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I'm rebuilding a Differential out of a '68 for my buddy Brett. He's going to use the car at the track from time to time so he wanted a strong differential. One of the mods I'm doing to it is fitting a Tom's steel cap. I've wrote about these before but this one proved to be a bit of a job and got me thinking about what guys would do if doing this at home. The instructions are very brief and the process is basic. Depending on the cap you get and your housing you may be in for fun. After the last 3 steel caps I fit, I would not recommend doing them without access to a grinder and maybe a mill. Here is what I found with this one.

First this is the cap compared to a stock one. I've posted this before but in case someone new doesn't know about this, here it is.



First the holes for the LH cap in the housing need to be tapped for 1/2-13 bolts- up from the stock 7/16-14's. Now you can step drill like the paperwork says but I was concerned about keeping the holes perpendicular and not oval them out with a hand drill. I set up the housing the Bridgeport and indicated the pads. Drilled the holes out with the tap size drill and tapped them all in one place.




Ok, now the cap should be fit over the new race and the gap under the pads should be set to .001". Here is where the fun started. I put the cap on and like the last 3, the pads had way too much gap to start with. The only thing to do is work on the ID of the radius. In the past I polished the ID little by little to drop the cap down on the race to set the .001 gap. This one I just couldn't get the gap set. Finally it occured to me:lookinup: to check the pad gap without the race in place. It should be flat- metalon metal.The pad gap was .002 to .000" it was not flat and I wasted time trying to fit the cap. I had enough of that cap and put it aside and got another new one out. This time I checked the pads and again the was a gap? So I had to get the pads flat first. I set them up in a vise, indicated them as flat as I could get them and ground them until they were flat across. The cap fit nice on the pads now, .001 feeler couldn't fit in between them. Then I checked the fit with the new race- Ha, the SOB had .020" gap now- No Good. I set up the cap in the mill, swept the ID to center it, then setup a boring bar to sweep the ID. With it centered I moved it up .020", bored the ID to fit and then had to go back and final fit the pads in the grinder again. Don't ask how long this all took:thud:

Here are the pictures as I went along. I have the cap fit now. With it on the race there is a .001 gap under each cap leg and this will allow to crush the race just enought to pinch it from spinning under load. The steel cap is much heavier then the cast cap and I like them but I can see where this would be a real problem in your garage if faced with what I was.















With the first one I decided to toss it out rather then use it in a job. What I did was to set it up and polish it like you might had home. I just wrecked it more but it did prove my point.





I then set it up in the mill and tried to dress the ID back to size by moving it up in the cap. I had to go .120" to get a nearly full radius that would work but then I would have had to set it up and machine the pads down to set the gap. I didn't bother I'll use it for a demo at Carlisle instead.


Here is another job with a fit cap that didn't take me as long to fit.:smack




The same goes for fitting a steel solid sleeve in place of the crush sleeve. The shims in the kit are worthless, the shim is not parallel ground and needs to be fit to be correct. Just something to think about if you are planning this for a garage job. It can be done just be sure to plan it out first.

:thumbsup:


Gary
 

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Here is that cap I bored to see if it would be ok to use. I had to go too deep with it and will not use it. You can still see where the blue dykem is and would not clean up enough.

The one to the left is a new cap and the caliper is on the bored one I'm tossing.






 

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Wow, you took off a lot of material there, dayum! You really moved the bore centerline up, looking at the radius and where the blue dykem is still there, meaning that's about the height of the bore center.
 

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TT, Yeah that's right about .120" deep. This cap was already trashed by attempting to polish it out. I was as careful as possible but I wanted to try and see how it would be if being done at home. Then when I started to clean up the ID I wanted to see how much it would take to get it back, as you can see - too much.
I could actually use this cap but I don't know how it would last. Probably ok in normal conditions but HD use may be a problem. It will hold down the tabel cloth at the swap meets!
 

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just wondering if you've ever tried a Mark Williams cap instead of a Tom's Differentials cap.

is it the same as a 12-bolt aftermarket cap?
 

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Discussion Starter #7 (Edited)
No I have not used a Williams cap, might be worth a try. Most jobs really don't require the steel cap. If you're going to push the car hard or just want the comfort of large bolts and a steel cap. It's not unusual to fit parts these days either. Even though they should be close it is not always the case.

EDIT: took a look at the Williams cap online looks just like the Tom's cap. Of course no way of telling how it would fit until you get one and try. I have some Tom's caps here on the shelf so I'll wait until one is needed and I'm out.
Cost was about the same,maybe $5 less.
 

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Let me share with you my Tom's Differentials cap horror story. I decided to use one of his "Super Trick" caps on my 72 LT-1 differential. I use the Sunnen rod cap grinder at my buddies machine shop to clip the cap for the 0.001" of "rock" Tom's instructions tell you to do (I love the precise instructions!) I found the cap mating surface went WAY OFF on the first pass of the Sunnen cap grinder (with only 1/3 of the mating surface showing grinding marks on it.) I then discovered that the bore was NOT perpendicular to the sides of the cap (this was an older cap without the raised part of the cap you show in your photos.) Now at this point I was really PISSED at Tom's and sat down for a moment to collect my thoughts, I then realized I could cut a bearing race in half place it in the cap (well almost in half, I left it 0.020 longer) clamp the cap back into the rod grinder and square the sides to make them perpendiculat to the bore and THEN begin to clip the cap for my fit (I used a dial bore guage rather than Tom's excellent method of 0.001" of "rock".) After clipping the cap face to full squareness, I was 0.005" under the diameter of the bore. I then did as you did and set it up in the mill to rebore the cap. I don't think very highly of Tom's Differentials and have seen a few of their "trick" 12bolt IRS differentials fail at auto cross races. Buyer BEWARE!
 

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Ah, a Last Word, a 120 caliper and I believe a 230 mic. You, sir, have chosen your measuring instruments wisely.

Starrett > all others

Edit: Whoa, that's a B&S mic! Too late in the evening for my own good here.
 

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45- yes B&S 1" mic and Starrett last word and indicator. Very good USA tools.

LT1- I have to agree that all parts need to be checked. The cap that is NG is one that I ASSUMED was flat and it wasn't. For this reason I hesitate on selling a part such as this to someone who doesn't have access to a shop. The instructions that come with it are vague but usable if the machine work is correct to begin with. That job was a PITA and took me about 5-6 hours to do it all. Most jobs don't require a steel cap. The diff my son blew apart had the stock caps with the SHCS and they were still tight after it blew.

This is the reason I posted about this. I don't see how a guy working at home will be able to set these up correctly given what I have seen.
 

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Very nice work and a great write up :thumbsup:

WIth all the machines and tools you own and your machining skills I'm really surprised that you're buying these instead of making your own from scratch.... maybe a bit more work but less aggrevation :cheers:
 

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You're right, in the long run I should have made my own. To save time and money I bought these. I have one left but will have to do the same thing I'm sure.
 

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Just a quick update, I have not had to rebore any steel caps since the ones shown here, 8 years ago. I still follow the same procedures but they have all been long in the pads. I have done a lot of them since those too.
 

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Interesting Gary.
Will read all later this morning.

Another way is to Offset bore.
Bob has done that for me.
I have access to my own Bridgeport Verticle Mill.
My little brother has nicer Bridgeports & Real Nice Kurt Vices.
 
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