BBC/TH400 Conversion

[geunther]

I currently run the original 350/350 combo. I would not do anything to the truck that I could not return to original in a weekend. That being said, because I want to drive the truck and keep the original combo in the best shape possible, I am considering an engine swap.

I am considering mothballing the originals and swapping in a BBC and TH400 that I already have. I see that my frame has holes to move the crossmember back, presumably for the extra length of a th400.

My question is: With the tail shaft farther back, is the angle good for a one piece drive shaft through the factory riveted in crossmember (the one that hangs the carrier bearing)? This would allow me to remove the original drivetrain back to the differential without modification.

I did this sort of thing with a 70LT1 Corvette and sold it with both engines.

[sick472]

I looked into a one piece drive shaft when I decided to install the TH400 in my truck, but ended up having a new front shaft made for it instead. I also changed the carrier bearing to the more heavy duty one.

I think you would find that the angles are good, but clearance through the carrier bearing crossmember hole will be real tight, if it is even possible for you truck's stance. The last thing you need is to have to modify the carrier bearing crossmember by cutting the hole larger or entirely out on the bottom side.

[Palf70Step]

Is it a short bed or long bed? If it is short bed, I'd do the 1-piece drive shaft. Long bed it is pushing to do 1 piece. You'd probably be better staying with the 2-piece, just beef it up.

Never understood why some shortbeds came with 2-piece drive shaft

[geunther]

I thought of mentioning the bed length when I was posting, but Id forgotten in the 10 minutes it took me to type.

It is a short bed so hopefully a one piece will work out. With the tranny in place its an easy figure out, but I am just trying to account for every little alteration before I commit.

And I totally agree on not modifying the crossmember. I even thought about running the th350 for even less changes, but I already have the 400 and obviously it is a better choice for the bbc.

[kwmech]

Question should be is: what are you using for the engine mount frame stands?

[geunther]

Originally Posted by kwmech

Question should be is: what are you using for the engine mount frame stands?

I'll either use the 72 only stuff for BB (stands, clamshells, etc) or 71 stands modified for attachment to the 72 SB isolators that bolt to the block. I almost pulled the trigger on 72 BB stuff but still putting it all together before committing.

[kwmech]

Placing the engine on the big block stands in the forward holes will move the engine towards the firewall 1-1/4'', then you have the length of the t400. All the holes are in the frame even with this conversion. The frame pivot for the transmission linkage will need to be moved also.

[geunther]

Originally Posted by kwmech

Placing the engine on the big block stands in the forward holes will move the engine towards the firewall 1-1/4'', then you have the length of the t400. All the holes are in the frame even with this conversion. The frame pivot for the transmission linkage will need to be moved also.

Thanks for the info. Thats the ins and outs that I need to know. Ive done the conversion on a 72 blazer but I just set the engine so the bell housing stayed in place. Valve covers were close but worked.

Ive not done in on 2wd so let me ask what are the differences in trans crossmember between setting engine where the factory would have (using 72 stands) and using 71 stands. I thought those would place the rear of the BB at the same plane as the SB?

[kwmech]

Originally Posted by geunther

Thanks for the info. Thats the ins and outs that I need to know. Ive done the conversion on a 72 blazer but I just set the engine so the bell housing stayed in place. Valve covers were close but worked.

Ive not done in on 2wd so let me ask what are the differences in trans cross member between setting engine where the factory would have (using 72 stands) and using 71 stands. I thought those would place the rear of the BB at the same plane as the SB?

No difference with the stands be it 71 or 72. GM needed to move the engine back so the fan wouldn't be too close to the radiator. I ran the 350 in my 69 with the big block stands for a couple of years before the tear down and conversion myself. It was easier for me that way not to have to deal with everything at once. I had the small block and t400 moved back in a couple of hours and had the driveshaft back the following day. When the time came for the 454 it just dropped in where it was supposed to be. That gave me time to accumulate the correct parts for the big block like the fan shroud, pulleys, brackets, hoses--etc.

[geunther]

Ill start by saying that I know my frame is not special and is just like everyone else's. I just cannot get the math to work.

Considering no other changes other than changing th350 to th400, I do not have holes that allow moving the crossmember back the required 5 11/16" (difference between th350 and th400 mounts).

Then considering the rear face of a BB is moved back an additional 1.25", I do not have holes allowing the crossmember to be moved back 6 15/16" (difference between SB/th350 and BB/th400).

