Technical pistons hitting head on 51 merk 8ba

this is hard to explain. a Frend of mine gave me this eng. he said it was running but had two stuck valves. so i removed the heads & repaid the valves. the eng looked very good no ridge i removed the oil pan & was surprised to see it had a merk 4 inch crank..so now i installed the heads which were merk heads . i started it up & never herd an eng sound so bad also the fuel pump bowl filed up with motor oil the eng was running on a clean gas can i remove the heads & saw the pistons were hitting the heads not the valves this levs me with two questions are ford head gaskets thinner then merk or have the heads been milled. what is the correct thickness of the merk heads. i know there is a lot of knowledge out there

 

You’re not alone
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Ford and Mercury gaskets are the same thickness. Maybe he used Ford pistons on the Mercury crank? That would cause the pistons to rise up .125" at TDC. Most stock heads I have seen have over .100" clearance if not modified. It might turn over and even run, but the noise would be there.

 

Maybe it had two gaskets on each head before?

 

How did you end up with oil in the fuel pump bowl?

 

Tubeman is correct, Merc pistons have the pin located higher up to condensate for the stroke. Merc and Ford rods are the same length. It could be the heads are milled too far. Did you maybe over fill the crank case and the engine has a hydraulic problem? Also look at the pistons at TDC to see if they are sitting flat, sometimes this can be the problem if the rods weren't aligned properly.

 

Yep, merc and ford rods are identical but the mere crank requires merc pistons. I bet your are out of the hole.

 

Sometimes people used Ford pistons with a Mercury crankshaft on purpose. It was a "poor man's" way to run "pop-up" pistons. The combustion chambers would be machined and recessed into the head to allow for clearance. Supposedly, this increased compression without compromising breathing. It was a fairly common practice back in the day. Perhaps the engine had different heads previously?

Kinda like these Kong dual plug heads :

 

All very interesting. Always learning something about these flatheads.

 

My 8ba has 4 1/8" stroke with .125 over 4" stroke pistons. They pop up 1 1/16" out of the bore. We fly cut the Offy heads to clear the pistons.

Dave

 

I hope you mean "1/16", not "1 1/16".