XJ12 5speed conversion

Mounted the flywheel (third time’s the charm?!). First, I needed to improve the fasteners. My first attempt, using grade 8 bolts and washers from the hardware store, dished the new flywheel’s bolt holes and chewed up the washers. The washers had to be small diameter because the bolt holes are close-pitched and locally available hardened washers are too big. Now, the stock setup (image below) from the auto trans used no washers. Instead, there’s a ring plate under large head bolts (which are too short to use on the new FW). Using this plate would prevent the dishing problem but I didn’t consider using it before because its outer diameter is too big to fit the new flywheel’s recessed middle. Now, I realized it’s just what I needed.
Flex plate Ring plate is under the tab plate

So, I carefully ground it down to fit. (I realize I did this before and showed a picture but did not talk about it. Now there is context) Then I found some flange bolts from McMaster-Carr with a perfect sized flange diameter. (two images below )

Note dishing around FW holes from yellow coated bolts & small diameter washers.

Ring plate OD ground down to fit.
New bolt flanges are close but don’t touch.

So, with the resized ring plate, and grade 8 flange bolts, I put on the flywheel, drove in the dowels and torqued the new bolts. …and measured radial runout at over .005". Same as before but this time I had rotated the FW 180 deg in hopes (ha-ha) of improvement. What’s going on? I took all the bolts and dowels out and just turned the flywheel on the stationary crankshaft while watching the dial indicator. It showed about .005" runout. I wondered about the sanding of the registration hole I had done before, with the first attempt. It is no longer a registration hole (never was, since it was too small to begin with). So, using gravity (and sandpaper) to my advantage, I experimented with finding a rotational position that minimized the runout. Then I turned the crank to match up the dowel holes. I put the bolts in but before tightening them, I pushed in the dowels which caused some movement. I torqued the bolts and measured the run out. No change from lousy. I loosened the bolts, removed the dowels and retorqued the FW in the minimal runout position. New runout measured .002". -and axial(lateral ) runout remains under .001".

Marks are axial runout 0 to .0005".
Radial is .002"-.003"

So, given the choice of less runout without the dowels or more runout with the dowels, I’m choosing the former. Here is where I know enough to be dangerous. I think the dowels are there for ease of assembly in manufacturing and to locate the flexplate concentrically. I doubt they are there to stabilize the flywheel torsionally. Ten 11/16" bolts should provide enough clamping force (over 50 tons if my chart is correct) for that. If I can get better radial runout without them, so be it. I don’t really care out why they don’t position this flywheel perfectly concentrically and I don’t have the instruments to probe that rabbit hole. The key requirements of secure mounting and acceptable runout are met, for me.

Good stuff. You’re either a genius or a worry wart, lol. I never checked any run out on the fly wheel itself, only axial run out with everything assembled. I don’t know if my fly wheel had any radial run out.

But, for some reason mine went together with no drama, dowel pins, center hole etc. everything fit the first time.

Regardless, I see no downside to having everything set up as close to perfect as possible as you have done. I would agree that the dowel pins are there to locate, and not to strengthen.

I hesitate to ask, and I don’t remember if it’s previous in the thread, but did you balance everything, the clutch and flywheel as a unit?


PS: I think we discussed this, but make sure the bolts you use for the pressure plate are not bottoming out in the fly wheel.

Probably more worry wart. Thanks for the encouraging words.
I checked several sources: RAM, local speed shop, engine repair shop, etc., and everyone said that the components are balanced individually and that unless I’m building a racer (not), not to worry about it. The nearest place that does that work is an hour away. I have changed clutches and flywheels before and never had a problem… I know, NASA thought the same thing about those o-rings and look what happened!
I have been warned!


1 Like

Doing that now with new shoulder bolts from McMaster. The Summit ones were too long.

1 Like

Just don’t start it in subZero weather… :slight_smile:


That’s what the engineers said!.. :wink:

What happened with the project? I hit an obstacle, lost momentum and stopped writing.

After installing the clutch and bell housing, I bolted the empty gearbox (I had the it apart for inspection) onto the bell housing to see where things would line up under the car. Everything looked OK for fit except the shifter. It was too far forward in the console! See pictures below
Gearbox to shifter dimension

Dimension transferred to floor under car and pilot hole drilled

View of pilot hole and distance to target (orange spot is centered on auto shifter location.)

