Okay guys, my TH700R4 has died after only 20,000 miles which i think means its time for a stronger tranny. I am leaning towards a 6 speed, which I know is not hard to swap into the car, but there is one problem. What do I do about the flywheel? I know my flexplate is counter weighted, so I cant use an LT1 flywheel. Could I just get a stock 400 flywheel for msybe some car which originally came with a 400 and a 4 speed? Does such a flywheel even exist?

 

It DOES exist... only problem is.. it's 14 inches in diameter and has 168 teeth. You need a 12 inch (okay 12 and 3/4) flywheel and 153 teeth. The ONLY manufacturer that I know of that makes this flywheel for a 400 is centerforce and it's about 400 dollars. Checkout www.centerforce.com On mine, I just paid 80 dollars for a stock 85 Camaro flywheel and had it balanced for the 400.

 

Using an '85 flywheel will not work to the best of my knowledge. The T-56 has a unique clutch in that it pulls on the diaphram rather than pushing on it like a conventional clutch. This means that the pivot point for the release fork is backwards -if that makes any sense. You need to use an LT1 flywheel, clutch, and associated parts.

I would (will, because I'm doing the same conversion right now) get an LT1 flywheel, take it to a machine shop, and have them balance it to match your current flex plate. At the same time, have them re-drill the flywheel, and bore out the inner diameter of the flywheel flange, since the LT1 has a smaller flange and bolt pattern than your older 400. I did this same process before on a 305 and it worked great. The LT1 comes externally balanced, so in that case, the machine shop had to neutral balance the LT1 flywheel AND drill the holes and bore the center for the older two piece rear main seal 305. Like I said though, it all worked fine.

 

the LT1 is externally balanced at the flywheel end just like the rest of the one piece rear main engines. the problem is the bolt pattern, not the balance. you would still have to have it rebalanced since the counterweight probably isn't in the same spot as it would be for the 400, but the bolt pattern/flange is going to be the issue. i'd recomend checking around like ebay etc. for a richmond or something of that type. the 700r4 shouldn't be having that hard a time with your 400. you likely need one a bit stronger if you stick with it. it's not that uncommon to find a 4L60/700R4 behind a 10 or 11 second ride.

 

Zippy, if you read the post preceding yours, you will notice that I addressed the bolt pattern issue; have the machine shop re-drill the bolt pattern in the flywheel flange while it is in for balencing. AND machining out the I.D. of the flywheel hub, since the O.D. of the crank flange is larger on a 2 pc rear main seal crank than on a one PRMS crank.

Ward, getting ahold of a used LT1 flywheel from a salvage yard or ebay, and getting it machined is your most ecconomical option. Buying a custom flywheel is certainly an option, but I'd never do it. That's just pissing money away unnecessarily IMO.

 

i see it now tom, the problem is it wasn't there when i was replying or it just didn't show up. on occasion i take a while to type in the reply when i'm also watching tv and someone's reply get's there before me... btw, your reply was good though, pretty much took care of it.

 

Yea thats about what I figured I would have to do, get an LT1 flywheel counterweighted to my engine. I did not know it was different where it bolts to the crank though. Hopefully I can just go to a good engine shop and tell them I need the LT1 flywheel blanced for a 400 SBC with a 6 speed behind it.