MX5 Miata Tech Forum

Transmission Identification Question, 5 spd Versus 6 spd

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O'Fallon, MO, USA   USA
Good afternoon everyone. I'm new here so this is my first thread topic.

My question will be better at the end after you all have the background on my build project.

I'm going to be building an open wheeled car powered with a Harley counterbalanced Twin-Cam "B" motor. It will be coupled via an adapter that's used in the manufacture of the new Morgan Three Wheeler in the UK. They have the adapter between the motor primary bolt pattern and the bell housing of the Miata 5 speed. There is an inset plate in the adapter that has a double row sealed bearing that supports a stub shaft with the flywheel bolt pattern. Inside the adapter is a coupling of sorts. They originally used an actual Harley compensator, but they wore out really quickly. Now they use a Centa rubber coupling that allows for around 5 to 10 degrees of wind up to "compensate" for the torque pulses of the Harley motor. Without the soft coupling the trans would get really beat up.

Here is the Morgan M3W with an S&S 2L motor with the drive arrangement explained above.

Actually this is close to what I was originally going to build.

Then someone on Talk Morgan did a little Photoshop on the M3W and came up with the M4W. WOW, what a great looking little beast... Well that's my opinion anyway.

I'm going to be adding suicide doors and will have more of a hotrod tubing main frame than Morgan's space age tube chassis. So I'll be running my exhaust under the body along the outside of the main frame rails. It will be exposed, but not in any way to get burned on. It also gives access for the doors.

I'm going to be using a MG Midget rear axle housing stretched about 3" to 4" per side to give me my rear wheel track I need.

Here are a few pics of the Morgan drive line.

Here's the motor with adapter and trans. This is a Miata 5 speed.

Here's an out of focus but better picture of the adapter area. The groove cut into the top is for the rack and pinion center section.

One last pic of the trans.

So with my build and running a 3.9:1 rear axle ratio and 18" wheels/tires, I'm thinking that the 6 speed would be a better choice for me and it also picks me up a little bit more on the first gear and then better gear spacing.

I "think" that Morgan uses new '06-'14 Miata 5 speeds. Is that what's shown in the pictures above?

They do NOT run the Miata flywheel and clutch, but they use a Volvo light weight setup and I believe Volvo's hydraulic throwout bearing assembly.

I plan on using the stock Miata clutch slave cylinder setup.

So here's my question. If this is a '06-'14 5 speed, which, if any, 6 speed will have the same bolt pattern? Are certain 5 speeds and 6 speeds interchangeable?

Because I'm going to be purchasing the adapter, Centa and the stub shaft assembly, I'm really locked into the Miata trans setup. Morgan has had LOTS of issues with their M3W's, but the Miata 5 speed has not been one of them. They seem to be holding up amazingly well.

If I want to go to the 6 speed, which I'm thinking I do, what year 6 speed would be a bolt in replacement for the 5 speed pictured above?

I've seen on eBay quite a few light weight flywheel sets and some aluminium flywheel sets. Are there any better than other sets that I might look at? I'm going to be running around 80 to 90 hp when completed.

I greatly appreciate whatever information I might be able to get.

Thanks in advance.

Dan Lockwood
O'Fallon MO

Dan's Photo Albums

Edited 1 time(s). Last edit at 2018-02-05 02:34 PM by Dan Lockwood.

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1996 Mazda Miata NA

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