I recently picked up a 425 engine and transmission from a 67 olds 98. The transmission is original and a three speed auto. Is this transmission a th400 or is it something else? Anyways, I would like to put the engine in my 70 cutlass, which has the olds 350 and original th350 transmission. Is it possible to bolt the 425 to my th350? I know that on the 425 the bolt pattern between the crankshaft and the flywheel is an older, different style, but the original flywheel for the 425 came with the engine. Can I use the torque converter from the olds 98 transmission in my th350? Can the th350 handle the hp of the 425? If the 425 cannot be used with the th350, can I use the original transmission from the 67 olds 98? One problem with that idea is it has a 9" tail housing that has no place for a speedometer cable. Apparently the olds 98's speedometer cable ran to the front wheels. Does anyone know where I can get a tail housing (and tail shaft) for the olds 98's original transmission that has a speedometer gear and is preferably 6"? Is the transmission from the 67 olds 98 the same length as my th350? Will it bolt up to my cross member within the range of holes on my frame? Thanks in advance for any help anyone can offer

 

So many questions....the 425 will bolt up to the 350 trans. You will need to use the 425 flywheel and 350 torque converter. The turbo 350 is not super strong but for daily driving it should hold up. If you pounce on it with the 425 it will fail. The turbo 400 out of the 98 is pretty much not worth using as it has a special torque converter since it is a switch pitch trans. It is longer than your turbo 350. A short tail turbo 400 is what you should use from 68 upwards....since these were not switch pitch transmissions. Then you will need some driveshaft swapping and another yoke for the turbo 400.....The swap is not hard since the factory produced the 442 which was a bigblock and turbo 400 equipped...you just need to round up all the parts. The kickdown is also different between the turbo 350 and the 400.

 

More questions, sorry... I read up on switch pitch transmissions and that seems to be an extra nice feature of the transmission and not a bad thing at all. I'm an electrical engineer so I shouldn't have any problems figuring the wires out. I have the original switch on the throttle for the transmission from the olds 98, and I bet I could get it to work for me in my 70 cutlass. I also have the original driveshaft from the olds 98 (scooped up the whole package dirt cheap). Since I WILL MOST DEFINITELY be stomping the 425 (hehe) I will probably need the th400. So here are the questions: Will the short tailhousing from a newer th400 bolt to my th400? What is the measure of the tailhousing that I will need, or does it have a name/number/letter code? There is a small range of holes drilled in the frame for the crossmember, will the crossmember still fall within this range when I bolt the th400 to it? Any ideas on how much it would cost to have the th350 built to handle the 425, if possible? I fully understand how the kickdown works and I already have the correct switch for the throttle, so it shouldnt be a problem. Thank you VERY much for your reply and the great info!

 

I'll try to remember what I did when I put a 425 in my rallye...
First off, I installed a trans with a long tail shaft.
The cross member will work, I think it will share the last bolt hole on the frame and then one further back will have to be drilled. As for finding
a new shaft, just put it all in, take your measurement and go to the salvage yard. I think I wound up pulling one out of a '76 camaro for this combo.
But just remember, the 400 yoke is different from a TH350.
Just bring your tape measure.

 

You really shouldn't be using the tranny or torque converter without a full rebuild and once you are spending that kind of money I think that you may as well go buy a tranny to go behind the 425. Which, by the way, is an excellent motor. I imagine that you might get any range of prices from different shops for a performance rebuild but just doing a quick internet check I found your basic B&M street/strip TH350 for just under $1000 (plus tax) and you'll need a converter as well. Last (and I mean last) rebuild I paid for was $600 and that was doing nothing but a damn basic rebuild with which I got a 30 day warranty.... but, I won't go off on that tangent. Though I've always had TH400's, for the extra money you can have a strong, light tranny to fit right in behind the big block with no (?) modification.

 

The switch pitch control solenoid on the 67 worked on the mechanical carb linkage; you have cable so I hope you have worked out something there, like a link to the carb throttle lever. Changing the tailhousing and main shaft is a job, so you could use the long tranny as is, if in good condition. Olds uses bigger u-joints than most chevies, but you may be able to find a driveshaft. If you can't find the correct length, choose one that is solid meaning no rubber sleeve with one tube inside the other. Get a driveshaft shop to cut one end and re-weld the forged end on it, and rebalance. Don't cut the driveshaft somewhere in the middle!
Physically the 425 engine is the same size as the 400 used in 442s for exhaust, etc. Many 350 parts interchange like water pump, engine mounts. You can use the water pump and pulleys from the 425 or the stuff from the 350 (the water pump is probably 5" deep on the 425 and 5.62" on the 350).

