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.