Swapping SM465/NP205 from '81 1-ton to '74 blazer

[AgentOrange74]

I just obtained a SM465/NP205 combo from a donor '81 1-Ton dually. My '74 blazer originally came with an SM465/NP203. The original trans needs rebuilt and the 203 is in unknown condition. I figured I might as well upgrade while I have everything torn apart.

I have a few questions about the swap. First, It's my understanding (haven't verified yet) that output yokes are positioned differently between a 203 and 205, meaning the driveshaft lengths front and rear are going to be different. Is that true, or a misunderstanding?

Second, the donor truck, being an '81 1-ton (single cab, long bed dually) had much beefier drive shafts. spaghetti vs summer sausage. The '81 also has a carrier bearing and slip yoke. Why did GM switch to this design? Is it better? Since I have to get driveshafts modified anyway, would it be advantageous to try to make the 2-piece later design work in the blazer, or just keep it simple?

Lastly, I didn't grab the clutch off the '81. I assumed it would be a heavy-duty version, maybe incompatible and undesirable for my application. Was I right in my assumption? The '81 had a 350, but I read somewhere else that the 1-tons had a 12" clutch regardless of the motor, whereas the trucks/blazers had an 11" clutch. Is there any reason anyone would want the 12" clutch? Is it a direct swap? Is the pedal pressure heavier?

That's it for now. Thanks!

[blazer74]

Yes, driveshaft lengths will change with the 205.

Replacements or have yours lengthened/ shortened as required.

If the 205 is a slip yoke style rear output that's going to require a different driveshaft than your existing fixed yoke 203.

If your going to lift and run bigger tires and axle, you might want to upgrade the shafts anyway.

Longer vehicles used the carrier bearing style 2 section driveshafts. No need for that in a blazer.



Don't know about your clutch but I would think that is determined by your existing flywheel size and spline count on the trans.

[AgentOrange74]

Thanks for the reply blazer74. I think my blazer is already lifted... It's got 33" tires already and there's plenty of clearance. I'll take pics of the shackles/axles, etc. when I can to verify.

I don't know what the slip yoke rear output looks like, but I'm assuming it has that. To eliminate the carrier bearing and slip joint,  would I just take the two pieces to a driveshaft place and have them combined into one? Would the yoke on my rear axle need to be changed?

Can someone post a pic of what the slip yoke rear output on the 205 looks like vs. the non-slip yoke? I've attached a pic of the id tag on the 205 (how do you decode these?). Unfortunately I don't have a pic of the output housing.

Thank you

[Peter S]

My 79 K10 had a 12" clutch from the factory and it lasted 300K miles. The flywheel is the same for 11" or 12". When I put my 78 K10 together I used a 12" clutch setup that I had gotten from Summit many years ago. The peddle pressure isn't a problem. The 12" disk has a greater surface area so it will last longer and be less prone to slip.

[1967KaiserM715]

Fixed yoke 205


Slip yoke

Any competent driveshaft shop should be able to take the old driveshafts and reuse the yokes to match the axles and transfer case

Image failed to work, trying again, both were taken from this site

[1967KaiserM715]

Note that the second is actually a 208 transfer but slip yoke is similar on 205

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[AgentOrange74]

I was able to check my 205 this morning and it's the fixed yoke version. That's somewhat of a relief (I think), as that simplifies things a bit. Although it sure looks like the yokes are different sizes. I'll take some measurements and report back when I can. Thanks for the pics @1967KaiserM715, that helped a lot!

[1967KaiserM715]

1 ton yokes are bigger, a 1350 normally, although I have seen a few rare cases of a 1410, 1/2 ton trucks and blazers used a smaller joint, normally a 1310, although I have seen ONE 1/2 ton with 1330, it was unclear if it was an upgraded truck though.

If you don't need to lengthen or shorten driveshafts, you could get conversion joints, but I am sure you need to drop off the shafts for length changes, just tell the shop to make the tranny end for the one ton size(bring the old u-joints) and the axle side blazer size. Bring a written explanation telling them what you want/need, that way they are liable for mistakes.

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[AgentOrange74]

Good advice. Thank you!

[Irish_Alley]

Conversion u joint numbers

[AgentOrange74]

@Irish_Alley: Does this mean I can keep both the 1-ton yokes on the driveshaft and just get a u-joint to convert to whatever axle I have? If that's the case, that sounds like the best route. Thanks for the tip!

[Irish_Alley]

yes, you just have to measure what u joint you need to the axle/driveshaft