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View Full Version : What model is my Merc, Mark 25? 20H? mk20? Hurricane



Mark75H
09-26-2002, 03:54 PM
Jason, the serial number appears to be a 1955 Mark25. The highest serial number for a 20H is 709612.

All of the external racing parts easily bolt on to the Mark25 block, but the 20H internals are different. The 20H crankcase is tighter, the porting and cylinder head shape are different, the piston shape is different, the reed block has bigger passages, the crankshaft is supported by roller bearings on the center main.

Your Mk25 originally had a smaller Tillotson AJ series carb; the passage behind the carb may or may not have been enlarged to take full advantage of the larger KA carb. (KA7A's are starting to bring over $150 consistantly on eBay but do not offer much {if any} advantage over any run of the mill KA series Tillotson.)

The KA7A Tillotson carb did not come on the 20H's, but was part of an update kit that came out when Tillotson introduced the larger KA series. The original 20H carb was a Carter N series carb that was known to be a bit finicky.

Mark75H
09-26-2002, 03:59 PM
Does your flywheel look like this?

Instigator
09-27-2002, 07:56 AM
Been a while but I think that fly wheel is from a Phelon (sp?) ignition (preffered) where the flat wheel is off the Bendix system (fishing motor).
20H blocks had "padded" cylinders which was the raised area around what would be the head to raise compression.
Not sure if any other 20 cu.in. blocks had this??
Some of the 20 cu.in. motors had the larger Carter carbs as well.

LakeRacer99
10-01-2002, 10:45 AM
Well...thanks for the replies so far, looks like we lost a few of them and I didn't get a chance to read them. Please repost-thanks.

Sam, I don't see a pic in your post. Here is a pic of my flywheel, you can see the ring to it on the stand. I was recently told this was from a 20H, so that's what made me curious. If I remember there was some gold paint that was under black, yellow and a coat of blue. When I painted this I didn't know a 20H from a KG7H.

Is the intake opening in the crankcase(behind carb?) shaped differently on the MK25 than the 20H? See picture in my next post.

Thanks,

LakeRacer99
10-01-2002, 10:45 AM
Intake pic...

Mark75H
10-01-2002, 05:54 PM
Yes, yours is a Mark20H type flywheel. Externally the crankcases are the same. You can't tell if the 20H type mod work has been done without removing the carb and looking at the throat and reed block opening without the carb on. Just the same, you probably don't have a 20H crankshaft with the full roller bearing center main. You have to take the motor completely apart to see that......about an hour's work.

Ooof. we did loose some posts.

The Carter N was used on other makes of outboards, but only on the 55H and 20H Merc's. The Carter N was used as a fill in while the larger KA series Tillotson was under development.

Some Mark25's especially electric start had padded heads as well, but not as deep as the 20H.

LakeRacer99
10-04-2002, 01:15 PM
Thanks for the help again Sam. I was told the Mark 25 crankcase had a flat surface where the carb mounts and continued out of a vee shape out to the cowling mount studs, all at the same heights. And the 20Hs had an oval shaped intake that protruded out from the crankcase 1/2-3/4".

This all has me confused, I am assuming mine is mostly Mk25 and I wanted to get a cowl set for it. I have the rewind and one side cover from a Mk25 and a box full of Mk25? brackets. the rewind is mounted to the starbd side rear and front and I have no clue how the Starbd side cowl mounts.

One other question is, if I remove the pipes, is the hole in the back of the KG housing (above cav plate) large enoght for the exhaust to exit?

Thanks for the help.

Jason

Mark75H
10-04-2002, 09:34 PM
The Quicky mid section exhaust outlet is more than big enough for any motor you can bolt to it. The same size outlet is used for the 20H. Consider this: the recommended hop up for the 63 cubic inch McCulloch involved approximately a 1 inch round exhaust outlet....much smaller than the outlet on the back of a A/B Quicky