New pistons in 68 318

[Confusedcuda]

Hello all, wanting to put new pistons in a 68 318. I noticed the KB 167 pistons weight 526g and the stock pistons weight 592g. Does this mean I need to have some material removed off the crank for balancing once I install the new pistons? What does this apprx. cost at a machine shop thanks.

 

[4spdragtop]

Depending on pin weight (if going new). Rods and pins have a bearing on "rebalancing" as well. Total weight would need to be added up??
66 grams per piston difference. I would yes to rebalance. It can be pricey. My machinist quoted me approx $600.

 

[Plymouth 65]

You should have the rotating assembly balanced if you're going with that much of a difference in piston mass. 65'

 

[1966DDart]

Hello all, wanting to put new pistons in a 68 318. I noticed the KB 167 pistons weight 526g and the stock pistons weight 592g. Does this mean I need to have some material removed off the crank for balancing once I install the new pistons? What does this apprx. cost at a machine shop thanks.

Short answer...yes, you should get it balanced.

Long answer...the pistons are not the only difference. The wrist pins for KB pistons will be around 132g and factory was around 154g. So, now you are looking at 66g (piston difference) + 22g (pin difference), for a total of 88g...or 0.194 lbs off each piston/pin. Multiply that by 8 and you are essentially taking 1.55 lbs off the reciprocating mass. That's quite a bit!

Balancing runs around $150-300 IF they don't have to add weight to the crank...which should not be needed in your case. Since you're looking at balancing now, I'd strongly urge you to NOT reuse the old rods. Scat makes some nice I-beams (~$350/set) for around the same as you'll spend to rebuild your old rods. The added bonus, large rod bolts and even more weight savings (595g vs. 726g). Factoring in the added weight savings on the rods (131g x 8) and you will have taken a total of 1752g (3.86 POUNDS!!!) off the reciprocating and rotating mass of your engine. It probably won't show on any dyno pulls...but it will rev a LOT faster!

 

[Confusedcuda]

The 318 I am using has around 60,000 miles on it. It's been sitting for a while and really I was just wanting to freshen it up a little. It wasn't hot rodded at all(know the guy who had it) is there any way of just popping the pistons out, checking the cylinder walls, cleaning up the old pistons and installing new rings?

 

[stumblinhorse]

If you are going through all the trouble to "popping the pistons out:, you should go with new stuff. My 318 has over 100k on it and the pistons and cylinders look just fine. I pulled the heads and am having them done. I am leaving the short block alone for now. But I was ready go to the next step and pull the engine and do the pistons if they didn't look good.

If your heads are original 68 heads. And never redone, the soft guides etc for leaded gas probably need some help. And 60k your short block should be just getting broken in

 

[moper]

Sure. It's called re-ringing. It's in service manuals.

 

[TrailBeast]

Sure. It's called re-ringing. It's in service manuals.

Agree, as it's been accepted practice since the cave man days.

 

[1966DDart]

Here's a pretty good overview...

Weighing in on Balancing Work - Engine Builder Magazine

 

[Confusedcuda]

Nice article, Thanks guys!

 

[Rapid Robert]

iirc 273 pins are extremely heavy due to the thick wall thickness as they were deliberately made that way for the purpose of not haveing to rebalance the 273 rotating assy with the crank it used (I'm sure I am missing some info on that) but you might be able to use them to restore the reciprocating (pistons/rings/rod small end/locks (if used) weight gram amt to what it was. when you press off the old pistons then clean one & its rings & HF has a cheap digital gram scale for $9.95 & it is just as accurate as my now deceased buddys' high dollar one. compare this to a new piston/273 pin & you can drill out the 273 pin ID (thats where the extra weight is) till the weight matches. this'll save you from rebalancing. it sounds harder than it is tho the redrilling would take some time. Holler how it turns out.

 

[pishta]

273 pins have same OD as 318 pins but they are much thicker walled/weigh more to compensate for the 273's smaller piston with the same 318 balance of the poly motor the crank came from. Put 318/340 floater pins in the OEM 273 pistons and youll save weight. shop around and you can find wide range of balance pricing. I found a guy that will do crank only for $75 if I bring a balanced "across the board": piston, pin, rod, cap and ring pack. Thats to drill/lighten to balance, no tungsten. I have said HF 1kg digital scale and balanced everything to each other to .1g, Even the nuts. Pistons were hardest as some were really out of balance and I was looking for places to lighten so I ended up taking some off the pin ends as they were pressed by now. I think some of my pistons were 15g over weight of lightest one. Rods were tough, tried to balance little end on knife edge at parting line to scale but it was not repeatable so ended up getting them pretty close this way and then balancing the entire rod itself to all the others and then the caps to each other. Import engine builder said this was sufficient, although they usually deal with single plane cranks.