XJ12 5speed conversion

Greetings JL,
Nearly three years ago I took a trailer to pick up this '79 S2 V12. It had a T5 in the trunk which I wanted to eventually install to, as they say, wake up the V12. But before I did that, I wanted to get the rest of the car straightened out. It took a while and a lot of work -and parts - but now it runs very smoothly, accelerates well, and I feel it’s reliable enough to leave the extra water jugs and fire extinguisher at home. And so now it’s time to wake it up and have some real fun!

Last week I removed the GM box and started procuring parts. Right out of the gate, I’ve hit a snag. The first thing I bought was a Fidanza aluminum flywheel. When I compared it with the freshly removed flex plate I could immediately see that the ring teeth would not align in pitch (diameter OK though). I counted 160 teeth on the flex plate. The new flywheel has 162. Also, the center hole, which is a close fit to the crank on the flex plate, is much larger on the new part. However, on the plus side, the mounting holes are the same bolt circle diameter and size, and the dowels are the same.

I called Fidanza and they were puzzled by all this. It’s the only flywheel they sell for Jag V12. They speculated that the 2 extra teeth wouldn’t matter but I’m not willing to make that gamble. It guess it WOULD be possible to swap ring gears but it’s extra work and I would be rendering the new part unreturnable. Alternately, I could get a starter that mates with a 162 tooth flywheel but I don’t know what cars had those. I think I’m going to return it and look for a correct 160 tooth flywheel.

I looked at many catalogs and parts sites for more information about ring gear tooth count but most don’t provide that detail. Has anyone else stumbled into this thicket??

Tony H.

Check out : BobPhxBob Brackney - 89 XJS V12, 5 speed, Marelli. It’s around post 19 he speaks of the fly wheel.

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Hi Tony-
The extra two teeth don’t matter. Don’t worry about it.

As you observe, not a problem. The alignment is handled by the dowel pins.

If you are using a hydraulic TO bearing, you MUST get the bellhousing and engine block center to less than .01" TIR (total indicated runout.) I thought mine was close enough, it was not, result was leaky bearing about 3 weeks in. I used readily available offset dowel pins and a better dial indicator setup and got to to .0045 TIR. Maybe you already knew this, but if not, now you do! :slight_smile:

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Hey Bob,

Thanks for the tips. I followed NIB’s link and found your project. I’ve been reading it all morning and I must say congratulations on a job well done! If you wondered if posting your work might help anyone, the answer in my case is a resounding yes!! I searched for conversion projects before I started this new thread but I only looked in V12 category and missed your XJS project. Intellectually lazy, I guess!

My T5’s history is unknown. I believe it is an early world class box from a Ford but has a Chevy S10 tail housing which so far looks like the correct placement for the shifter just in front of the radio. It also has a strange looking front bearing retainer. This will be replaced with the Ford type with 4.9" diameter that will match the bellhousing which I just ordered from Dellow Conversions. (I tried to get the 5speeds bell you used but they have no stock and they estimate end of this year before they are available.)

While I wait for my bell, I am going to start on the pedal box conversion and look for more parts. Theres lots to do!

Best regards,

Tony H

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Pedal box notes. Yours will be different but might be helpful.

Hi Tony-
Glad to hear the write up was useful!

Here are a couple of links that might help you figure out exactly which T5 you have:

Also Paul’s book (no affiliation) has a great section on T5s:

Tony, one more editorial comment:
Almost any used T5 is likely to have had a hard life. They are all pretty old by now.

My original plan was to re-build a T5 because I thought it would be cheaper and I had never done it before. I bought a cheap used “good” WC t5 and broke it down. It was a mess inside, lots of pitted parts. Paul sells rebuild kits for reasonable prices, but I needed things like input shafts and counter gears. In talking with Paul, many of available aftermarket parts are poor quality. All were rather expensive. My rebuild was going to be $1300 or so in parts, plus the $200 I paid for the trans. The stretch to $1900 for a brand new trans was not a hard decision.

You can also buy re-man T5s for just about anything. But, you must know the MMY that you want. There are so many variations, and the critical dimensions are not provided on the reman sites. The assumption is you are going like for like, so everything is sourced by MMY.

IMHO your options are:

  1. Use existing trans and cross fingers. If it’s noisy or otherwise flawed, rebuild or replace with same reman model, assuming you can figure out exactly what you have.
  2. Tear down existing trans, inspect, based on condition, decide to rebuild or replace.
  3. Go with a reman right out of the gate, assuming you can figure out exactly what to order
  4. Buy new, which is what I did, but there is only one option that I am aware of (American Powertrain.)

Apologies for the unsolicited advice, just thinking out loud here. I spent a lot of time planning and thinking about my conversion, which I think paid off. Not only do you want it to work right the first time, you want it to be maintainable and serviceable in terms of parts, ease of install / disassembly, etc.


Bob, your advice is welcome!

I went with your option 2. I cracked the box open soon after I got it and it is very clean inside with no signs of abuse or wear. I checked all the things per Paul’s book and it looks like a safe bet. I bought a spigot bearing and it fits with about .010" clearance to the pilot shaft. Seems like a lot -not sure if its too much, but the input shaft is mainly supported by its taper roller bearing and as you noted, only turns with the clutch is pushed in.

Next step is to work out the clutch. Your discovery about GM vs Ford pressure plates fitting the flywheel is useful here. From RAM’s application chart, all GM 10 spline input shafts are 1-1/8" diameter. My input shaft is 1-1/16" which matches Ford. It looks like I will have to get the Chevy pressure plate and a Ford disc. I’m going to see if a local parts store has these pieces in stock so I can bring my stuff in and check the fits.

