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Saito2

Madcap diff shims

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I have a couple of Madcap diffs I'm rebuilding. Whats a good starting point for the number of shims to install? The manual calls for 2 (one on each side) but but states more can be added for adjustment. I was thinking of starting with 4 (2 on each side) as I'm working with old, but usable parts. I'm aware over-shimming will simply distort the pot metal, cast diff side housings so I'm cautious about going over 4 (also know to use proper ball diff grease). Also, what grit paper should I use to sand the diff rings flat. Thanks.

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It's best to build the diff as shown in the manual as a starting point.  If parts BH3 and BH4 are worn on both sides, it's probably best to discard them and get some new ones.  If only one side of the rings have a groove worn into them you can flip them over to use the other side.  Part BH7 is a super thin shim that you really only need to use if or when the diff starts to slip excessively.  I'd say that if you need to use more than one on each side, it's probably time to install new components throughout.

For sanding diff plates, I'd use finer grits or wet/dry.  Start with an 800 or 1000 grit and go up from there until you're satisfied that the surface you're sanding is as smooth as possible.  Sanding on a very flat surface such as a sheet of glass would also help you get good results.

 

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Yeah when I first got my madcap I rebuilt the diff with 2 shims on either side straight off the bat. To say it distorted the housing would be an understatement...it actually sheared the side off one of the diff halves completely. Not good...

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3 hours ago, Silver-Can said:

Yeah when I first got my madcap I rebuilt the diff with 2 shims on either side straight off the bat. To say it distorted the housing would be an understatement...it actually sheared the side off one of the diff halves completely. Not good...

After going through my pairs of diff housings, I can see why its possible to shear of one of the diff housing halves. The casting is incredibly thin at the recessed "ears" where the three screws go through. One of mine actually had a small hole in the casting at this point. The next pair I had were badly warped. The pair after that were perfect, except it was run its whole life (by the PO) in a Madcap with the stock bronze bushings. Now the area were the bushing/bearing rides is worn about 0.010" undersized. The forth set looked usable. Sigh.:mellow:

Incidentally, I grabbed an Astute manual when going through the diff on the bench as I am using the Astute/King Cab style of thrust bearing and now I see it differs from the Madcap manual in respect to adjustment. The Madcap doesn't use the shims in the standard setting (though one is depicted in the assembly picture, probably to show orientation should one be added) and states one could be added for adjustment if necessary. The Astute (which uses the same diff except for the "assembled" full metal, thrust bearings), simply shows two shims going in as standard and states more or less of them could be used for adjustment. More certainly sounds like a bad idea. 

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I found the diff in the Madcap to be tight enough without the use of additional shims.  I only added them once I noticed the diff starting to slip, affecting acceleration of the car.

I wonder why the assembly instructions for the same diff is different between the Astute and Madcap.

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3 hours ago, Saito2 said:

After going through my pairs of diff housings, I can see why its possible to shear of one of the diff housing halves. The casting is incredibly thin at the recessed "ears" where the three screws go through. 

Yep, that is exactly what happened. Once I replaced the housing I didn't shim at all initially. Around 6 months later I added a single shim either side and have left it at that. I also managed to score the 'upgraded' internals but have not yet used them. Not really sure I ever will as I don't run my madcap often. I have more modern/race kits to scratch the performance itch, as well as more robust re re's for the nostalgia so I never really found my niche for the madcap. Apart from looking awesome 😁

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7 hours ago, Fruitfly01 said:

I wonder why the assembly instructions for the same diff is different between the Astute and Madcap.

Me too. The Saint Dragon is the same as the Madcap. The King Cab is the same as the Astute. My only guess is despite both having 540 silvercan motors, the Astute was seen a more "racy" and might need a tighter diff with a hotter motor that might be installed at a later date. The King Cab might be down to the greater load imposed by the bigger tires. All just a guess.

7 hours ago, Silver-Can said:

Not really sure I ever will as I don't run my madcap often. I have more modern/race kits to scratch the performance itch, as well as more robust re re's for the nostalgia so I never really found my niche for the madcap. Apart from looking awesome 😁

I've backed off running Madcaps due to the diff but I have so many spares and no one shows much interest in buying it so I decided to run it anyway. Beats sitting around collecting dust.

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5 minutes ago, Saito2 said:

I've backed off running Madcaps due to the diff but I have so many spares and no one shows much interest in buying it so I decided to run it anyway. Beats sitting around collecting dust.

I'm in the same position. I do have some spares and I do give it gentle exercise on occasion, as it does run really well. I guess I'm just a little hesitant about 'giving it the beans' as I don't like to abuse my models but, by the same token, I do like to run them to appreciate their movement (how the suspension soaks up the bumps, how the chassis rolls and leans in corners etc) which necessitates some degree of speed. At least it looks very pretty on the shelf between runs collecting that dust 😁

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4 hours ago, Saito2 said:

Me too. The Saint Dragon is the same as the Madcap. The King Cab is the same as the Astute. My only guess is despite both having 540 silvercan motors, the Astute was seen a more "racy" and might need a tighter diff with a hotter motor that might be installed at a later date. The King Cab might be down to the greater load imposed by the bigger tires. All just a guess.

I've backed off running Madcaps due to the diff but I have so many spares and no one shows much interest in buying it so I decided to run it anyway. Beats sitting around collecting dust.

I have the same thoughts about the build differences, although it still doesn't make sense.  Adding more shims to a new diff will only add more pressure to the components and make them wear quicker.  That might not be an big issue with the Astute, but I can see the extra weight and driving potential of the King Cab causing premature failures.

I'm surprised that no one has shown interest in buying your Madcap or spares.  Recently I've seen these cars promote frenzied bidding wars and attain good, even ridiculous prices on Ebay.

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I've never really done Ebay for selling. I've had it on here for many months awhile back but the cost of shipping is a deal killer, plus I'm in the US. The best offer I got would have netted me $40 US, so I kept it. I'm going to give the trans one more chance and keep the motor a silvercan. If it fails, I'll replace the trans with something else (and the chassis too, as I won't cut up an original Madcap one.

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It pains me to say this as for decades ive been a fan of the Astute and its differential its shortcomings are HEAVILY over stated.  That being said with dwindling spares and high associated costs id recomend piecing together the rere super astute TTC. I did that for my Astute, Kingcab, and Monster Racers.  I added  differential putty to the gear diffeential, turns it into an AMAZING modern bulletproof trans (that when paired with a metal idler gear brace).

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