couple of pics of an Okie style setup though.

The first pic is to just show how solid of motor mounts Steve Sharp
used to run in his Blazer Buggy.
The second pic shows how he had a single, solid (welded) crossmember
which incorporated 2 poly mounts. He ran this setup hard for several years behind a T350 great success.

When he parted the rig, I ended up with the doubler setup and the remander of the buggy. I am presently installing the doubler behind a 700R4 in my trail rig Blazer. I plan to emulate the style of crossmember he built in the buggy but it will bolt into the GM frame in numerous places.

The buggy will be pulled into the shop soon for a winter project.
Good luck with your crossmember build.
Be sure to post up plenty of pics and let me know on the bell.

Sharps blazer w leafs- front-r.jpg

Sharps blazer 203 inverted-r.jpg
 
Jethro:
I won't have any pictures until I get done. Kinda defeats the point of asking for suggestions then don't it? ;)

BigDaddy:
I'll think about the bell. For now, I was keeping it "just in case. With the sealant on the bolts (contrary to instructions, but based on the local tranny guy), already go a "midplate" type shim, and all the rest. If this new tranny breaks the bell (and there is a small crack, nothing unusual though), with all that money invested, I will try it again. I'm hoping this new rig keeps it intact. If so, there will be a massive shop cleaning come Spring, and you may just get a PM. :D

Never seen the cross member before, can't really make out much. But I do think that stiffer engine mounts are a must if you're going to run stiff trans/case mounts. That's why I said I would go to poly engine mounts if I go poly case mounts.

I'll certainly post pics if it turns out ok.
 
Russ, I'll stop by sometime this week and tell ya that you're doin it all wrong so you'll have to throw your hands in the air and give me all your stuff. Tools included.

Sound like a deal?:haha:

Seriously though, I'll see if I can swing by to give a possible lame suggestion or two.
 
Russ,

For the life of me... I don't know exactly why you've had such rotten luck w/ this whole "mounting" issue.... Considering all the Doublers I've assembled, seen assembled, and generally hosed around w/ - I've seen way less breakage w/ not near as much analysis.... :dunno:

I won't get into the flex/hard mount discussion, but either way, going T400 will give you a much better connection at the front of the 203. If you really wanna buy a cheater stick, what if you took the time to make an "over the top" brace for the tranny out of tubing..... Roughly I'd say it might look something like this:
Russ_doublerbrace.jpg


Marv
 
I don't know either, that's why the analysys. As discussed before, I think the biggest problem is bent springs leading to compressed square shafts that couldn't really slide under high torque.

While I think (hope) that is in the past, I wanted to do something better than what I had, which was only mounted under the doubler adapter. So here we are.

Your frame work thing is close to what I had in mind for the top braces. But I don't think it needs to be that complex. My main desire is to stop the verticle up/down forces in the rear and middle (long span between rear mount, and far forward engine mount) from being contained only by the trans case. And straight rods will do better at that than a close fitting framework due to the bends creating a flex point. Do you see it otherwise?
 
You must log in or register to reply here.

Latest Posts

Top Bottom