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I am in the middle of an engine build and need transmission advice. The engine will most likely produce a lot of torque (big bore, X-flow, supercharged, fuel injected etc.) and is intended to be used on the street. Used block is a 1275 pre A+ one and to it I have the corresponding clutch housing, gearbox, transfer gears and so on. Except from that I have a DAM 5626 gearbox housing with an unused Jack Knight SCCR gearset on the shelf. I think I can live with the whining on the first three gears and would preferably stick to helical transfer gear. (Out of what I have read it should be doable as long as I am driving with my brain)

So what is the best route to pick combining my A+ gearbox housing with the pre A+ block? (Clutch housing, bearings, idler gear, primary gear and input gear, straight cut or helical transfer gears, idler roller bearing conversion)

The block doesn't care which box or housing you use, so not much to worry about there.

And 1-3 straight with helical drops is perfectly fine for road use. I drove many many miles like that and noise was of no real concern.

I broke many things ( many many years ago ) within the gearbox...but never the drops, so they're decently strong. I'm sure you'll be fine.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Im currently running SC gears and helical drops without issue and significant power.

Shane

Thank you for your opinions @stevieturbo and @shane. Any preferences considering choice of clutch housing (pre A+/A+), i.e. should I aim for the larger idler gear shaft and bearing (A+)?

The idler gear needs to match the gearbox and then the drop gear housing needs to match the idler gear.
The primary gear, idler and 1st motion gear need to be a set - ie same tooth profile.

So you will need an 1275 A+ drop gear set and housing.

As Per Grahams response with the 7|8" idler bearing over the 3/4".
Use the later Torrington needle roller bearings also.
Set your idler end float tolerance as close to bottom limit as you can.
Choose a transfer case (idler housing) that the idler thrust mating face is in good condition.

Shane

you need to use the housing to suit the idler gears and bearings in use.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will