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EFV-8 Club Forum / General Ford Discussion / Piston Size Versus Crank Size

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Posted By Discussion Topic: Piston Size Versus Crank Size -- page: 1 2     choupie 04-15-2011 @ 7:06 PM reply profile send p.m. Member Posts: 26 Joined: Jun 2010 Newbie-type question: Say you're holding a piston that was in a 59AB engine. Is there some dimension on this piston that would tell you what size crank was in the engine, or may a given piston have been used with various size cranks? If a given size piston only works with a certain size crank, is what is the telling dimension on the piston, and is there a table to determine what crank was used with that piston? Thanks, Paul sturgis 39 04-16-2011 @ 2:04 AM reply profile send p.m. Member Posts: 74 Joined: Nov 2009 I am not an engine builder. I think they change the length of the rod when the stroke is changed or they wrist pin location is changed in the piston. I hope some of the engine builders will respond.I guess I never thought about this issue. IF IT CAN NOT BE FIXED WITH BLASTING WIRE, DUCT TAPE OR JB WELD - IT CAN NOT BE FIXED 51f1 04-16-2011 @ 5:47 AM reply profile send p.m. Senior Posts: 573 Joined: Oct 2009 I'm really not sure what you are getting at. Unless there is something I don't know about a 59AB engine, generally, all pistons and cranks used in a particular new engine are the same. By "size crank," if you mean the journals have been reground, that doesn't affect piston size. The piston remains the same unless the block is rebored, and larger diameter pistons are required, but they are the same type of piston as used originally (they look like the original). Richard alanwoodieman 04-16-2011 @ 5:59 AM reply profile send p.m. Senior Posts: 873 Joined: Oct 2009 the telling of the crank "stroke" is the piston pin center line to the top of the piston. A ford has a 3 3/4" stroke, in a merc the stroke is 4", uses same connecting rod but different pistons as described above, the stroke in this case comes from the crankshaft throws being machined differently, with more offset ford38v8 04-16-2011 @ 9:07 AM reply profile send p.m. Senior Posts: 2883 Joined: Oct 2009 Paul, Here's an article with pictures, by Bill Boomer. It describes the differences between a Ford 3 3/4" and Merc 4" crank. Not the question you asked, bu maybe it will help you. http://www.goldengatev8.org/docs/merccrankid.pdf Alan carcrazy 04-16-2011 @ 10:58 AM reply profile send p.m. Senior Posts: 1979 Joined: Oct 2009 Piston pin location as measured from the top of the piston to the pin centerline will differ with the stroke of the crankshaft. This assumes that the rods have the same center to center length for both the Ford (3 3/4 in. stroke) and the Merc (4 in. stroke). The pistons for use with the 4 in stroke will have the pins 1/8 in. higher than those for use with the 3 3/4 in. stroke. This difference in pin location is required to keep the piston from poking out of the top of the block at TDC. choupie 04-16-2011 @ 4:13 PM reply profile send p.m. Member Posts: 26 Joined: Jun 2010 So, as it does seem to be the case that a given piston will fit either the Ford crank (3 3/4 in. stroke) or the Merc crank (4 in. stroke) but not both, does anyone know what the measurement between the top of the piston and the centerline of the piston pin is for a Ford crank and for a Merc crank? Stroker 04-16-2011 @ 5:04 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 Choupie: I think you have been given what you need to know by others. Given two pistons for a 59A, the Merc will have the pin hole 1/8 inch higher than the Ford. When you ask for a measurement, our problem is the reference point. Problem is: you can measure a piston from the top of the dome, the outside edge of the cylinder dome, the top ring land, etc. The only relevant measurement is the difference between pistons designed for 3-3/4 inch stroke 59A motors, and pistons designed for 4 inch stroke 59A motors. If you are in possession of a Ford and Merc. piston, you should be able to visually observe the 1/8" inch difference. choupie 04-16-2011 @ 7:15 PM reply profile send p.m. Member Posts: 26 Joined: Jun 2010 Okay, I'm feeling sufficiently dumb now, 'cause although I agree I should be able to visually appreciate a 1/8" difference, I consider it's hard to tell a difference when you only have one to look at. I also agree that problems with providing a meaningful distance arise when the reference point is not specified. So here's what I did: I put the piston upside down on a table and measured up to the centerline of the piston pin - 1 11/16". So now I'm anticipating that this measurement, too, will be insufficient data to determine what crank is in the engine because of the variety of dome heights that pistons may have (the inside of the piston is marked 80 and 1624 in a circle - for what its worth). So how about this: although the engine is not here, the connecting rod is still attached to the piston and maybe it has enough info on it to answer the question. The connecting rod is marked on one side: Ford U.S.A., what looks like a W with a circle around it, 31-1; and it's marked on the other side 29A, A2, 1. Is this connecting rod specific to a certain size crank? Thanks, Paul 37RAGTOPMAN 04-17-2011 @ 9:23 AM reply profile send p.m. Senior Posts: 1992 Joined: Oct 2009 if you put the piston on top dead center, and turn the engine over till it hits bottom, and you measure the distance it traveled and it is 4 inchs you have a MERC crank,and MERC piston, I do not think you can use MERC pistons without the 4 inch crank,if you used FORD pistons they would strike the head, they are 1/8 inch longer,in being the piston pin is offset 1/8 lower then the FORD piston the make up the difference in the longer stroke of the MERC CRANK. I am almost positive the rod are the same, this is what make of this,Lets hear how you made out, this will be interesting if , I am correct, hope this helps 37RAGTOPMAN This message was edited by 37RAGTOPMAN on 4-17-11 @ 9:26 AM

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