Tailhousing installed with shifter core and console placed. Dummy “shift-stick” all-thread can’t go into
first, third, and reverse.

I knew from the measurements it was going to be tight. But I cut a hole in the floor anyway to see how close it really was. -too close without some serious shifter modifications! This was unknown territory for me. I’ve done very little welding and I don’t have the equipment or skills of a good fabricator. Nor was I confidant enough to design something and have someone else make it. Looking for a solution on-line, I found Core-Shifters, who specialize in this area. Their first advice for this kind of situation made perfect sense: Start with the right transmission! -or the correct model for your needs! I knew T5s had interchangeable tail housings because of the many makes-models of cars that used them but I had no idea what would work. For example, Mustang tail housings are very easy to find but the shifter position is too far aft. I just needed a few inches to put the shifter where I wanted but didn’t know which T5 donor tail housing might be the answer until I happened upon Core-Shifters. They have a table showing the shifter position for about a dozen cars. Eureka!! This is just what I was looking for! The answer is a tail housing from a Chevy Astro van. Its shifter box is 2.6" farther back than the S10’s. I found one on eBay and it just arrived, longer shift rail included.

Side by side, only difference is shift box location

S10 tail on the right. Astro on the left is over 2.5" inches aft. Should be on target.

So, I feel safe to say I’m back on track. The T5 has its guts back in, I need to install the Astro shift rod and tail housing, and shim the input shaft bearing. I have a final plan for connecting the hydraulic lines to the clutch bearing. The supply line is already clamped inside the bell. See below. I will attach it to the bearing before I spear it using the Brackney method.

Holes and grommets added. Supply line clamped before installing bell.
The bleed line will go thru the starter side (Left in picture)

There’s still lots to do!

Good Stuff! Glad you are back on track! Great tip on the site about the tail housings. I used a mustang tail shaft in my XJS, it’s about as far back as I’d want it. It’s not too far back, it’s perfect for me, but any more further back, I don’t know that I’d like it.
Funny that you are attaching the lines to the bellhousing. I did not do that, and on my one year inspection under the car a couple of weeks ago, I noted that the braided lines were just occasionally touching on the pressure plate. Just enough to leave a little bit of a shiny spot. So, I did something similar to what you did, except I had to do it with everything in the car, which required some creative language. But at the end of the day, it’s all good. Glad you thought of it ahead of time. Onward!

The transmission is ready to install, though the Astro tail housing was not as plug and play as I had hoped. I assembled it with the longer shift rail that came with it and found a problem. There’s a metal block -a shift lug -which is pinned to the back end of the rail with a roll pin. The shift lug has a socket on top that accepts the ball end at the bottom of the shifter. The motion of the shifter is translated to the shift rail via the shift lug. Inside the transmission cover at the forward end of the shift rail, there’s another metal block with a finger that engages the shift forks. It’s a very simple system, but everything has to fit perfectly, or it just won’t work. It won’t even assemble. When put all together, with the shift forks and shift lug in their neutral positions, the roll pin holes in the shift rail and shift lug should line up to accept the pin. Mine was off by about the diameter of the pin as shown below.

Rail & forks assembled with top cover off for clarity. Arrow points to misaligned shift lug hole

…and the rail is just too long enough to interfere with the shifter fit in the lug socket.

So, I had apparently found an obscure limitation of T5 parts interchangeability. Ideally, I wanted to find the correct parts to make this work, but I could not even ID the parts, let alone search for them. The quickest solution was to get a longer shift rail from a Camaro, cut it off to the correct length, and drill a hole in the correct place. This wasn’t as simple as it sounds but with some careful measuring and fixturing and a carbide drill bit, it’s done!

New modified rail, 3/16" shorter with proper pin hole placement.

Now, in the two steps forward, one step back nature of this project, I discovered another problem by accident. When I installed the bell housing, I realized that if I ever had to change the starter on this beast, access would be a real PITA. I decided to do a preemptive replacement and got a rebuilt starter from Rock Auto. After I put it in, I thought it would be a good idea to test it before I returned the core. So, I sat in the driver’s seat where I could see the clutch through the hole in the floor and turned the key. Yep, it spun the engine nicely, but I noticed something else: The motion of the pressure plate spring center, where the throw-out bearing would push, was eccentric. Its rotation was off center (??!) ( I tried to upload a GIF but can only upload a .jpg.)

