Mark 30H help with a block serial number...

[rjparksjr]

I acquired a Mark 30H. All the correct pieces are there. The case seal is intact. My other H motors all have either a 6 or 7 digit serial number. This particular engine has a three digit stamped where normally the serial number is stamped. The serial number is 7-- XH and then an additional H is stamped at 45 degrees to the serial number.

Any ideas what this block is? Service replacement block? doesnt show up the the Merc dealer catalogs.

Any insight or help is appreciated,

Kindest Regards,

Bob

[larry herman]

It may have been a factory test and development engine ?

[foot_doctor]

Remove the starter top, and look at the top of the block. The serial number should be stamped there, near the rear.

R.T.

[rjparksjr]

Thank you for both responses. The serial number, 7-- XH is precisely at the top of the block- near the rear. Stamp number and lettering size is correct.
Interesting.

[25xs]

Bob,
Just to be clear in your first posting, is the 7--XH stamped on a 30H factory serial tag located between the transom clamps, riveted to the swivel bracket? ...and then the second location is on top of the block?
I'm kind of reading that both stamps are located on the block, one on the side and one on the top, in which case it could just be an engine builders' markings on a replacement block. Pictures would be cool, and what other markings or stamped in info was on top of the block, and is there a factory 30H serial tag on the swivel bracket at all?
Tom

[rjparksjr]

Thank you for your input. The tag that is located between the thumb screws is not there. Normally the tags are riveted in place. When you remove a tag the holes are hogged out a bit. The rivet holes do not appear to have been used at all. Just odd.

The only serial number that I can find on the top of the block. The top of the block appears not to have milled or shaved. I sanded it lightly to make the serial numbers easier to read.

Appreciate you all!

Bob

[25xs]

Interesting for sure. The little mark between the two "H"s is a bell. It's a foundry mark. Is the pictured stamping the only location of that particular stamping? Your posts never mentioned the 736 XH rather just a 7--XH
A brand new swivel bracket would not have the serial plate so that's not uncommon to have the drive screw (drive rivet) holes without damage. Can you tell if the lead seal has been tampered with..? It would certainly be odd to have a factory sealed engine after all this time (was never in the winner's circle for inspection or re-ringed between race seasons...)
I have attached a picture of a Mark30 block and a Mark55 block with their stampings. The 736 X and H were likely identifiers of machines or machinists involved in the manufacturing process since they show up on these two fishing blocks as well. I think you have a replacement service block with no serial number since serial numbers were stamped with characters 1/2 as tall as the 736 XH on your block (see the Mark55 serial number stamp).

[25xs]

Look on your cylinder block, at the flange between block and waterjacket cover on intake side of cylinder #2 for a serial no. stamped in... Should be something there if the block was a factory 30H or Mark30 fishing engine.

[rjparksjr]

First of all, thank you for your input 25XS.

I checked the block by the #2 cylinder as you suggested. There is a serial number 1023440 works out to a 1956 Mark 30. But wait! There is more!... reached out to Mercury Marine Performance. They indicated the X that is stamped in the block is the internal code for the quicksilver lower unit and the H is for the H block/ hydro tower. Regarding the three digit serial proceeding it, and were going to do some more research. The Tech indicated that the early documentation of serial numbers left a bit to be desired- but felt confident about the X and H designation.

What I do know for sure: It is a 1956 block, the seal is intact, the rest of the parts are H motor and correct. Whether or not it is a "real H Motor", I will probably never know. Other than that I think I am at the end of what can be determined. And lastly, it is a welcome addition, that will be run on a very nice Dawe Craft Hydro and Desilva Runabout this summer.

Kindest Regards,

Bob

[DeanFHobart]

Isn’t 1956 the first year for the 30H?

[rjparksjr]

Dean, Yes it is a 1956 block.

I have some more information from a very knowledgeable 82 year old ex- Mercury Racer / Collector that called me last night. When I explained to him the serial number dilemma (and before I could finish), he said, " Bob, I know exactly what it is." In 1956 they were selling more of the 30H's than they had powerheads. So they pulled Mark 30 engines off that production line, added the H hardware, stamped them with a number and XH. The number stamp is the number of engines they pulled from the regular production line that they converted to the H model.

Very interesting. Makes sense of the two serial numbers.

[DeanFHobart]

Cool.... but was 1956 the first year for the 39H?

[25xs]

The case seal is intact. My other H motors all have either a 6 or 7 digit serial number. This particular engine has a three digit stamped where normally the serial number is stamped. The serial number is 7-- XH and then an additional H is stamped at 45 degrees to the serial number.

Ok, before this thread goes further into speculation, it should be updated to clarify a couple things... All the Mark30H powerheads came off the Mark30 production line. They are 100% the same powerhead but 30H had light flywheel, full carb butterflies, shift linkage delete and a non-electric Mk55 lower pan. Those are the ONLY difference between Mk30 & Mk30H powerheads. You don't have a "763 XH" serial number, it's a standard serial number 1023440 stamped in the same, correct location on the water jacket flange as ALL the 30ci Mark30 blocks. (40ci blocks were stamped on top of the #1 cyl water jacket) The 763 you see is just a standard factory machining stamp to match the block to the crankcase and you should find a corresponding 763 stamped into the crankcase if you look down, between the crankshaft & magneto drive pulleys. Those three numbers match on all the Mark30's & Mark55's in some manner from before, after, and during the factory "H" racing engine production. The X is also shown above on my own Mark30 fishing powerhead in my previous posting which has been professionally restored to 30H spec and I have confirmed with a couple friends who also have X stamps on top of their 30ci block. I know my block did not come with a Quicksilver racing tower & gearcase. Could you shine more light on the Mercury Marine Performance? (Mercury Racing?) tech discussion because that is specific info none of my own mentors or collector experts have ever mentioned. Who was that tech because it would be nice for that person to share that knowledge from a memo, chart, blueprint, document of some sort to help others I.D. their blocks.

Nothing I'm saying above is either first hand factory knowledge, just fairly well documented info from collecting and studying old Mercury engines for about 30 years.

Oddly enough, I've heard the exact opposite story as your 82yr old racer friend. I heard Mark30H engines were parted out at the factory to sell the tower & gearcases separate and the 30H powerheads longblocks were installed on fishing engines...
First batch of Mark30H engines were in the serial range 997920 thru 1166568 1956 to 1958

[DeanFHobart]

Cool.... Thanks.

[ctpdsr]

A few photos that may help out. This block, #1025774, was sourced from a late 1956 MK30 service powerhead. Again, there were no physical alterations made to the MK30-H blocks as was indeed the case for the OEM MK55H blocks in 1956 & 1959. The machinist marks shows as B220 on both the top of the block and crankcase cover. The serial# is harder to read and stamped into the side of the water jacket casting adjacent to cylinder #2 on the intake side. These marks do have significance as they indicate the block and case cover are a matched set. The other stray marks including an "X", "M", "F", "Z" and even a cute little 3-leaf clover are irrelevant. Someone at the plant carried a stamp and marked the block when they completed a specific task. No Urban Legends here.