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1994 s600 coupe
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Discussion Starter #1 (Edited)
this topic deserves its own thread. after buying 2 sets of caps/rotors from Autozone,
another 2 sets from Rock Auto,
then a single set from an Ebay seller,
in addition to individual caps, and rotors, from Rock Auto and Ebay sellers,
I have a pretty good idea of what's out there for the M120 V12 as replacement parts.
also delved into buying the 300SL straight 6, 3 liter parts.
sometimes you get 6-cyl parts for the v12, sometimes you get slightly inferior bona fide v12 parts, and vice versa.
at this point the parts chain is completely intermixed, mix and match,
they are selling anything and everything that will bolt onto the V12, as replacement.
bottom line- the late model 1993-95 MERCEDES cap and rotor, is the best bolt on quick fix.
it will give an approximate .050" rotor to cap contact gap when installed.
to get an even closer cap/rotor gap, the early 1991-93 rotor, with late model 1993-95 cap,
will give a .025"-.035" gap, or smaller, meaning it will stay in tune longer before the gaps open up, from burning deterioration or cleaning- and have less current drop.
but you have to put a bushing on the early model cap, to make it work.
more about that at end of thread.
note- NONE of the caps/rotors are made in China. original Benz is made in Germany, replacement Bosch made in Spain, replacement Bremi made in Italy.
 

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1994 s600 coupe
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359 Posts
Discussion Starter #2
first 2 caps I bought, just ordered as V12 replacement from AutoZone,
they sent me a bona fide Mercedes V12 pair cap/rotor, and a 3 liter 6-cyl pair.
the 3.0 liter, 6-cyl cap has thinner contacts, with no bakelite plastic reinforcement,
6-cyl cap also lacks the heavy plastic flexible black shield on top.
the 6-cyl cap also has further apart contacts, as measured side to side across cap with calipers,
meaning the gaps will be greater.
6-cyl rotor also has exposed metal contact strip on it to tip,
where the original is completely insulated inside the bakelite rotor.
no contest I took the 6-cyl stuff back for refund, cuz it was $156 plus tax.
here's a comparison of the 2 caps, the OEM V12 cap is obviously a heavy duty unit,
with added insulation, contact post bulkheads, and protection against moisture.
would the 6-cyl parts work ? most definitely.
but they cost the same or nearly as much as the V12 parts, depending on vendor, and are somewhat inferior to the V12 parts.
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1994 s600 coupe
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359 Posts
Discussion Starter #3
next up, cap/rotors ordered from Rock Auto, ordered for 300SL 6-cyl. these also lacked the bulkheads reinforcing the contacts under cap, and had thin contacts. when the setup gets to the point there are gaps in the spark chain, that are actually larger than the spark plug gaps themselves, problems will occur. both sets of 6-cyl caps were also returned to Rock Auto as well.
in addition, one of the 6-cyl caps was delivered with a snapped off center contact, the black carbon spring loaded piece in middle of cap, bottom of coil post, that is supposed to contact rotor. it was rattling around in bottom of bag, I taped it to rotor when I returned it. 6-cyl cap also lacks the protective black plastic insulator cover.
again, it will work in a pinch if no other parts are available, and in this case the 6-cyl parts are cheaper, but you get what you pay for.
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Discussion Starter #4 (Edited)
yet another pair of cap/rotor sets obtained from online vendors for the V12, made by BREMI in Italy, these were ordered as early 1991-93 model replacements, for comparison sake, and have the solid brass stud in center under cap, not the spring loaded stud like late model. it also has a very short rotor tip. the rotors and caps are from Bremi, they DO have the top outer black plastic cover.
problem- this creates another gap under the cap, the spark must jump from the center brass stud, to the rotor, it's not a close interface there, there is about .045" or so of gap. not a good thing. it should be solid contact. this is most likely WHY Mercedes discontinued them in the first place. it places undue stress on the ignition system, driving the coil to maximum voltage all the time, to jump these extra gaps.
in addition, the outer 6 brass contacts do have plastic backing support into cap edge, but the contacts are VERY thin, like paper thin. the gap with the Bremi rotor supplied, would be .090", way too large. both of these were returned to vendors as well. Bremi caps come with a rubber band holding the bolts on the cap, to be removed upon installation.
again, yes they will work, but not the optimum setup, being the original late model Mercedes parts cost around the same, and are superior- if you can find them.
the 2 rotors shown, show the difference between the Bremi rotor (left), and the early 91-93 Mercedes rotor (right). the early Mercedes rotor gives a closer outer contact gap when used with any cap, OEM or aftermarket. but the hole inside the early rotors, creates an extra gap in the middle of the cap, between the center of rotor and center of cap. so it's counterproductive.
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1994 s600 coupe
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Discussion Starter #5
here's the difference in rotor lengths, this is most critical, you want to strive for the smallest gap possible. the Bremi cap will work but the gap is very large with the Bremi cap. again we're talking .090" on what I measured here !
the late model 93-95 Mercedes rotor is somewhere in between the Bremi and early Mercedes rotor in length. it will supply an approximate .045"-.050" gap, with a rock solid contact on the center stud on the late model cap. in that respect it is superior.
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Discussion Starter #6 (Edited)
here's the ultimate setup I'm working on. Being the early rotor has a longer tip by about .020", improvement can be made by adapting early model rotor, to late model cap, with brass bushing tube from local hardware store, for $1.29 each, TrueValue brand. rotor is replacement Bosch, made in Spain. tube must be hand filed on straight end, until it fits between cap and rotor on car. flared end contacts cap. I started hand fitting it, filed it down twice, had it almost perfect, but didn't have time to finish. It's still a little too long, not allowing cap to fully seat against distributor housing. In the meantime, I bolted on the one new late model rotor I got free from AutoZone, with the new late model cap, because I had to move the car in the driveway. I have sourced another new late model cap for the other side, it arrived yesterday, price was $75 shipped. next step will be adapt the early rotors to the late caps, using the TrueValue spacer bushing tubes. that will yield a .025" or smaller rotor tip to cap gap. ultimately I'd also like to find a solid dowel pin to put inside the tube, to rest solidly on the spring loaded carbon contact in the cap. as it is now, the carbon contact fits inside the flared end of the tube. stay tuned will post results when I have time.
after messing with the cap/rotor contact gaps, it's obvious why Mercedes and the auto industry as a whole, went to distributorless ignition, coil on plug.
the early model 91-93 M120 engine, had generous gaps at the center contact/rotor interface, as well as the rotor/outer contact interface. the 93-95 late model cap/rotor eliminated the center contact/rotor gap, but still had a .050" rotor/outer contact gap.
all gaps are eliminated entirely on the 96-up coil on plug ignition, leaving only the gaps at the plugs.
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