I felt like Sgt Schultz in Hogan’s Heros -“I see nothing!” I stewed on it for a few weeks while I figured out how to mount the clutch m/c reservoir. Then, as I was getting ready for the final preparations of putting the trans in, I thought about the radial clearance between the spring fingers and the support tube of the t/o bearing. It’s very small and not easy to check before actually installing the trans. So, it has to be addressed.

Note to self: next time go old school with an external slave cylinder and fork setup!

It may sound like I’m hopelessly mired but I actually feel pretty good about my progress. I found a neat way to mount that m/c reservior…

…can you tell what that chrome rod is repurposed from?

…and my exhaust system finally arrived from AJ6!

The test fit went well and I’m confident I can resolve the off-center pressure plate without too much pain. -just need some warmer weather and spring is coming!



I’m missing some thing here. Those clutch fingers must be perfectly centered, not just because of the transmission input bearing retainer that the throwout bearing rides on, but because if they’re not, it will wallow the throw out bearing. I know you’re aware of that, I’m just a little lost as to how you’re going to address the issue? Is the clutch not centered on the flywheel?

Hi Bob,
Although I did say in a previous discussion that an off-center clutch would wiggle the t/o bearing, I’m not so sure about that. As long as the centers of rotation between the two parts are aligned (which is why we take pains to center the the bell housing registration hole to the flywheel), I think the shape of the contact area is irrelevant. It could be square or oval or in this case, a circle slightly off center. It can’t be so off center that it hits the support tube or exceeds the area of contact with the t/o bearing face. It definitely looks funky! In any event, I did measure the runout of the inner lip and got repeatable but highly variable numbers from 0-.018" and not describing eccentricity, but rather an imperfect circle. Since it is a stamping, and not a machined part, I’m not too surprised. Just looking at the edges of the P-P “feet” where the bolts attach it to the flywheel, there is very little variation. They almost match the edge of the flywheel within a few thousandths. I’m going to recheck the seating of the shoulder bolts, I had to grind a couple of threads off the ends so they wouldn’t bottom out. I dunno, maybe the spring in the P-P is not centered.

OK, I see your points. Agree that it shouldn’t matter, except that where the fingers make contact with the throw out bearing will vary with every shift. Sometimes they will be slightly to the left, sometimes slightly to the right, sometimes slightly to the top, etc. There won’t be any wallowing because the center of rotation of the TO bearing and the center of rotation of the fingers are concentric.

I have never seen the fingers be noticeably off center, but I’ve also never paid that much attention. I don’t know who’s clutch you’re using, but I have found the RAM helpline to be pretty solid. One thing that I would be a little concerned about is if something is off inside the pressure plate, the balance is going to be poor. I don’t recall if you had the pressure plate and the fly wheel balanced as a unit or not.

With the right shoulder bolts, not bottomed out, and with you visually checking the edge of the pressure plate in relation to the edge of the fly wheel, it’s hard to imagine how the pressure plate could be off-center on the fly wheel.

Well, that’s the thing…how many people spin the starter and look at the center of the spring? I just did it incidentally because I was checking the starter.
I am using the same clutch assy and t/o bearing as you.
…and you’re right, RAM does have responsive helpline. I asked them about this and they suggested that the clutch was off center on the flywheel. That was before I saw that it is very well centered at the edges to a flywheel with <.003" run out. So, I’m going to use some different shoulder bolts (the ARP ones I got from Summit have a taper from the threaded part to the shoulder and maybe that is allowing some slop).
In any case, I will check the clearance of the t/o bearing support tube to the spring fingers. I think that’s the key element.
Thanks for your comments!

Been thinking about this, the only other thing that is in the back of my mind is that the throwout bearing will be very slightly side loaded with each activation of the clutch. I don’t know if it’s a big deal or not, but we are dealing with thousands of psi, so even a tiny bit might make the throw out bearing unhappy over time.
Will be curious to see where you land.
Nice job on the shifter rails by the way glad you got that worked out!

You might consider confirming the clutch is actually concentric with the bellhousing. Just to be sure.
Here’s how I did it…