Stroker pistons

[my4dv8]

Just a Query , is it possible or practical to run 4" 1/8 stroker pistons with the shorter 6" 7/8 French rods on a std. 4" merc crank?? Is the gain worth the effort? I know you can run std. 3 3/4 stroke ford pistons on a 4" merc. crank with these rods.

[Karl Wolf]

If I had one, I wouldn't walk across the street to get the other... It will work though. Haven't done it, but all you gotta do is get the piston to the top, don't think there would be a significant advantage to a 1/8" shorter rod.
Karl

[Barn Junk]

The French rods and stroker pistons will result in the pistons being about 1/16 inch from coming to the top of the cylinder and you will lose compression and power. Not a good idea in my opinion. Run the stroker pistons with the stock Ford rods and clearance/ machine the heads to about 40-50 thousands and you should gain a little power. To do this the block in the area of the cylinder can not be relieved much.

[Ol' Ron]

The shorter rod is a mystery to me. Rod angularity is important for several reasons, it has an affect on the torque curve and piston acceleration. among other things. Take the SBC they like a longer rod like the 6". The Flathead would do better with a longer rod, unfortunately it is mechanically limited.
In my opinion the short French rod should never be used in any flathead.

[HotRodmicky]

Originally Posted by Ol' Ron

The shorter rod is a mystery to me. Rod angularity is important for several reasons, it has an affect on the torque curve and piston acceleration. among other things. Take the SBC they like a longer rod like the 6". The Flathead would do better with a longer rod, unfortunately it is mechanically limited.
In my opinion the short French rod should never be used in any flathead.

Ron is right.
Over 4inch stroke the ratio goes bad.....with the 6 7/8
Even 4 1/4 with 7 inch rods the ratio is no good....

I wouldnt run the 4 1/8 with 6 7/8 rods

[51 MERC-CT]

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Without taking in to consideration any mechanical disadvantages.
The piston dome to piston pin dimension on 4 1/8" piston is shorter than that on a 4" stroke piston.
This will result in a loss of compression if used with 4" crank.
Using the 6 7/8" rods and 4 1/8" stroke pistons with the 4" crank will result in a further loss of compression.

[Tim Ayers]

Originally Posted by my4dv8

Just a Query , is it possible or practical to run 4" 1/8 stroker pistons with the shorter 6" 7/8 French rods on a std. 4" merc crank?? Is the gain worth the effort? I know you can run std. 3 3/4 stroke ford pistons on a 4" merc. crank with these rods.

Which French rods are we talking about? Are these the ones that Tardell/SF Flathead sells?

What is the standard length of a Ford rod-7"?

[51 MERC-CT]

Originally Posted by Tim Ayers

Which French rods are we talking about? Are these the ones that Tardell/SF Flathead sells?

What is the standard length of a Ford rod-7"?

Believe that Tardell/SF Flathead sells 7" standard rods. (no need for anything else)

[Tim Ayers]

Originally Posted by 51 MERC-CT

Believe that Tardell/SF Flathead sells 7" standard rods. (no need for anything else)

Got it. OK, thanks. Why did the French engines use the shorter ones?

[51 MERC-CT]

Originally Posted by Tim Ayers

Got it. OK, thanks. Why did the French engines use the shorter ones?

Just a theory, supposedly they built both the 239 c.i. and 255 c.i. engines.
Perhaps on the 255's with 4" stroke they opted to manufacture new 6 7/8" rods and use the 239" pistons.

[Tim Ayers]

Originally Posted by 51 MERC-CT

Just a theory, supposedly they built both the 239 c.i. and 255 c.i. engines.
Perhaps on the 255's with 4" stroke they opted to manufacture new 6 7/8" rods and use the 239" pistons.

I think you are right.

[Ol' Ron]

I'd think it would be more expensive to forge a connecting rod , than relocate the wrist pin 1/8"

[my4dv8]

Thanks guys, I thought it was too crazy to work. Just thinking out loud as i was trying to economise and avoid having to purchase a stroker crank. No real loss as it's still a pipe dream. So will have to build the big motor the correct way. How does the extra piston speed and dead time at the ends of the stoke affect things with 4 1/8 stroke?

[Bruce Lancaster]

Get hold of Smokey Yunick's "Power secrets" book for a long speech on that. He like 2 to one stroke to rod length, and opined that longer rods might likely be better but were outside what could generally be accomplished with available engines.
Ford carefully discussed ofsets and piston travel rates in an engineering text they published in the '30's.

[Mart]

I don't know the why's and wherefores, but when the French tooled up for manufacture, they wanted a 239 and a 255. They made the engines common in most ways and did indeed opt to use common pistons between the engines and make up the difference in the longer stroke by using a shorter connecting rod. I suppose one consideration would be that as a military engine, only one range of pistons would need to be stocked, whereas in many cases the rods can be re-used.

The rods use the 8BA shells.

I would imagine, also that as this was a military/truck engine (most times governed) the limited rev range would make the rod length ratio a moot point, and they were obviously satisfied with the durability of the engines in service.

I have a 4" French motor in my roadster.

Mart.

[51 MERC-CT]

Originally Posted by Ol' Ron

I'd think it would be more expensive to forge a connecting rod , than relocate the wrist pin 1/8"

Ah,yes maybe so but you have to consider that these are military/government built engines.
The tax payers are footing the bill so common sense is pretty much an unknown commodity.

[34PKUP]

Originally Posted by 51 MERC-CT

Ah,yes maybe so but you have to consider that these are military/government built engines.
The tax payers are footing the bill so common sense is pretty much an unknown commodity.

Ain't that the truth.