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Shell Shock – Transaxle/Engine Combo

At this point in time the chassis is framed out. As of this writing the LCA bracket jigs are being used to install all the front lower control arm mounts. But that’s not what this all about. We’ll touch on that next time. When I used to have the Instatrash, I was handing out teasers of CAD modeled transmission adapter plate for my application. Well the prototype is in testing phase.

Let’s mate and engine and trans.

3D Modeling of bell adapter

What I started with was a Ford Coyote Gen 3 block and a Subaru STI 6-speed. I can hear all the Subaru butt pumpers moaning. Keep it in your pants you’re all shitbird queers. Subarus in and of themselves are complete junk. I drive one. I know. Anyway…as far as I’m aware this swap has never been done. I made several emails and phone calls to Australian shops who deal in all sorts of shennanigans with Subaru junk and came to the conclusion that there are no “off the shelf” kits or parts for that matter.

Why this combo? Why do I have to make parts?

I needed a transaxle to suite my mid-engine layout. What seems to be a trend, short of dumping large cash on a racing transaxle, is to take a Porsche rear engine transaxle, rotate and flip, then call it a day. Personally, I couldn’t find a Porsche trans I wanted to use. I started to not be able to stomache the thought of using one either. I had a Subaru transmission laying around, a donation left by a previous customer. My original plan included beefing up an MT5 with a gearset from PAR and disabling the rear output shaft with a spool from Subaru Gears. After some stark brooding and consideration I realized that no matter what, the MT5 will always be a boat anchor. So I did a repair job and took a pile of parts in exchange. The pile included everything to assemble a YF856 6-speed.

STI 6-Speed parts pile

I believe I’ve done a write up on why I chose the Gen 3 Coyote, so I won’t cover that. (It is the finest example to date of an American made V-8)

Like I said earlier, as far as I know, to date no one has put this combination together. I’ve never done a trans to engine adapter set before, so I figured it’d be interesting.

The basics are simple. Combine the bell pattern of the engine and the bell pattern of the transmission onto a single plate. This plate attaches to the engine, then the tranny (I don’t give a shithouse rat if that word is not allowed. Fuck off!) attaches to the plate. Now, what to do about the clutch and flywheel? Flywheel will be Subaru with a spacer on the crankshaft taking up the distance created by the adapter plate. Clutch will be the typical triple disc style. Starter will be a high toque variant of the Subaru unit. Basics. Initial considerations. All had to be decided upon before proceeding.

All decisions made, what would be next? Materials. 6061 T6 24mm thick. (That’s .945″ to most of us here in the US)

Other than location of all the bolt holes, most important in this process is the alignment of the trans input shaft and the centerline of the engine crankshaft. Misalignment of the trans input shaft and crankshaft center bore will laterally overstress the input shaft and front bearing. This is the whole point. Don’t stress the shaft. Desirable tolerance is on or around .005″ (.127mm). I’m shooting for less.

Next, methods.

How will I build this thing? CNC or hand cut?

In the days of yesteryear, when men were men, the best method to do this was through process of layout and transfer punch. I looked into it. Very tedious. Very susceptible to error. Very labor intensive. Can’t say I even want to delve into the subject. So I wont.

If that upsets you, too fucking bad.

Option 2 was the totally high end method. The CMM method. This method involves probing the X, Y, Z axis of each part and digitally plotting a “map”, so to speak, of the parts. This would allow a layout in CAD to be generated. Bang lets mill some aluminum.

I don’t have access to that. Zippity fucking doo dah.

Duck Duck Go was initialized and the search began. How would the average stroker accomplish this? What I found led me to believe that there are a lot of overconfident strokers out there who have no fucking clue during a shitnado of what the fuck is going on.

After hours of reading and video watching of losers trying to impress themselves I had to invent my own way. It all lead to triangles. A low pressure shit system of them. Triangles.

The input shaft center has to point directly at the crankshaft center, so why not plot points around each common center and then overlay them using that common center as the reference, or home point? That was the plan.

Inlut shaft hole to crank centerline

I first needed a large enough hole to hole digital caliper to handle the task, so one was acquired. Its a nice unit. Does metric and barbaric. Also has an offset on one end for height difference. Caliper zeroed and guns were ready to fire.

Next I designed and 3D printed an exacting center plug with counter sunk hole to act as a support for the caliper. This piece was inserted into the bearing hole at the ass end of the crankshaft.

Step one was to plot the base triangle of the engine bellhousing. This involve measuring between the lowest bell bolts (which lay on the same line at the bottom of the engine) and the crank center. This established my base triangle. Next, using some trigonometry, I acquired my three angles and my height. Then I used the height measurement to split the main triangle into 2 right angle triangles. This established two major things. Left bottom bolt hole distance left and distance down from centerline. And right bottom bolt hole distance right and down from centerline. On paper I hade my first 3 points of reference. I could then use the two bottom points along with any other point on the bolt circle to gain my lengths and angles of points from my corner, “home” bolt hole. Repeated for all the remaining holes, using trig, I established every bolt and dowel point in the pattern. Using the exact same method I established these points for the transmission. When plotted on the same design component, using the same centerline, this became my adapter bolt pattern.

Design 1

What a mouthful. If you don’t get it, don’t worry. Only one person I explained it to could even talk to me about it.

A DXF was plotted in Fusion 360 and as opposed to 3D printing, was laser cut mostly for speed and somewhat for accuracy.

Plenty of failures

Adjustments were made three times and I finally had the perfect pattern.

Shameless C/S Speed LLC plug right there. Final Pattern.

From DXF I immediately went into 3D modeling, making all cleaeance holes and counter bores for bolts and the srarter. With the 3D version designed and 3D printed I test fit to block. Satisfied with the result I bolted the empty trans bellhousing on.

3D printing

And that’s where we are right now. The bellhousing adapter is a plastic protoype. I still have an adjustment or two to perform pertaining to the counter boring of a few of the holes and their diameters for bolt/washer clearance. Next up is a prototype of the flywheel spacer, stay tuned for that. I’m also waiting on a shipment of aluminum from Midwest Steel to make the mock jig for the engine and trans assembly placement. The rear frame rails are designed, made, and ready for mock-up. Stay tuned for some insite on the jig making process and a run down on installing the lower control arm brackets, so be prepared for more on that in an upcoming installment.

The video series is in production, but publishing it is stalled. We wont be using ShitTube as we are not in agreement with their policies. So expect that on a different platform.

theDoc View All

Chassis builder, engine builder, cynic

2 thoughts on “Shell Shock – Transaxle/Engine Combo Leave a comment

  1. Nice work! I agree that the Coyote engine is the best choice for a Subaru. But instead the most popular V8 swapped into a Subie is an LS. Did you get further along in the project? Looks like you got the hard part done.

    Liked by 1 person

    • It is further along, I’ve just been too busy to get on here and do a post. The chassis is not actually a Subaru, I’m just using the transmission.


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