 

I was told by the guy I got the 425 from that the 9" tailshaft on the th400 would be too long for my cutlass and that it would create angles within the driveshaft that were too large to use. Is this true? Doesn't seem too logical because if I were to add an overdrive unit the whole thing would be 13.5" longer. (the overdrive replaces the tailshaft) Not that I am getting an overdrive, just using it as an example. The main problem I have with the tailshaft is it has no provision for a speedometer. If I got a shorter tailshaft, would I need a shorter yoke for the driveshaft? I have the original driveshaft from the olds 98, which I would have shortened to fit. I'm all set with the wiring portion of the transmission, I will be able to control the switch pitch and the detent correctly. One last thing I forgot to mention before, the car is a convertible, so it has a boxed frame. There is only a small range of holes in the frame for the crossmember and I believe the crossmember cannot go out of the range of those holes because of the shape of the frame. I'll double check it today. If I cannot put in the th400 I guess I will have to have the th350 built up, but I was looking to make the swap as inexpensively as possible... Anyways thanks again for the great info, this is a big help.

 

You can order adjustable upper trailing arms if you are real worried about pinion angle. I think you'll be ok however. If it will fit, it will probably work. Just make sure that the yokes on your driveshaft end up being matched with the correct yokes for the rear end and trans you choose.

 

Engine swap is now complete! The 425 dropped in with very few problems. I had to buy big block A body exhaust manifolds because the Olds 98 exhaust manifolds would not work, but other than that everything went fine. In order for me to mount the th400 with the 9.5 inch tailshaft I had to drill a couple holes...but not in frame. The front holes in the crossmember lined up with the third to last pair of holes in the frame, and then I drilled two holes in the crossmember which lined up with the last pair of holes in the frame. It was a lot simpler than drilling new holes in the frame, especially since the convertible frame is boxed. I used 350 motor mounts and everything lined up perfectly with absolutely no trouble. Even my old dual exhaust pipes lined up. The best part is the engine and transmission work great and with LOTS of power! The switch pitch torque converter also works great with no problems. Anyways, here's a pic. Thanks for all the advice, it really helped me through this project!

 

Congrats, nice loking install. The added power must be nice. enjoy!

 

Just curious--what did you do about the speedometer?

 

well...I have a 3.55 posi so the speedo never worked right anyways...right now the cable is just hanging loose. On the transmission there is a spot where it looks like it may be possible to add a speedo gear (with all the right parts) but I know at minimum a bolt hole would have to be drilled for the bolt that holds on the forked holder plate. As for the hole itself right now there is a plate in it that looks a lot like a freeze plug. Maybe this part can be knocked out? I'm not sure. My friend has a th400 with the speedo gear and it is in the same place that I have the freeze plug looking plate. It is not casted into the tailshaft housing, it is in the main transmission body right before where the tailshaft housing bolts on. Maybe another way to solve the problem would be to get a tailshaft housing with a speedo cable gear housing casted into it, but I am unsure if such a thing exists or if a gear can be added to the main shaft to drive the speedo gear in this location. Also I don't think the speedo cable is long enough to reach the tailshaft, but I'll check next time im under there. I'll take some pictures too.

If anybody has any info on what is required to attach a speedo to a th400 which did not have a speedo gear originally, I'd love to hear it!

 

Hi can you post pictures again. Clicking on the one above just sits and spins and does not show.

Cheers,
Gary

 

ernie's s.p. th400 came out of a '67 delta, i took it to the transmission shop, and had them put the appropriate drive and driven gears, taking into account the tire size and rear end ratio. i also had them re-seal it.


bill

 

I bought a $100 GPS for my work truck and use it for a speedo if you want a temporary solution to keep you from getting tickets. It's great for checking accuracy and all you need is a cig lighter.

 

You should have kept the 67 Olds 98 and scraped the 70 Cutlass. Lol

 

Old thread, but, for those who may come across it, there is at least one company that makes a system that will take the speed off of the driveshaft with a magnetic pickup, and transmit that to an electronic box that will spin the speedo cable and run your speedo like original.
Not cheap, but it's adjustable in case you change rear end or tire size.

- Eric