Mercury Mk55H-S 'Merc Challenge' motors

[trident]

The last Mk55 Stock Outboard race motors were very interesting. Originally we had the Mark 55H and the Mark 55H-1. Those were built in 1956 and 1959 and both versions had +.060" higher intake and exhaust ports and big carbs. The first models had Carter carbs, which proved troublesome and the H-1's had Tillotson KA7A's, plus there was an update kit of KA7A's for the earlier motors. We ran those motors to death... literally.

Mercury made some replacement blocks and gearcases, and parts like pistons and rod bearings, etc. were still readily available new. Then we ran out of nice 55H blocks. An experiment was done, by individual racers, to substitute the lower ported service motor blocks for the worn out H blocks. I saw some of those tests, BITD. That block change, to what should have been a slower ported version, but combined with all the tricks we learned over the years as to what a Mk55 liked for individual parts and internal setup, gave us a motor that was, as expected, slower on top speed than a good 55H, but IT TURNED BETTER LAP TIMES AROUND THE RACE COURSE! What? 1 to 2 mph off on sheer top end, but quicker around the course? Guess who wins? Obviously the service block had a better torque curve and got to speed earlier on the straight. Oh, the service block was less fussy to prop, too.

The APBA rules were changed to allow the service blocks as replacement blocks on the Mk55H for the D classes. Now the question was which service block is best? We quickly found the later single cut 5/8" round port (think Mk35A, Merc 300) was NOT the answer. In Mod, where the ports can be significantly altered, these can be made to really run, but in Stock, where the ports have to be OEM stock, the earlier double cut 9/16" port was better. OK, they made those blocks from 1955 (predating the Mk55H) through the 1958 models. 1959 was the 5/8" round ports, the Mk35A.

I was building lots of service block racing Mk 55's. I call them Mk55H-S, as in Service block, to not confuse them with the high port original H's. Some guys sell those motors to collectors as H's, but that is misrepresenting them. Dishonest or ignorant, you decide. (I've seen basically anything on a H unit called a 55H, even 44's) We would bore the service blocks, remachine the case cover for the KA carbs and build in everything we knew to make a Mk55 run. Measuring these pre-1959 blocks showed they were well made, ports were machined well within spec. I noticed certain blocks were unpainted, a very dark gray color... if you scratched it with a scriber, it was shiny aluminum. Quickly figured out these were early models, 1955 or so... Apparently something they used in manufacturing etched these early blocks that gray color, and they became known as 'black blocks'. Also noted the very earliest so-called 'black blocks' were cast differently with the sleeves actually protruding, standing proud, with no aluminum casting at the bottom end where the transfer passage starts. Later 'black blocks' have the normal casting of aluminum to the ends of the sleeves, and a blunt edge to the passage. Found those really early blocks, serial number below 89XXXX, 'black blocks', with the odd sleeves, properly built, ran like 'Jack the Bear'. ALL my best service block D's used that type block, bored oversize and built to my specs. My Merc Challenge D motor, the best 40 D I have or ever had is one of those. Its an early 1955 block with everything I know how to do that passes inspection in there...

These were the D motors we were all running when the Merc Challenge came along at the end of the Mk55 era. The 44XS was not released in time for the first year of the Challenge, so Mercury made the black Challenge wraparounds with current graphics and paid us good contingency money to paint our D motors Phantom Black and run them fully cowled with that wraparound. Mine were already black. I just had to paint my silver towers black and switch wraparounds. Next year, 1987, the 44XS was introduced. End of an era. The 55's had a good run, 1956 thru 1986. Its still one of my favorite motors... I first drove one in 1960 and was instantly in love. Looking back now, as I raced both, I think the Mk55 was the better motor. Yes, the 44XS IS faster, but the Mk 55H was a better driving motor, IMHO. Sounds better, too.

Funny story, very early '80's I got a bright idea to try a true +.060" ported 55H block, on our newest pickle fork hydros, with the new three blade props, running tucked, high on the transom... Put the best parts I could find in there with four new selected, matched pistons and fresh bores... Oh, that motor ran big test numbers, but it never won a race the whole season. I dropped that entire crank train into a freshly bored and prepped early numbers 'black block' service block and went right back to winning races. Disappointing experiment, but point proven!

I have a 'kit' of the critical parts I was setting aside for myself to duplicate my Merc Challenge service block powerhead, should the need ever arise. It didn't so the parts just sat. Still sitting...

Any interest out there for these choice and 'correct' parts to build a fresh duplicate 'longblock' to my 30+ year old MK55H-S Merc Challenge motor? Drop me a note. Let's talk...

The 'kit', For Sale:

I have a very nice early 'black block', serial # 859365, right off the service motor. Standard bore, matching block and case cover assembly. Nothing done to it yet. Needs cleaning, boring, finish honing and needs the carb holes opened for KA series carbs.

I have 4 brand new, OEM 'flat top', selected, matched by weight +.015" pistons and new rings, a nice, selected, measured, race prepped 400-2 small journal crank, 4 matched 600-4 rods, selected, checked for straightness, lapped and honed to match the crank, 8 nice wrist pin spacers, 2 nice 1 7/32" brass reed cages with labyrinth seal ID's, a solid, cut down stator flange top bearing cap and a single seal bottom bearing cap to match the small 400-2 crank. Also have a cutdown 2# cast dog type flywheel, painted, ready to install, a new reproduction 55H gasket set, a nice Mag drive with a good bearing (these are NOT replaceable), a late 'bump' style mag actuator plate for a Mercury or Fairbanks Mag, a nice, clean, straight inner stainless exhaust cover plate, a freshly rebuilt rewind starter assembly with welded pins (Roundtop cowl version)...

