Swapping out old figure 8 bolt pattern 205 with a new round style bolt pattern 205

[Leadfoot]

As some may already know I torqued the living daylights out of my motor and transfer case mounts (Too much HP and torque) and broke the cast iron adaptor that mate the tranny and transfer case. I previously bought High Energy Suspensions transfer case mounts and new rubber motor mounts (that was half my problem), so I just purchased ORD's motor mounts which should help. My question is whether or not they have a stock adaptor that will mate the newer (round bolt) 205's to a TH350/700r? The reason for this is the old adaptor was tall (leverage) and it broke where it necked down. I figured with a newer style 205, I could use the newer/shorter adaptor (if they make one for the TH350/700r) with the humped crossmember and lower my chances of breakage. Any info would be appreciated.
P.S. Does anyone have a round bolt NP205 mated to a TH350/700r?

If you didn't build it yourself, how can you call it yours.......?

 

[prjt_blzr]

I'm not 100% certain of this but I believe the round pattern on the 205 was only used on manual and TH400s. GM changed to the round pattern in the late 80's on their one ton trucks. Since GM never ran the TH350 in these trucks I doubt that it exists.

 

[HarryH3]

If anyone makes a custom adapter, it's probably these guys: <a target="_blank" href=http://www.advanceadapters.com>www.advanceadapters.com</a>. You might want to browse their site and see if they have something that you could use.

<font color=black>HarryH3 - '75 K5</font color=black>
<a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
It's a great day to be alive...

 

[Leadfoot]

I know the round bolt pattern NP205 was never used with a 350, but I didn't know if an aftermarket company (I already checked with Adv. Adapt. and they said no) made one. I would assume I could put a 27 spline male input shaft in the t-case instead of the 32 spline female. Then take two adaptors (the old figure 8 NP205 adaptor which mated to the TH350 and one of the new circular ones), the female 27 spline coupler, measure and cut both adaptors and reweld them together to get the round t-case pattern with the TH350 tranny bolt pattern, and have it work. I have plenty of machining buddys and could do it, but A.) it's seems like alot of work B.) I would hate to cut up two perfectly good adaptors.

If you didn't build it yourself, how can you call it yours.......?

 

[Blazer1970]

The round t-case bolt pattern on the 205 is not the same as the NP208 / NP241 pattern is it?

Tim

70 Blazer CST 4X4 350 SM465 NP205
87 Burb 4X4 350
01 GMC 2500HD 4X4 Duramax/Allison

 

[prjt_blzr]

WOW, This seems to be a lot of work to get the 205 bolted to the TH350. Why not use a TH400 or swap your round pattern for the figure 8 pattern?

I believe that the round patterns are the same between the 208/241/205. The 205 has some shift rails sticking out that the others don't You can machine the adapter to bolt them together though. That is how I got my 205 to bolt up to the NV4500 with only factory parts.

 

[Seventy4Blazer]

in the case of wanting to use a TH350 with your motor... WHY? if you are snapping parts you will eventualy snap the Trans. you might as well replace both the trans and case. that way you know they match up. and the trans can take more. Lots More.

1974 Chevy Blazer Cheyanne. lots of plans for it....
but it is on 33's now, with saggy old springs. i am in so cali if ya got any parts for me.

 

[Leadfoot]

Well if I had a stock or only mildly built TH350 I would be worried. The one I have has a trussed case (same principle as trussing an axle). The TH400 is a much beefier case stock and that is the main reason for using it behind the big blocks. My tranny guy says the internals of the 350 (most aftermarket) are stonger than 400, but the cases on the 350 flex under severe torque causing the internals to self destruct. By trussing the case and some creative internal bracing, the 350 can be made to withstand supercharged, nitrous, big-blocks (although it is allotta work). It has a lower first gear than the 400 and with a NP205 I need every little bit of gearing advantage, and the other reason for staying with this is because I have several 350's and 700r's laying around. I actually have the 350 in the basement and have the 700 he built for me in it's place. That was the tranny I was using when I snapped the adaptor (and people say 700r's can't be built to handle 600 hp). I am not worried about the tranny breaking. So if I could pick up an adaptor for &lt;= $200, it would save me from having to aquire a TH400, spend money to build it right, convert to the kickdown under the gas pedal, get the proper tranny to t-case adaptor for it, etc., etc. I agree for other people it is the way to go, but with what I have already aquired it is not worth it (Plus I can be damn stubborn at times

). Besides, I want to save my money and be one of the firsts to put one of the new Chevy Big Blocks and Allison tranny's in a 73-87 rig. I figure it is going to be two to three years before I can find a reasonable donor. I know somebody will probably beat me to it first, but I want to do it before it becomes the "New Fad".


If you didn't build it yourself, how can you call it yours.......?

 

[Leadfoot]

I have the figure '8' pattern now, but it uses the tall adaptor. With the HP/Torque I put through it the tall adaptor has to much leverage on it and will keep breaking. I figured if I could somehow use the short adaptor it may be worth it, but while I was writting this is just occured to me that I could probably machine my own custom adaptor to use with the new style crossmember and do it less than Adv Adapts. Well thanks for getting the wheels turning in my head. I will see how it works and keep you updated. Now I just need to find a newer style NP205.

If you didn't build it yourself, how can you call it yours.......?

 

[Stephen]

Why machine a custom adapter when all you really want to do is mount the t-case? Just find somewhere on the stock adapter to bolt a new mount up higher. Maybe you mount the 205 using the pad on the side and the PTO cover. You could also cut the factory foot off and weld a new mounting pad on the bottom of the adapter in a higher location. Fabrication work is generally much easier than machining a new adapter!
Also, have you measured the distance from centerline for the motormounts and the t-case mounts and checked the deflection under load of each? It's easy to assume that a shorter foot will help you, but some more research into it might save you the work of building something to find out you had another problem. Figuring out drivetrain mounting is one of the great brain twisters of 'wheeling.
The problem you're going to run into on the round 205 is the input is 32 spline and you have a 27 spl output. You could go to a custom bearing to use the 27 spline input (I assume there is one that will work since you can get a bearing for just about anything) and use a special seal to go around the drivesleeve to seal it off.

But, once again, why? You may be able to solve it a simpler way. I don't know what it is, but keep looking!

Making the world better, one truck at a time.
SW-ORD

 

[Leadfoot]

Thanks Stephen, I sometimes cannot see the forest for the trees, you know what I mean. I tend to try and reinvent the wheel at times. I think I am going to just modify the existing (broken) adaptor, and maybe use a newer style crossmember or fabricate my own. I just have to mock up the existing drivetrain to get height measurements.
Again that's why this site kicks a$$! Thanks for everyones input, and if you think of anything else, keep it coming. The more ideas the better.

If you didn't build it yourself, how can you call it yours.......?