heavier pistons/ problem with balance
01-03-2011, 09:29 PM
#1
TECH Fanatic
Thread Starter
iTrader: (8)
Join Date: Nov 2009
Location: phx the cactus patch
Posts: 1,095
Likes: 0
Received 13 Likes
on
13 Posts
heavier pistons/ problem with balance Hello cam crazy poeple . I am rebuilding a 5.3 and the Speed-Pro pistons and pins are weighing 12 grams more than stock pistons and pins . And the ARP rod bolts are roughly 5 grams/ 2 bolts . So each piston, pin, rod, cap, bolts assembly weighs 17 grams more than stock . Will this be a problem ? If so , what can I do , buy a LS1 crank ? Thanx for You're input .
01-04-2011, 12:01 AM
#2
On The Tree
iTrader: (1)
Join Date: Dec 2010
Location: Ft. Worth
Posts: 133
Likes: 0
Received 0 Likes
on
0 Posts
The extra weight of the bolts should not be a problem as they are on the big end of the rod and don't disturb the balance as much.
The pistons weight is more important.
Did you weigh a complete package? Piston/rings/pin/locks-both new and old
If you can make the complete piston package weigh close to the old one I wouldn't worry about balancing. It is common to see balancing marks on the undersides of piston domes. The most common mark is the beginning of a drill hole. Use a large (3/8) drill and just "kiss" the piston in a thick spot on the bottom of the crown. It is common to see multiple marks and then a couple of smaller marks as the person doing the balancing uses smaller drills when they are getting closer to their target weight.
12 grams is close enough to balance by this method in my book. 50 grams is not although I remember a thread on my Z car site where someone removed nearly 80 grams per piston from a complete set (for lightening)
Personally I would NOT make the pistons lighter than the stock one to compensate for the extra rod bolt weight.
Match pistons to stock weight and assemble. You should be good.
Most production engines are balanced to a "range" and not a specific weight.
It is more common to see factory stuff "overbalanced" IE the balance pads on the crank throw are "heavier" than the pistons/rods they are balancing.
Adding a small amount of weight is usually less detrimental than removing weight.
Many machine shops would use pistons that much heavier and never mention balancing.
Your close already.
Good luck
The pistons weight is more important.
Did you weigh a complete package? Piston/rings/pin/locks-both new and old
If you can make the complete piston package weigh close to the old one I wouldn't worry about balancing. It is common to see balancing marks on the undersides of piston domes. The most common mark is the beginning of a drill hole. Use a large (3/8) drill and just "kiss" the piston in a thick spot on the bottom of the crown. It is common to see multiple marks and then a couple of smaller marks as the person doing the balancing uses smaller drills when they are getting closer to their target weight.
12 grams is close enough to balance by this method in my book. 50 grams is not although I remember a thread on my Z car site where someone removed nearly 80 grams per piston from a complete set (for lightening)
Personally I would NOT make the pistons lighter than the stock one to compensate for the extra rod bolt weight.
Match pistons to stock weight and assemble. You should be good.
Most production engines are balanced to a "range" and not a specific weight.
It is more common to see factory stuff "overbalanced" IE the balance pads on the crank throw are "heavier" than the pistons/rods they are balancing.
Adding a small amount of weight is usually less detrimental than removing weight.
Many machine shops would use pistons that much heavier and never mention balancing.
Your close already.
Good luck
01-04-2011, 10:31 AM
#4
Anytime the weight of a rotating assembly is different from before I would recommend rebalancing. It may cost a little extra money but when that assembly is turning at 6800rpm you don't want it to be vibrating because you didn't spend the extra $200 to get it balanced.
01-04-2011, 10:48 AM
#5
Anytime the weight of a rotating assembly is different from before I would recommend rebalancing. It may cost a little extra money but when that assembly is turning at 6800rpm you don't want it to be vibrating because you didn't spend the extra $200 to get it balanced.
01-04-2011, 12:41 PM
#6
Since the big end is typically counted twice when calculating the bobweight, the rod bolts weight are significant since you're looking at about a 10g difference.
The stock balancing also isn't that great to begin with, so I wouldn't hesitate to have a set up like yours balanced before it goes together. When you do take it to get balanced, all they'll need is the rotating assembly. The LSx engines are internally balanced so the flywheel and dampener are not required.
The stock balancing also isn't that great to begin with, so I wouldn't hesitate to have a set up like yours balanced before it goes together. When you do take it to get balanced, all they'll need is the rotating assembly. The LSx engines are internally balanced so the flywheel and dampener are not required.