I can move the crossmember into 4 additional positions, just none of them line up where it needs to be if my measurements are correct.

Unless the BB being moved back 1.25 is at the mount, and accounting/adding for the difference of .62" between rear face of engine to engine mount of BB vs SB? If thats the case then I believe the front crossmember hole would be the hole measured at 7 11/16. My math is off an 1/8 but could be my measuring.

[Pitpig]

Swapped in an LS with a TH400. No problems with single drive shaft or driveline angles. Got the driveshaft from Amazon of all places, around $250 with all u joints and slip yoke

[kwmech]

Bolt the stuff in, truck came with a big block and a 400 from the factory. I think you're over thinking it. Have a little faith grasshopper.

[70STOVEBOLT]

I used all factory holes, but had to have my driveshaft shortened to 59.5”. Took out 250/3OTT and put in 402/TH400 using all factory big block components including the shift linkage, 68-71 mounts and frame stands, and one piece driveshaft. It has an HEI distributor and there is enough clearance to remove and install it with the engine in place. I don’t know if 2WD and 4WD frames are the same, mine is 2WD SWB.

[geunther]

Originally Posted by kwmech

I think you're over thinking it.

I totally agree.

Thanks everyone. I think I have it all figured out now from your help and looking at factory BB/400s. I knew I would have to have a shorter DS and will measure that once everything is in. I think the only thing I need to source is the smaller linkage frame pivot and engine mounts.

[kwmech]

Originally Posted by geunther

I totally agree.

Thanks everyone. I think I have it all figured out now from you're help and looking at factory BB/400s. I knew I would have to have a shorter DS and will measure that once everything is in. I think the only thing I need to source is the smaller linkage frame pivot and engine mounts.

I may have the linkage piece, I'll take a look tomorrow--pretty sure I saw it last week

[geunther]

Originally Posted by kwmech

I may have the linkage piece, I'll take a look tomorrow--pretty sure I saw it last week

Thank you for checking.

[Caddylackn]

Remember there are three different length T400s, but the longest one (13" tailshaft) is only BOP bolt pattern.

If I were to do a one piece driveshaft conversion on a 350 motor, I would use a BOP to Chevy bell housing adapter then use the longest 13" tail shaft BOP from a T400 big cadillac, buick, or olds tranny. That would get me the shortest one piece driveshaft possible. The long shaft 13" T400 was designed for the long monster big boat cars of the 70s so they could use a one piece driveshaft without the driveshaft being too long.

I would pull a driveshaft from a square body long bed and have it shortened if needed. You might need the slip on yoke from the BOP driveshaft, so be sure to grab that from the donor car.

The big boats usually ran 3.07 or 2.73 gears so expect to change out the speedo gears to match your rear end.

[geunther]

Thanks caddylackn. I already have the TH400, currently bolted to a 454, normal chevy pattern.

I plan on ordering a new shaft from Tom Woods. Sadly, I am a frequent customer (jeep).

[kwmech]

frame linkage

[70STOVEBOLT]

That looks short, is it off a 4WD?

[kwmech]

Identical to the one on my 69 flatbed, who knows what it originally came off of

[Caddylackn]

Originally Posted by geunther

Thanks caddylackn. I already have the TH400, currently bolted to a 454, normal chevy pattern.

I plan on ordering a new shaft from Tom Woods. Sadly, I am a frequent customer (jeep).

You seem to know what you are doing, it should work out great. I somehow missed that this was a BBC swap too. The worst thing would be to have to do driveshaft shortening and you have that covered. You should at least check and possibly swap out your speedo gears now and replace the o-ring. Much easier now, then later when its dripping on your face.

I always put a shift kit in all my transmissions. Shortening the shift time and getting rid of lazy shifts really helps prolonging clutch disk life. Again, much easier now then in the truck, plus you get that great chirp into second

[geunther]

Originally Posted by kwmech

frame linkage

That's the one. The smaller ones were used on big blocks with th400s. The larger ones were used on small blocks whether not it was th350 or th400. Let me know what you want for that.

[geunther]

Originally Posted by Caddylackn

I always put a shift kit in all my transmissions. Shortening the shift time and getting rid of lazy shifts really helps prolonging clutch disk life. Again, much easier now then in the truck, plus you get that great chirp into second

The trans will get a rebuild before going in. Its in a 4x4 so there will be some work that needs done along with all the sealing up.

[kwmech]

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