Friday, February 14, 2014

Five Lug Conversion

These pics are from a few years back when I first did this on my '91 BRG. The object of this exercise is to convert the Miata from 4-lug hubs to 5-lug with the 5X4.5" (5X114.3) bolt pattern. I used RX-7 hubs on the front, Mustang Cobra hubs on the rear. Later, I used re-drilled Tbird hubs on the rear.

91 spindle on left, then RX7 for strut type suspension, then RX7 for swing arm IFS.



RX7 is considerably larger but uses same design for bearings, just a little bigger. About .200" longer and .200" larger diameter. Calls for a spacer and slight modification of the Miata spindle.


 Modified spindle with spacer and RX7 5-lug hub.

Spindle with spacer installed


Then the hub. Spacer is shorter than inner race of bearing so full clamp load is on the bearing. The spacer essentially floats. Slip fit on shaft, slip fit on bearing.



Modification consists of cutting the original diameter down .200" into the base.

 Modified Miata on left, stock Miata on right.

Following pics are from 2014, did another set. Pretty much the same, slight difference in that the cut on the Miata spindle base was to the diameter of the Rx7, rather than the Miata. I figured there may be a slight strength advantage to having the larger diameter of the RX7 at the base.

  Left and right side Miata spindles modified with spacers. Note the diameter at the base of the spindles, same as the OD of the spacer, which fits the ID of the bearing. Slip fit, not a press fit anywhere.

Spacer dimensions: ID=1.101", OD=1.298", Length=1.764"
Dimension from top of installed sleeve to base=2.00"
Dimension from top of shaft to base=3.00"

 



 Same idea, the spacer is shorter than the inner race, so the races are clamped by the nut, the spacer floats.




Stock Miata 4-bolt front hub on left, RX-7 front/Probe rear hub on right



When I did my '91 in 2007, the Cobra IRS hubs were available for $50.00 ea. As of Feb 2014, they were in the range of $150-180. So it seems reasonable to redrill Tbird hubs. However, the 2002-08 Explorer has an IRS design using some of the same components as the old Tbird/Mustang Cobra IRS, so the hubs may work in this application. I haven't tried it yet. A quick search on Rockauto shows the Explorer hub with bearing for $38.00. If it works, that would be the way to go.

The following pics show modification of T-bird rear hubs to get the 5 X 4.5" pattern. The stock T-bird has 5 X 4.25". Also snout diameter is reduced to that of the Miata so this hub now fits into the stock Miata upright/bearing, and has the Ford splines which work with the Factory Five Roadster axles.

Drilling was done on a milling machine with the hub held centered in a rotary table. My approach was to center on one of the stock holes with a probe in the drill chuck, with a line thru that hole and the center hole perpendicular to the x axis of the machine. Then rotate 36 degrees, offset .125" and drill. Rotate 72 degrees, drill, etc. Center drill, then 5/16" pilot drill, then 37/64" drill, then 19/32" ream.



The Miata and Tbird hub bearing snouts are virtually the same length. I measured 4 of each and the average dimension was the same for both at 1.424" long. The variation was less than .005". Some may have been pressed out of the bearing with the "socket and hammer" approach.

I measured the diameter of the Miata hub at 1.574", the Tbird at 1.654". So the mod consists of cutting the diameter of the Tbird hub down to 1.574" with the appropriate radius at the base to mitigate stress fractures.

 








27 comments:

  1. Mike, is there anychance you would be willing to create a set and sell it to myself? if so, how much? i have been wanting a 5 lug conversion for a long time so i can finally run my favorite wheels. please let me know!

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  2. If you are re using the rear upright, you should look into the BJ Protege front hub. They're 5x114.3 and use the exact same wheel bearing as the miata. Although i figured this out on my own I am not the first to notice this.

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    Replies
    1. What axle/cv joint do you use with the Protege hub? I thought I looked at this and they also needed to be resplined, which is more expensive to me than modifying Tbird hubs.

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    2. I use the BJ Protege5 5 lug hub and kept the Miata upright and halfshafts. I've had them pulled apart but i'll double check to make sure the splines are the same. Will get back to you soon

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    3. sorry for the late reply. I have checked and the hub mates with the driveshaft/CV joint.

      Mazda BJ Protege5 5 lug
      New bearing
      Miata upright
      Miata driveshaft/CV

      nothing needs modifying. I used the timken parts catalog to cross reference the bearing which is how i realised that the protege5 uses the same bearing as the miata.

      Timken Parts Catalog
      http://www.showmetheparts.com/timken/#

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    4. I understand. I tend to think in terms of V8 conversions and a heavier duty rear end and axle. But for those who simply want a 5-lug conversion, and retain the Miata drivetrain, that'll work for the rear. What did you do for the front?

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    5. with the amount of time i spent researching. i found nothing that was going to be easy. the only way was the route you took with the rx7/probe hub and sleeve onto the modified miata upright as you have done. I'll eventually find an easier alternative

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    6. I just did this on my K24 miata. In case people can't find Protege parts, there's something else that fits: I used a Mazda 323F (Astina) BA V6 2.0 5lug. Front is still in progress, I'm using Mazda 626/MX-6/Probe/RX-7 stuff there.

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  3. Found some front hubs on Amazon for under $9!
    http://www.amazon.com/Dura-International-295-12119-Rear-Assembly/dp/B00279H1UY/

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  4. What brake calipers do you use when you convert this?

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    Replies
    1. I would also lile to know what calipers you have used.

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  5. This comment has been removed by a blog administrator.

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  7. How did you turn down the spindle base? A lathe?

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    Replies
    1. Yes. The spindle has centers front and back, so I supported it between centers on the lathe, drove it using a "precision machined" dog that straddled one of the legs and was driven by 2 of the chuck driving cam lock pins. The only real difficulty was getting a decently smooth finish cut with the radius tool.

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    2. Exellent info. Thanks a lot.

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    3. Also by any chance do you know the spindle nut thread size? I made a hub to adapt 4x114.3 wheels and I'm trying to make a sholdered nut to fasten it due to increased bearing thickness...

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  8. I'm in New Zealand, TBird and Mustang Cobras arent everyday cars here.
    Would the hubs on Ebay auction no. 113391847296 be correct?
    Thanks in advance

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  9. What brakes do you use for the front? Rx7 all the way?
    Thanks

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  10. Didn't get to that ebay auction in time, the hubs are as shown, Raybestos 712119 or equivalent. Basically front hubs from an RX7, which are the same as the rear hubs from certain year Proteges. Rockauto is a good reference for this sort of thing.
    The actual brakes that I used are in the post on this blog entitled "Big Brakes". I started out with the cast iron calipers from the Tbird and ended up with aluminum PBR calipers from I think a 2000 Mustang. A friend who did the same 5 lug conversion used RX7 4 piston calipers.

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  11. This comment has been removed by a blog administrator.

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  12. Hello! Mike did you do the machining yourself or take it to a shop?

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  13. Anybody can recommend front hubs? Hard to come by RX7 hubs these days. The RX7 hubs have to be 1986-1991?

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