Stay tuned,

Sounds good! The only thing about the spigot bearing is that it determines how well the disc and thus the input shaft are centered when the clutch is engaged. If you wanted to tighten it up any good machinist could make you a new one.
Interesting observation on the shaft diameter, will be curious what you learn.

I measured and then installed the spigot bushing I got from SNG Barrat. The ID shrunk about .006" but that still leaves .006" difference between bushing ID and shaft OD. According to what I see from various opinions on-line that is borderline too big. Machinery’s Handbook has a chart for shaft size vs diamertral clearance showing a range of .001" to .003" for applications above 600 RPM. That makes it 2x too big!

I’m thinking of getting a new bushing and ream from McMaster and reaming it to 002" larger than my pilot shaft. (varies from .492"-.493") Trouble is, what ID should the bushing be before reaming? Since they are sold in 1/8" ID step sizes, I’ll have to get a 3/8" ID bushing and drill it close to but less than .494". Should I just get the next size drill bit down from .494"? How much material can a ream remove?


.010” or a bit more is a good target to remove with the reamer. Best results will be when you start with a quality hole. So if you get good hole with a 31/64 (.4844) drill, reaming to .494 should give you a good result.

Sounds like this is not your first rodeo, so I assume you know that you’ll have to sneak up on the 31/64 hole, stepping up through a few smaller bits . Don’t try to hog that thing out all at once from 3/8 to 31/64 :slight_smile:

PS. I talked to Dad and he said the best approach is on a lathe with a boring bar if you want a perfect hole. Cut to fit. He said “any back-alley machinist can do it.” :slight_smile:

I hadn’t thought about drilling in step sizes but I’m sure it would have become apparent.
I had also concluded this would best be done on a lathe to ensure concentricity and parallel. I could actually end up worse trying to fixture it on my drill press. I’m going to find someone to do the rough bore on a lathe but I’ll still get the reamer to finish it after installation in the crank since it will reduce about .006".

Thanks for the input!

If you have access to someone with a lathe, have them do the final cut with a boring bar, not a reamer. You’ll get a better result. And you won’t have to buy a reamer. My .02.

Well, I’d love not to spend $50 on a reamer I’ll probably never use again but that means I’ll have to estimate how much the part will compress and reduce the ID. It’s worth a try. I ordered 2 bushings yesterday. What tolerance would be reasonable for the machinist? (If experience is any guide they’ll ask.) I would think +/- .001 wouldn’t be hard.

I don’t think the compression should be an issue. My throw out bearing is quite thin, and we did not account for any compression when we machined it. It didn’t discernibly tighten pre/post install. Respectfully, I might suggest that any compression that you were seeing could be a result of measurement variances. We use a precision set of pin gauges for work like this. .001 tolerance is very reasonable. He ought to be able to get to .0005.

Another answer might be to pull the input shaft from the tranny and give it to the machinist. Tell him you want a very nice slip fit over the nose of the shaft. That removes any discussion about the accuracy of measurements, it becomes a cut to fit job. He would measure the nose of the input shaft, cut the throw out bearing to maybe .002 undersize, and then take a fine cut until it’s a perfect fit.

Or, have him make you two. One .003 over, another .001 over. Try them both and see which works best. Of course then you get the joy of installing and removing pilot bearings, for which I use the wet toilet paper method.


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WTP?? Do tell!
I’ve only used the grease pack method.

Also, I know I shouldn’t worry about this but how can the pilot shaft be turned down? How can the shaft be fixtured? -and I do have it out of the tranny.

Instead of packing it with grease, pack it with wet toilet paper. It’s a lot less messy. Just keep jamming toilet paper and squirting water in there until it stiffens up, just like you would with grease.

The only way that I would turn down the nose of the input shaft (pilot shaft) is via precision grinding, like a crank grinder would do. You won’t be able to get a true enough polish on a lathe. I looked into that as a potential solution for my problem, couldn’t find a local crank grinder that I trusted. Depending on the depth of the hardening, you might also be faced with re-hardening it.

There is one company, in Michigan, that I think could do it right. They are the ones who produce the shortened input shafts for the series 1 and two E-type conversions. I don’t know the name off the top of my head, but if you run a search in the E-type forum, you’ll find a lot of discussion about it.

This is the first time you’ve mentioned doing anything to the nose of the input shaft, did I miss something? I would think messing with it would be unnecessary, just machine the pilot bearing to fit, you should be good to go. Again, unless I’m missing something.

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One more editorial comment on pilot bearing crush. You only need about a .001 interference fit between the OD of the pilot bearing and the ID in the back of the crank. Anything more than that will make the pilot bearing difficult to drive in, and could result in some crush.

I did not need to do anything to the OD of the pilot bearing that I got from Moss. I just opened up the ID to accept the nose of my transmission input shaft.

I’ll try it. This reminds me of the plumber’s bread trick.

I unwittingly ran with your suggestion to take the input shaft with the bushing to the machinist. But now I do remember your concern about surface hardness with your IS. In any case it is a good idea to give the machinist both parts but I won’t have the IS changed. Although it has some wear, I think it would be overkill to have it turned and re-hardened. As long as I can get a good running fit with the bushing, it should be OK.

Looking at my notes, I had measured the crank bore to be .745" and the bushing OD at .753". However, to your point, since I don’t have pin gauges, my crank bore measurement (with a vernier caliper) is not as accurate as I wish it was. Judging by the fairly easy effort to install the bushing, it is probably closer to .75".
My new bushings arrived today and they too measure .753" OD. I am confidant of that figure because the tool (micrometer) is better.

BTW, I was looking at the picture of your pressure plate bolted to the flywheel in your project, post 74. Does it have locating pins, or just the six bolts to center it?