I ran across people selling Mark Willaims 8-3/4 main caps on Ebay and had no idea such a thing existed. Are these a worthwhile purchase, or save my money?

If you run a 8 3/4 they are worth it

It depends...what's the application?

I have an cap that a friend did for me. I think it is an good upgrade, 8 3/4 need all help it can get

Remember, this is a good time to analyze whether you're ever going to swap in a 9" or Dana. If yes, any money spent keeps your 8-3/4" going but subtracts from your upgrade fund.

When I installed a set I tapped the existing holes to the bottom of the factory holes and went with longer cap head screws.

Understand that the caps need to be machined
to fit your pig so they are not a bolt on out
of the box

Who is selling them.. I'd be interested in a set of caps too too.

W2 408 in a full street trim 70 Dart. Mostly street use. No transbrake. 235/60-15 drag radials.

Being in street trim you need them or a better
diff.. I have a lite car with the alum diff in
it and its doing great.. in my Rampage I run a
9" with a 416 and W-2s.. I like the take out
diffs and wont change

I did OK w/ MW caps on both sides (some people only use one on the right side, I think), a 742 case w/ a Power-Lock (clutch-type) sure-grip, and a small back-brace for the housing. It SOUNDS like you'd be OK, but nothings a sure thing with an 8.75", IMO.

Left side / Driver side......... I hope

Been in the 9's and 1.3 60 ft. for years from 3200 lbs. now down to about 3050 w/a 470 BB stroker and a stock capped 742 but recently went to the Nodular from Dr. Diff w/forged caps so it should out last me, but I don't race much anymore............

Mostly street use, I'd say you should be OK w/ the 8.75. IMO, the caps are a waste. If it has enough steam to go 10s or so, I think you should consider upgrading the whole rear.

I've seen more than two stock cast steel caps break on 8 3/4, I would put at least one on the driver side
The stock 8 3/4 center section are cast steel so adding a billet steel cap will stop breaking the stock cast caps
How much weight and how fast has your car gone at the track?
Spinning and hooking along with medium to heavy weight is what causes the stock caps to break

Every engineered item has a weak link. Making the weakest link stronger (in this case I believe consensus is, that I agree with is the DS Cap), which means the next link is now the issue, and so on. Does this billet cap make an 8.75 a more robust rear, yes, is it robust enough for you, don't know, at some higher power level will it fail, absolutely, have I made this two sided cap upgrade to all my steel 8.75'x, yes, would I do it again, yes.

I went the direction of adding a MW steel cap on my 8-3/4 a couple of years ago. Be aware, as Mike noted above, that they are not a bolt on. They need to be machined to fit your pig, and that process cost me more than the cap did. It's notable that on the nodular Iron units, they upgrade to steel caps from the get-go.

I don't throw a lot of power at mine, but I do hit it with a lot of weight (3800+). I see a lot of 10-second cars running them, and believe they can be a decent option if we'll prepped. I'm of the belief that a 742 8-3/4 with a back-brace, spool, steel caps, and good axles is more durable than many here will have you believe.

I will say that if you put the cap/caps on it
that you should put a back brace on the diff
to keep you from bending the axle tubes(try to
make them as long as you canto support the tubes)

So by the time you buy the MW main caps and have them machined to fit, you could have bought a junkyard Dana 60 and had it narrowed to fit your car. The gear, spool, and axles cost the same for both rears.
I just don't see the point.

So you can buy a Dana and have it narrowed for $200-..... I doubt it. Nobody is arguing that a Dana isn't a good option, it is. but I think you're over simplifying the Dana swap by a pretty large margin. Especially if you already have the 8-3/4.

Perches, brakes, brake lines, gearset, spool or sure-grip diff., yoke / driveline work, narrowing..... these things all add up and aren't free when doing a swap. If you already have an 8-3/4 with the correct gearing that fits your car, you can have that unit prepped to survive at the drag strip for around $500. I'm guessing that's around the cost of the junkyard Dana you are starting with. Not to mention you can pick away at it and not have to come up with all of that $$$ at once.

Each option has its pros & cons. Having an 8-3/4 under you car isn't a kiss of death like many will make it out to be.

No, not for $200. Looks like the MW caps are $130 EACH (quick ebay search). Not sure what it'd cost to machine the centersection for them, but I'd guess around 100 bucks. About the same as narrowing a rear. Even w/ the good caps, the main problem w/ the 8.75 is still not addressed and you'll still kill gearsets.

I've bought 2 junkyard Dana 60s for $125-150 each. Both came w/ good gearsets and one had the good powerlock diff. So after the initial cost, a pair of axles and housing ends, narrowing, and a pair of perches...ready to bolt in. The brake lines are what, 20 bucks max at the parts store. One of mine already had a 1350 yoke too.

