Started tinkering with my new 255 Merc Flathead, preparing myself for the rebuild. I pulled off the oil pan and discovered something weird. One of the pistons looked like it was cracked really bad. Got looking closer all 8 pistons had the same exact crack all in the exact same spot. There's no damage anywhere and it the previous owner said it was a good runner when he pulled it. It also looks like it's been rebuilt sometime in the past since there are numbers stamped on the rods in their specific order. Showed this to a pro mechanic friend of my dad's who just so happened to be there checking out my dad's 37 Plymouth. At first we all thought there was some major damage done to the engine, but after looking at it closer it's like someone in the past did it on purpose! Why would someone cut a groove on the skirts in each piston almost all the way up to the rings? Was this some old timer's way of allowing more oil up to the rings or something? The previous owner also said it never smoked when it ran either. I'll take some pictures in a couple days to show everyone. Really struck me as weird.

Also what's the easiest way of measuring the stroke of a crank? Since this engine's been rebuilt in the past, I wanna make sure I still have the mercury crank. It didn't have the typical "jellybean" on the front lobe like most American mercs had. It did have a casting number on the second lobe from the front that said either "K81112" or "K31112" or "K?111?". The first character was definately a K but the second number was either a 8 or 3. The three characters in the middle are either a L or 1 or 4, hard to tell. The last number I thought looked like a 2 but everyone else said it looked like two dimples. Other than that, theres no other identification. I just hope they didn't pull the merc crank out and relpace it with a ford.

Sorry for the long write up. Just have a lot on my mind about this engine! :-)

Sean Kelly

 

Sounds like a split skirt piston to me. Very common in automotive.

 

the 3 3/4" crank has 3/8" plugs for the oil galleys and the merc 4" has 5/8" plugs

 

What's weird is our mechanic friend said he's seen nothing like it in his career as a mechanic. Was this common back with these flatheads or is it still being done today? I still haven't had a chance to take pics yet. I'll post them some time sunday.

Sean Kelly

 

If my memory serves me right - they were split to allow for expansion.

 

here is a picture of a 4" merc crank on left and the ford 3 &3/4" crank on right.

notice the merc has larger journal cleanout plugs as Effie notes above.



also measure the widest part of the thick throw, ford measures about 5 &1/2" and the mercs measure close to 6" across.

Read this it covers it all,
How to Identify a 4" Mercury Crankshaft

Josh

 

Josh, thanks for that website! Should help out some!

Still haven't found the time to go out to the garage and take pics of those pistons.

Sean Kelly