Merc or Ford rods ?

[nbrskn]

When using a Merc 4 inch crank shaft can the stock ford rod be used ?

[JSeery]

Yes, the difference is in the piston pin location.

[rotorwrench]

It also depends on what rod bearing application you plan to use.

[51 MERC-CT]

Originally Posted by rotorwrench

It also depends on what rod bearing application you plan to use.

Thought that 4" Merc. cranks only used one application (2.139") unless it was stroked and then sometimes used another smaller dia. application.

[Ken/Alabama]

Originally Posted by 51 MERC-CT

Thought that 4" Merc. cranks only used one application (2.139") unless it was stroked and then sometimes used another smaller dia. application.

That's true, but if you use the 8ba rods you'll need the locked in type 8ba Bearings. If you use the full floaters you can use the 99A or the 29A rods.

[51 MERC-CT]

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Originally Posted by Ken/Alabama

That's true, but if you use the 8ba rods you'll need the locked in type 8ba Bearings. If you use the full floaters you can use the 99A or the 29A rods.

Understand the insert bearings.

[56markII]

Now that you brought this up, not knowing what year your flathead is? and in my case of mostly building 255 instead or 239,(1949-54 Yes we used the flatheads until 1954 up here in Canuckastan!) the crank and piston is different but the block and pretty much everything (except the heads?? IE chamber size and model number casted on top) is the same leads me to wonder if there is a way that these 239 pistons which are very common can be slightly machined down on a lathe to be used in a 255 instead. Why? you ask other than being easier to find 239 pistons at a lower cost is my 50 Mercury has a pathetic stock compression ratio of 6.8/1 and 239 pistons pop up beyond the block and would hit the cylinder head (by how much I haven't determined or tested) but if one could reduce the protrusion slightly to prevent impact you could increase the compression ratio Sorry for the off topic on this question but milling the heads was not the best solution due to reducing the flow area of the valves within the heads and pop up pistons are available but harder to find and expensive. I have three complete cores in my hoard, one with severe cracking rendering it usable for parts, one with very slight otherwise harmless cracks from the water passage straight to the head bolt hole (my Merc has these and I have been driving it for 35 years) and lastly a complete engine core with zero cracks but I have not determined if any are 255 out of the collection. If one is I will take a 239 piston assembly without the rings and place it in the block and rotate to see how far out it will be or other than placing one wristpin through a 239 and 255 piston to lock and align them together and measure the comparison. Has anybody else thought about this or researched this?

[waldo]

I used a 4" crank and 3 3/4 pistons. 2 head gaskets make for the proper clearance and just right compression. I run in my flathead vintage dragster. I can leave the line at 3500 rpm and spin the tires too much. 3000 is about right. No problems YET!

[JSeery]

The difference between 4 inches and 3 3/4 inches is 1/4 inch. So half of that is 1/8 inch, that should be the difference.

[rotorwrench]

The French SUMB engine, in it's 255 variation, had shorter con rods so it could use the standard Ford type pistons but when using them on a US cast Merc crank, you have to check them for clearance of the piston skirt to crankshaft. They can make contact on some crankshafts. I don't know if the French manufacturer used different design pistons than the US manufacturers or not.