I am in the process of having my 1954 Mercury 256 Y-Block engine rebuilt. The standard size cylinder bores were good so the mechanic lightly honed the walls and I did not have them bored.  One of the pistons had a small piece of the bottom skirt casting broken off but no damage was done to the cylinder wall.  My mechanic recommended that I replace all eight pistons which bought from Summit.  The machinist who did the work on the cylinder heads and installed the new rod bushings noted that the replacement pistons are 63 grams heavier than the OEM pistons.  He recommended balancing the entire engine claiming that the increased weight of the new pistons is likely to make the engine run with a vibration.  I have already put over  $2K in this engine and I don't really want to go much higher if I can avoid it.  This engine is going in a car that will be a low-stressed "Sunday Driver".  Is the use of these slightly heavier replacement pistons likely to result in an engine vibration or is the machine shop just trying to get more work for themselves?    Thanks, Alan S. Freeman

Balancing is one of the most important things to do, do not skip this, also pay close attention to the deck height, replacement pistons are .020 shorter, replacement head gaskets are .025 thicker (approx), failing to correct this will result in a slow, gas using engine, you may want to check the cam that you're going to use, some of the replacement grinds are copies of a 1964 Ford truck cam (newest design) this is not in your best interest, it cost no more to pick the right cam (grind) than the wrong one.

I have a slightly different opinion, I would hold off until someone like Ted can address this.

I understand what you're driving at, Alan.  $$$

Cliff's points regarding the value of having the rotating assembly re-balanced in light of changing the piston weights is a good one.  What is also true, is that the original Ford / Mercury engine balancing work was not THE best.

Cliff's second point regarding replacement pistons having a different pin height coupled with using composite gaskets that have a different compressed height, is true, too.  IF your quench dimension, the distance between the top surface of the piston and the flat deck surface of the cylinder head, starts crowding .060 inch, you'll be flirting with detonation problems.

I've run into replacement pistons from both Silv-0-Lite and Sealed Power that have piston pin heights .020 less than Ford's original dimension.  My understanding is that the piston manufacturers' expectation is that the engine block is going to be re-machined in the rebuilding process.  Two y-blocks I've been through that were previously re-manufactured by Ford authorized re-builders had the mains line honed and the blocks decked as part of their process.  

With 2K in your rebuild already, addressing these other issues will serve to support what you've already invested in the rebuilding process.  A friend who is currently having a 312 rebuilt by a very capable machine shop here in N.Y., JUST got digging into the quench issue with his machine shop.  The build-up of tolerances with the crank having been turned (SLIGHT stroke reduction), replacement pistons having a .020 different pin height, use of composite head gaskets in place of the original FoMoCo steel shim ones yielding a new .046 compressed height, and the original block deck height being different from front to back, the engine came apart again to square and deck the block head surfaces.

A re-balanced engine rotating assembly will yield a smoother running engine AND one that will wear less rapidly going forward.  That said, the quality of the equipment and the accuracy of the machinist doing your work, is critical.

Click the link below to see Ted Eaton's article on cylinder head gasket compressed thicknesses.
http://www.eatonbalancing.com/blog/2012/06/30/head-gasket-volume-calculation/

Hope this helps.  


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

jim, thinking the same.



  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

Does anyone make standard pistons that are more appropriate for this application?


Paul

Alan.
I’d love to say that you would not necessarily need to re-balance a stock rotating assembly but that’s not the case.  I’ve balanced a number of stock oem engines and most need extensive work to put the rotating assembly back into a state of balance that’s acceptable.  The factory variances alone will warrant even an original engine with no replacement parts to be rebalanced.  Anytime parts are replaced or pieces are remachined, then balancing is recommended.  Here’s an analogy.  You wouldn’t install new tires on wheels without rebalancing so in the same train of thought, you would not install new parts within an engine without rebalancing.
 
The fact that the pistons are considerably heavier than the originals puts the crankshaft in a state of underbalance which is the wrong direction for the balance attributes to be in.  If the pistons had been slightly lighter than the originals, you might have gotten away with the piston weight difference assuming the connecting rods and crankshaft balance attributes were in the neighborhood.
 
As with most budgets, you’ve run into a couple of items that will be worth the extra money in the long run by doing them.  Balancing will improve the engine performance and overall life as well as reduce any perceivable vibrations that would come about from existing imbalances.  Decking the block will also restore the compression ratio to the original factory specs and also keep the engine running within its original performance expectations.
 
Thanks to everyone who commented prior to this as the comments are spot on.

Lorena, Texas (South of Waco)

For all interested there is a very easy program found on the KB Piston site for static and actual compression ratios of your engine. You need to know the cc of the head chamber, where the piston is in the block and diameter/thickness of the gasket. Cam timing if actual is wanted.
I've used the program countless times and was shocked to see what different thickness of gaskets did to the compression along with the compression height. We put an engine together with no rings measured the piston height, disassembled to machine the block for what we want. To me this is normal with off the shelf pistons in racing. Being we assemble our own engines this "blue printing" is our way saving costs. Good Luck

56 Vic, B'Ville 200 MPH Club Member, So Cal.

An unbalanced engine...

https://www.dropbox.com/s/m6xql51nelz6ej0/2014-08-08%2012.41.54.mp4?dl=0


Hyvinkää, FI

I've been unable to find info on what the OEM pistons weighed? I need to check the weight of the new pistons I'll be using soon. I didn't have my last engine balanced, it was a budget build just to get the car running temporally, it turned out vibration free and has 6,000 miles on it now and runs like a Swiss watch.