340 REBUILD QUESTION

What are the old pistons?

If they're stock or stock replacements, you can swap in new stock replacements and be within a factory balance job. Mopar balancing was well-known to be the least accurate of the Big Three, yet the engines were not known for being rough. Shows what cost-cutting engineers can do.

However, IMHO it would be well worth your time and money to buy lightweight forgings and have it balanced again. Stock pistons weigh 720 grams without pins, a decent lightweight forging should come in at 500 grams or maybe even less. Taking this amount of weight off the ends of the rods will be noticeable.

R.

It had the stock 8.5 to 1 pistons. I replaced them with Kieth Black's.

weigh your new: pistons/rings/pins/locks (if used). HF has a gram scale for $9.95 on sale and it is just as accurate as my buddys high dollar one. record your weight of the (8) assys. Clean one of your old piston/pin/rings assy & weigh it. Post (here) what you get. If your new assy is heavier you can grind material off of the rod small end pad(s) and if the new assy is lighter, iirc 273 pins can be obtained and are heavier than std SB pins. Use you magnetic dial indicator stand or similar to get the rod horizontle to weigh the small end. the small end/piston/rings/pin/locks are all added together to get the reciprocating part of the bobweight formula and you want all (8) assys the same & a particular piston can be for example 5 grams heaver than the next one if the rod used with it is 5 grams lighter as the crank only sees the total weight of the assy and not individual item weights. For a regular balancing job you would weigh the rod big end and the bearings for the rotating weight part of the formula but you have already been balanced and are only changing the reciprocating weight part of the formula (piston/pin/rings/locks) so you only need to get that part back or very close to where it was when it was originally balanced with the original stuff. If you cannot because of the new piston weight difference then you need to have it rebalanced and if you can weigh/equalize the parts (including rod big end/bearings tho these will/should be the same as each other as you ain't changing anything on that end but you do need that number for a new balance job at a shop if you dont have the paper from the orig balance job) then give those 2 numbers (rot/recip weights) to your crank man & let him do the math (one gets doubled/one gets halved). this will drastically drop your bill as he has to do no repeated grinding/weighing to get em all equal, all he has to do is spin & balance the crank. #1 #1 #1 (A) clean/weigh what you have now (B) weigh your new pistons/rings/pins/locks. (C) compare the difference (grams/%). you want your new assys to be equal to each other and to the old stuff it will be replacing (you have a certain leeway). If you can get the parts equal then no rebalancing neccessary. EDIT you grind on the rod small end pad to manipulate the piston/rings/pin/locks for the recip part of the formula and grind on the rod big end pad to manipulate the rod big end and bearing weight for the rotating part of the formula as you cannot grind more than a hair on the pistons/none on the rings and the pins can be shortened a bit (within reason) as (if) needed.

THANK YOU