273 commando rebuild question

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zhandfull

zhandfull

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Rebuilding this little 273 Commando engine The goal is low budget and keep it as original as possible.

Today I picked up some new Egge .020 over pistons and as has been discussed in other threads they are lighter than stock. I have a decent postal scale that says the stock pistons and pins are 750 grams each. The new pistons are 682 grams each. So the new pistons are 68 grams each lighter.

That got me thinking about the rods. The stock 273 rods I was going to reuse without having any work done to them. They weigh in at 728 grams each. I also have a set of stock 340 rods that have ARP bolts, been resized, and the beams cleaned up a bit that weigh in at 748 grams each. They're 20 grams heavier than the 273 rods.

Not knowing a lot about the balancing. I usually just take it in and say balance it. In this case I know it is still going to need balancing regardless of what rods I use. With either combo it appears to me the bob weight will be lighter requiring balancing the crank is that right?

My next question is should I keep the stock lighter 273 rod? Or should I put in the better 340 rods?

Also noticed there is a couple grams difference on a couple of the original pistons and the new ones. What is an acceptable difference in weight between the 8 pistons and and even the 8 rods?
 
If the pistons/rods are lighter the machinist will drill the crank to balance the weight. The 273 rods are plenty strong and I would use those. The machinist will weigh all the pistons and use the lightest one and make them all weigh the same by removing material inside the piston. They do the same with the rods. They make them all weigh what the lightest one is. tmm
 
toolmanmike said:
If the pistons/rods are lighter the machinist will drill the crank to balance the weight. The 273 rods are plenty strong and I would use those. The machinist will weigh all the pistons and use the lightest one and make them all weigh the same by removing material inside the piston. They do the same with the rods. They make them all weigh what the lightest one is. tmm
Click to expand...

On the 273 rods I think my main concern is the 45 year old bolts. Of course if I have new bolts installed I think they need to be resized. Then next thing I know I have more money in them than a new set cost at Jegs. The 340 rods I have a lot of money in from 20 years ago but they have very little run time on them.
 
zhandfull said:
On the 273 rods I think my main concern is the 45 year old bolts. Of course if I have new bolts installed I think they need to be resized. Then next thing I know I have more money in them than a new set cost at Jegs. The 340 rods I have a lot of money in from 20 years ago but they have very little run time on them.
Click to expand...

I see. Yes new bolts are in order and they should be resized for sure. New ones or the 340's will be just fine as long as you balance it. Check with your machine shop. I don't think it was that much to install new bolts and resize. I'm sure many engines were built/rebuilt with mismatched parts with no problems. Comments anyone?
 
I aways thought the 340s main advantages was a lighter rotating assembly. If the 340 rods are heavier than the 273 rods I have to wonder if they are also longer.
Shows how much I know about engine building and their OEM specs.
Tagged the thread though, hoping to learn something . Good luck
 
zhandfull said:
Rebuilding this little 273 Commando engine The goal is low budget and keep it as original as possible.
...

That got me thinking about the rods. The stock 273 rods I was going to reuse without having any work done to them. They weigh in at 728 grams each. I also have a set of stock 340 rods that have ARP bolts, been resized, and the beams cleaned up a bit that weigh in at 748 grams each. They're 20 grams heavier than the 273 rods.

Not knowing a lot about the balancing. I usually just take it in and say balance it. In this case I know it is still going to need balancing regardless of what rods I use. With either combo it appears to me the bob weight will be lighter requiring balancing the crank is that right?

My next question is should I keep the stock lighter 273 rod? Or should I put in the better 340 rods?

Also noticed there is a couple grams difference on a couple of the original pistons and the new ones. What is an acceptable difference in weight between the 8 pistons and and even the 8 rods?
Click to expand...

I would use the 340 rods since they are done. I do not think they are required but they are heavier duty. You are going to ballance everything anyway. I usually ballance to plus or minus .2 grams, but my machine shop says I'm over the top.
 
RedFish said:
I aways thought the 340s main advantages was a lighter rotating assembly. If the 340 rods are heavier than the 273 rods I have to wonder if they are also longer.
Shows how much I know about engine building and their OEM specs.
Tagged the thread though, hoping to learn something . Good luck
Click to expand...

340's were known for being strong, and making power. Pistons are heavier, rods are heavier, great heads, intake, and carb from the factory. Rods are the same center to center as all small block Mopar.
 
zhandfull said:
On the 273 rods I think my main concern is the 45 year old bolts. Of course if I have new bolts installed I think they need to be resized. Then next thing I know I have more money in them than a new set cost at Jegs. The 340 rods I have a lot of money in from 20 years ago but they have very little run time on them.
Click to expand...


So just use the 340 rods, maybe as is. For all we know those 20--25 year old bolts might have been made better than some new rod bolts you can buy today.

Have the caps been on or off all these years ? I would check them out for being round, if not run them as is.
 
Don't mess with the balance, have your machine shop install new bolts in the 273 rods and resize them. Probably $100, well spent. Those reconditioned rods and new bolts will outlast us both. toolman
 
toolmanmike said:
Don't mess with the balance, have your machine shop install new bolts in the 273 rods and resize them. Probably $100, well spent. Those reconditioned rods and new bolts will outlast us both. toolman
Click to expand...
I agree, the 273 rods are just fine and as long as they are re bushed & studded everything is fine as is. The rods will all be the same weight and the pistons will be as well unless the manufacturer was really bad. It wouldn't hurt to weigh everything. The best results I have seen is using all the same parts from the original assembly, I have even went as far as stamping the rods and caps 1 thru 8 and reinstalling them in there exact order again, this eliminates the caps from being mixed up which has to be done anyway. Usually a crank turned and rods re bushed and re studded will not alter balancing of a good rebuildable core. What usually messes things up is when the original harmonic balancer is changed out and or the clearances are to much or to little. If the balancer is changed out from original equipment the assembly will have to be rebalanced. Most of the rebuilt equipment is a little lose, except for skirt to wall piston clearance, that you want just right which requires good machining meaning truly round as well as the same diameter cylinder holes.:salut:
 
zhandfull said:
Rebuilding this little 273 Commando engine The goal is low budget and keep it as original as possible.

Today I picked up some new Egge .020 over pistons and as has been discussed in other threads they are lighter than stock. I have a decent postal scale that says the stock pistons and pins are 750 grams each. The new pistons are 682 grams each. So the new pistons are 68 grams each lighter...
Click to expand...

He is 68 grams light on each piston. Do you really think he should not Ballance the engine???
 
66fs said:
He is 68 grams light on each piston. Do you really think he should not Ballance the engine???
Click to expand...
I have built many engines small & big block and have always put a lighter piston in replace of the heavier ones and never had a problem unless I didn't run the factory harmonic balancer.:tongue7: Revolution!
 
65CudaFish said:
I have built many engines small & big block and have always put a lighter piston in replace of the heavier ones and never had a problem unless I didn't run the factory harmonic balancer.:tongue7: Revolution!
Click to expand...
I mostly used Badger pistons with speed pro rings, unless I wanted higher compression then I used Ross!:burnout:
 
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