I agree that if you already have a well set up 8.75, swapping to a Dana doesn't SEEM like a cheap option, but it is. Sell the 8.75 to someone who thinks they can use it and build a Dana. Then you can sleep well knowing you'll likely never break your rear again.

Machined as in having the mating faces shaved so the inside is perfectly round when it goes together? There is a diff shop here that knows their stuff, but I don't know if they can do the machining depending on what needs done. I have not talked to them about the caps.

I have a 489 case with 3.55 gears and a solid spacer. It was clear powdercoated years ago and it started to rust under the clear + it was weeping oil in a few spots so I took it out and removed the clear. I was just going to bolt it back in until I saw the MW caps.

I know the ring gear teeth can shear off and the axle tubes will bend inward under heavy racing. I know a local guy who ran one forever in a first gen Cuda and it wasn't until his car became capable of pulling the front end off the track did he start to have issues making him go to a Dana 60.

My dad had a 8 3/4 in his duster back in the mid 90's. (440 ran 7.30's in the 1/8th has been in the 6.80's more recent years) Would break a ring and pinion every year. Finally put a Dana in it in the late 90's and hasn't broke anything in the rear since... He did build a duster for my sister in 2011 with a 360 run's 11.80's with the 8 3/4 and hasn't had a problem with it...Keep the build simple and low torque and you'll be fine. Increase torque and power, you'll find the weak link.

I run high 8s in the 1/4 with a alum 8 3/4 but
the car in light.. I wouldnt waste my time with
a dana 60.. just go to a 9" which I did for my
Rampage being its a bit heavier.. its 2860 race
ready with a 18 gals of fuel.. but thats me..
the guys on this site think you HAVE to run a dana
when you dont.. JMO.. if your not running a tank
or have a ton of power then your fine.. it will
run a lot of years... its up to you what you want
and so far its been a 8 3/4 being that your looking
at caps

Please explain then how my stock capped 742 case has lived 14+ years and most of that at 3200 lbs. w/a BB 470, running low 6's in the 1/8 and 9.70`s(5 years ago)in the 1/4 w/1.3 60 fts..............

When I did it I went to my engine builder with the empty chuck and the cap, he did the gross cut in the milling machine and the final cut in is rod cap grinder. At minimun do the left side, ring gear side, that side sees the higher loading.

I have no issue running 8 3/4. 2 of my cars use stock parts without failure (12.50 stick, 11.0 auto). But if it was going in a car that was going to be raced often I'd use a junkyard Dana. Mine and Dizusters turbo car got new ends, axles, and a spool. I cut the ends and brackets off and welded on new perches and ends. Both started out as a 8 lug full floaters. Dizusters still uses the junkyard gear set. Mine uses recycled 8 3/4 factory housing ends. Spool, axles, and gears cost the same either way. With a Dana you'll get either a 3.55 or 4.10 so you may not even need to buy a gear. I'm cheap. Junkyard Dana's are a great bang for the buck.
Doug

Yes, but you will have to size them....

My car runs mid 10's on a drag radial, footbraking. I don't have money to upgrade to a Dana or 9" right now so I bought a brand new Yukon Nodular iron center section with nodular caps for $250 With a spool and 35 spline axles I think it's good enough for now with my setup. Just another option for you.

FWIW, I sell a new, super duty nodular iron 489 case with forged steel (counter-sunk allen bolt style) caps, and billet steel adjusters for $300.

You will spend more than that to put MW caps on a used case.

Just a bit more info on this, these caps are $105 if purchased from MW enterprises direct, someone is trying to make a fast buck out of a great manufacturer, and as far as I can find out the only one still producing this item.

I run one on my 489 center section for 20-something years lol, been 1.20's with a best of 1.26@3350 with no problem, but if I had to to it all over my choice will be a 9".Like Mr.P Body say I like the removable center section

If you ever think you will use a transbrake, or go to a bigger motor (bb)? I would lean to the Dana rear rather than spend $$$ on an 8 3/4. Shock loads and high torque, and or poor gear setup will kill ANY rear eventually. That is why they sell pro gears.

If I were going to run an 8 3/4 hard this is what I'd run. My 408 broke the 8 3/4 in my Dart on the second pass it hooked up. The transmission had to come out and apart along with rebuilding rear. I sold the rear end complete after I went through it and ordered a new S-60 from Strange.

Looking back I wish I'd been patient and found a deal on a used 9" aluminum center section and bought a new housing and axles.

Mine has survived 200+ passes with stock caps, 35 spline spool dead hooking radials with a transbrake and no issues so far. No bracing either. This while working through traction issues so it took lots of abuse. I do however plan to upgrade to Dr. Diff’s 489 nodular case with forged caps this winter as I was starting to get a little concerned about the caps.