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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     avrotom 04-19-2011 @ 2:22 PM reply profile send p.m. Member Posts: 71 Joined: Apr 2011 I have a new EGGE L991 Piston that is for a 4" stroke Merc and it measures 1.045 inches from the top of the piston at the OD or outer edge to the top of the wrist pin hole. An original Ford piston measures .017 more, so 1.062. Hope this helps. supereal 04-19-2011 @ 9:48 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 We do have a crank grinder, and it is also used to polish the bearing journals if they don't need to be resized. We open the oil passages in the crank, as well, to remove the usual deposits from years of use. There is nothing wrong with an owner doing his own engine rebuild, but there is more to it than just bolting parts together if you want an engine that will justify the time and money. TomO 04-19-2011 @ 8:05 AM reply profile send p.m. Senior Posts: 7385 Joined: Oct 2009 If the crank rod journals have 1 oil hole, you have a 29A crank shaft and it would have a 3 3/4 stroke. If it has 2 oil holes it would be a later crank shaft and you would use the reference Alan gave to determine if the crank is a 4 inch stroke crank. IMHO I would put more faith in a machine shop that has the crank than the memory of a previous owner, no matter what his age is. I would order the 3 3/4 stroke pistons or let the machine shop order them from their supplier. I also think it is a better idea to let the machine shop order all of the parts. If there is any problem with a part, they will stand behind it and you are not caught in the middle between your vendor and the machine shop. That is why it is important to select a machine shop that has flathead Ford experience. Tom 37RAGTOPMAN 04-19-2011 @ 6:37 AM reply profile send p.m. Senior Posts: 1992 Joined: Oct 2009 coupie if that machine shop has a Crank grinder, and can not tell you which crank you have, I would get my engine back, I am assuming they are just going to rebuild the engine with out grinding the crankshaft, if they had a crank grinder they would have to know what size the throws are to set it up, or they are planning on farming the crankshaft job out,which is what most shops do, you should be able to measure the throw of the crank if it is in front of you with a ruler, to get a rough idea it is a 1/4 inch longer, cannot understand what the prolblem is with your engine rebuilder my 3 cents 37RAGTOPMAN supereal 04-18-2011 @ 9:45 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 If you have chosen a machine shop, it is too early to order pistons until the block has been disassembled and thoroughly inspected. If the shop can't tell you which crank you have, I suspect they may not be capable. At our machine shop, we don't order parts until the engine has been completely disassembled, tanked, and inspected. When we determine the necessary bore size and the condition of the crank, we draw up a list of parts. To do so before the machine work is completed, is the wrong sequence, and would likely result in problems in assembly and add extra costs. This message was edited by supereal on 4-18-11 @ 9:47 PM choupie 04-18-2011 @ 8:17 PM reply profile send p.m. Member Posts: 26 Joined: Jun 2010 What I'm trying to accomplish is ordering the right pistons to rebuild a flathead Ford with 59AB heads. The engine was delivered to a local machine shop for evaluation of the viability for rebuild. They say it's rebuildable, and I'd rather fix what was in the car than put in a more modern engine as a replacement. The only parts I have with me are one piston and its connecting rod that came out of the engine prior to delivery. The fellow I bought the car from in 1988 (who is ~80 now) has over the years been confident that this engine had a 4" stroke Mercury crank in it. The machine shop that has the engine now believes the crank to be a 3 3/4" stroke. I was hoping to be able to determine what's what from the piston/connecting rod pair I have sitting in front of me - both of which were described with all they have to offer in the earlier posts (with the exception that the piston has 4 rings and is also stamped on the top "STD"). Although I appreciate the physical ways to check the stroke by measuring the travel of the piston in the cylinder - it's a little late for that now as the engine isn't here and the whole thing is apart now anyway. Apparently all late '30s to '48 Ford and Mercury flatheads used the same length connecting rod, used floating bearings, and had 3 3/4" cranks. I also gather, perhaps incorrectly, that '49 through '53 Ford engines still used 3 3/4" stroke cranks whereas the Mercury engines for these years used 4" stroke cranks, but both still used the same length connecting rods that were used on the earlier engines although the bearings were different. Consequently, I guess you could use a pre-'49 connecting rod with a 4" Mercury crank, but you'd have to use the earlier floating bearing setup as well. Accordingly, whatever info I might gather from the connecting rod won't be conclusive as to the crank shaft in my engine (which did have floating bearings in it). So, I was thinking that the only way to conclusively tell what size crank I have is from the piston - which apparently is a correct thought process (as the 3 3/4" crank pistons are 1/8" taller between the piston top and the centerline of the pin than the pistons that go with the 4" stroke cranks are). However, I haven't been able to get any data with points of reference for the measurement to be able to tell if the piston I'm looking at is for a 3 3/4" stroke crank or for a 4" stroke crank. Although my heart tells me to go with the report of the original owner, my head tells me to go with the machine shop's report. It'd be nice to have data to make an independent informed decision, which is what I was hoping for with the original post. Perhaps I'll toss a coin - I guess the vendor I get the new 60 over pistons from will swap them out if I got it wrong. Better yet, perhaps they were both right, it's just that the original owner is a little confused on the year the crank was made - maybe it's a 3 3/4" stroke Mercury crank from a pre-'49 engine (although I'm not clear on the advantage of using a Mercury crank over a Ford crank of the same stroke). TomO 04-17-2011 @ 5:42 PM reply profile send p.m. Senior Posts: 7385 Joined: Oct 2009 Paul, If you tell us what you are trying to accomplish, we may be able to give you better answers. In my experience, when a 49-53 crank shaft is installed, the engine builder usually installs the 8BA rods to use the less expensive rod bearings and reduce the machining to recondition the 29A rods with there floating bearings. Tom ford38v8 04-17-2011 @ 3:42 PM reply profile send p.m. Senior Posts: 2883 Joined: Oct 2009 Paul, although a 4" crank could have been installed at a later date, your 59A block originally had a 3 3/4" crank. Hopefully, the various answers you've had will all come together to make some sense to you. Don't feel dumb about this, it's a reasonable question and deserves the best answers we can muster. Alan supereal 04-17-2011 @ 10:38 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 Check your piston for a part number. The 3 3/4 Ford piston will be marked "49T 6108" with a suffix letter indicating diameter (standard or oversize), and will be aluminum with a domed top and split skirt. These pistons are used with a 29A crankshaft. 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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