Adapter Plate 101

Here we go Experts, time to school me in Adapter Plates.

I understand the basic idea, making a plate that goes between the transmission and the block to mate foreign parts together, but what are the basic rules of thumb?

Does the bellhousing on the tranny have to be smaller/bigger than the bolt pattern on the block? does it matter?

Lets say the plate is 1inch thick, wouldnt that set the transmission 1 inch further away from the block and mess up clearences with the transmission shaft making it to the clutch?

What is the basic plan of attack as to constructing one of these? Get a sheet of steel/aluminum, trace the bell housing pattern on one side, and trace the block bolt pattern on the other then drill holes? What else is there?
There's a nice writeup by Ken Saunders on the GT Forty Enthusiasts website on this subject. I believe you can access it via a link on the Forum.

First off...what engine/trans combo are you trying
to mate? If an adapter is commercially available,
you're better off buying it.

To answer just one of your questions, yes...the adapter
spaces the tranny away from the crankshaft. So people
like Kennedy Engineered Products make special flywheels and
spigot bearings to make up the difference. Not simple.


Russ Noble

GT40s Supporter
Lifetime Supporter
Clay, Maybe the trickiest and most important aspect of building an adaptor plate or bellhousing is ensuring that the input shaft is in line with the crankshaft. Unless this is spot on it plays havoc with your clutch. Like everything, easy enough to do if you've got the right gear. The size of the bolt pattern doesn't really matter but if the bellhousing ID is smaller than your ring gear OD it means a smaller flywheel and ringgear and relocating your starter motor.

There are several ways to make adapters but one of the easiest and most accurate when using boxes with integral bell housings is to use an internal main shaft/input shaft bearing seat to find the exact center line.
Strip down the gearbox you want to adapt to your engine and use a lever type dial gauge to clock up the exact center with the gearbox main case mounted on your adapter plate on a rotating surface of a mill or CNC, it should in theory be possible even on a large drill or lathe but a little tricky I should think.
Once you have this center, mark it and the dowel/bolt locations and then repeat the process with a gearbox that was origionaly used on that engine, or better still, if at all possible, a main bearing surface of the engine block.
By locating this onto the exact center of your plate and marking up the dowels/bolts locations again you`ve got your adapter, all that`s left is to cut the meat out of the center and trim the edges up, job`s a good `en /ubbthreads/images/graemlins/grin.gif
Reading this makes it sound easier than it is and it`ll take far more time than you may at first think to get it right but does work and is pretty accurate, it`s getting the dowel locations correct to your center that`s the key, you`ll have a little leeway with the bolt holes but the two sets of locating dowels HAVE to be spot on to the common center (there is an accepted tolerance but it`s so small you`ll be using it up anyway so go for as exact as you can).
If you get it wrong you`ll knock out engine and gearbox bearings like there`s no tomorrow, you`ll have clutch judder and wear issues and horrible vibration and get that real, authenentic `kit car` feel /ubbthreads/images/graemlins/grin.gif