Not a complete motor, but a kit of choice parts. The new owner can supply the labor, machining, fasteners and generic parts... I will supply machining instructions, finished bore dimension and advice...

Drop me a note to tridentracing@new.rr.com if interested...

Jerry Wienandt

Trident Racing

[trident]

I should also mention that whatever process etched those early 40 blocks that dark gray color was also used on early 30's. There were Mk 30H 'black blocks', too. And those also were very good blocks, although they didn't have the sleeves standing proud of the aluminum casting like the very low serial number 40's. Then, Mk 30H and Mk 30 service motors were 1956 models...

[Matt Dagostino]

Another 'Black Block Trick' later showed to me by Bob Thornton was his boring method of Clark Maloof's 'Black Block's". While many of us were busy testing props, Clark would have Thornton bore several Black Blocks and gauge the hardness of the metal by the color of the sparks when bored. Clark would then take one set up crank and rotate it from one block to another until the killer motor was found!! Bob Thornton in the 1970's-1980's was the best! My best Black Block won me several Merc Challenge Races and National Championships. We could run right at 80 mph and those motors were indestructible!
Our best "black block' was found on a Virginia Military base being used on a pleasure boat...go figure.

[ZUL8TR]

Interesting. Were all the ports at the same depth on those blocks Clark had bored? Also interesting if they were exactly the same since there was +/-0.010" tolerance on the A and C port position per the SO inspection manual? So maybe a combination of bore hardness and port position?

[rumleyfips]

I read about this elsewhere. My understanding is that he was looking for the biggest A-E measurement.

[trident]

Of course, when building lots of these motors I'd get as many early serial number 'black blocks' as I could find and I'd carefully select the block based on the following criteria:

Obviously, I wanted a block that’s in very nice condition with no damage anywhere, such as thrown rods, broken rings, etc. My choice would be a well seasoned, undamaged block; one that had countless cycles of starting, running up to temperature and cooling down again.

Working with my machinist, I'd check the OEM port dimensions. My preference was a block that measured above the nominal spec (but within tolerance) on the exhaust side and on spec or slightly below, on the intake side. I'd measure the head depth dimension, preferred block that is to the tight side of spec. I'd measure the split line of the case relative to the line bore cut depth to see if the crank centerline is parallel to the split line. If its off a bit, shim the block accordingly, face down, and take a very light cleanup cut off the back to true up, then flip the block over and bore the new holes perpendicular to the crank centerline and on the correct bore spacing... And so on... No rocket science, just attention to detail.

So long as we're reboring the block, why compound the original machining errors by just 'picking up' the original bores? Here was a legal way to true up that block in the process of re-bore. We could be fussier than the OEM production line. Round, straight, square, true to dimension is what we're after, here. And that's what my machinist, Jim Ruck, delivered every time.

Jim is now retired, BTW. Then, so am I.

[jeff55v]

I was told the black blocks were cast by Reynolds Aluminum, the silver ones, by Alcoa Aluminum.

[trident]

Had an inquiry about full butterflies for KA series Carbs... Steve Roskowski and Aeroliner both have very nice, correctly made ones for KA and KC Tillotson carbs. They are really oval in shape with beveled edges because of how they sit at a slight angle in the venturi. A home made round one does not fit right!

I soldered up the Merc 500 ones with 2 or three idle holes, BITD. I'd hit the holes from both sides with a cutter to create a sort of 'hourglass' shape to the brass, to give some mechanical fastening to the solder plug, as I didn't ever want one to loosen and fall out, then I'd solder to above grade from both sides and lap flat. Way too much tedious work, when you can buy perfect ones for a reasonable price...

Support our suppliers who support our sport.

[trident]

So long as the subject of carb parts came up, let's discuss the KA7A and subsequent versions we used on these motors...

At first, we all ran KA7A as those worked well and those were what was legal... then they loosened the rules to allow any 1" KA (BTW, the KA10 and KA16, maybe others, are 1 1/16"... same casting, marked 1" on bottom but they were OEM milled out in the venturi an additional 1/16")

At first, when I bought two Mk55's, I couldn't find a set of 'real' KA7A's for sale anywhere, so I ran modified KA9A's. I say modified, because I epoxied the choke shaft holes closed and drilled the castings for a 10-32 hole in front, like a KA7A had, so I could run front mounted chokes. I replaced that M. Mouse KA7A choke linkage with the later cast aluminum shutters from Merc 500's. Simple and robust.

Later model KA's, still 1" and legal per the rules, are already equipped with a front choke and also have a shorter airhorn, see KA19A, for example. I liked those carbs and had great success with them on my 40 Mods.

I also found updates that worked on the KA7A's, when I finally got a set... For example, the KA7A uses the idle tube from the KA2A, I liked KA9A tubes, which are a bit larger ID... and so on.

I had been told that carburetor is a French word that means 'leave it alone', but I found all sorts of interesting things, experimenting with various combinations of legal parts on these motors. Some worked really well, some not so much, but I learned something...

[daveracerdsh]

Very interesting piece Jerry....I was just getting started racing with a 55H purchased from Tom O'neill when the Merc Challenge series got started.....I remember the first 44XS I ever saw was at drivers meeting 1986 at Lawerence Lake, Yelm Washington. I remember people howling that the new motor would "kill" the D class due to it's cost ($2250 plus shipping if memory serve me correct) but of course the naysayers were not correct....pretty soon we had well over 20 D entries at Challenge series events......the 44XS revitalized the class! To this day I remember how excited I was to open the shipping crate and see my brand new XS motor! Again, thank you for the info Jerry...I learned some stuff I did not know.