Thread: Piston question (off set pin)

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Does a stock Small Block Chevy generally run an offset piston pin.

My engine is a 1989 350 (Late Gen-1 TBI) from a 2wd Suburban.

I've swapped the engine to counter rotation for my boat.
~ Simple Cam swap / crank seal swap. Cam and crank are gear to gear with no idler gear ~ Crank spins backwards / Cam spins forwards.

Now I am getting conflicting information (from the boat guys) that I need to flip the pistons as some folks believe that the Small Block Chevy has an off set wrist pin as standard design, Thus the part making the pistons directional.

Question 1. Are the piston pins off set?

Question 2. Will I need to flip the pistons to run the engine backwards?

Question 3. Any concern for the rods (oiling cyls) spinning the opposite direction.

[img] width=700 height=523]http://i9.photobucket.com/albums/a53/whitfimb/Dixie%20Super%20Skier%20299/P9060908.jpg[/img]

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Rods are directional for rod to crank assy.

Originally Posted by techinspector1

You mate the caps to the rods with the bearing tangs together. Then the rods are installed on the crank with the tiny little oil hole at the parting line of the cap and rod facing the camshaft, not the oilpan.

Originally Posted by techinspector1

If one of them has the oil hole aimed toward the oilpan, it's wrong. The caps and rods usually have a deeper chamfer on the side that mates to the cheek of the rod journal to allow clearance for the fillet.

Pull the cap off the one that looks wrong and see if the wider chamfer is toward the other rod or toward the fillet. If it's wrong, you probably won't be able to get the proper side clearance on that pair and you ain't gonna like it down the road, cause the shallow chamfer is gonna dig into the fillet and the extra heat generated could grow the big ends enough to spin a bearing.

John Lingenfelter: Modifying Small Block Chevy Engines

p. 44-45

Cast pistons are often built with what is called Piston Pin Offset. A typical offset would be .060 inch, where the pin is closer to one side of the piston then the other. This is used to ensure quiet operation, especially during cold start up when the clearances are the greatest. Some Hot Roders using cast pistons will reverse the piston pin offset which will be worth a small amount of power. As you can imagine the pistons will slap around a little when the engine is cold.

Still doesn't give me much. Sounds like I might get some start up clatter if they are offset but they will run and live either way.


How do the masses feel about reversing Cast Pistons?

i remember dirt track cars did this,and i don't remember my friends doing anything but changing camshaft. naturally change the firing order.

I may be wrong here but the piston is still in its orignal bore going up & down in the same maner as before I don't see the need to change pin offset If I'm wrong someone will shot me I'm sure
Charlie

It loads the piston differently due to the rotational direction.

Sounds like lingfelter believes it can work either way due to the fact he states that some people run them just like you are for small increase in HP with some piston slap while cold at start up........you might see a difference in wear over the years and it might shorten the life of the engine a little.

Sounds like it will work either way,I would let the engine warm up for a longer time with it the way you have it now,before I hammered it down.

I am no exspert ,I am just basing this on what was posted and how much work it would be to swap them.....sounds like almost a complete rebuild to swap them over.

I am not positive,depends on the build ,but I would be more worried about the oiling of the bearings due to some of the oil holes on the crank are chamfered to the trailing edge for improved oiling.....just listen for piston slap when you fire it up and determine how bad it actually is slapping.

In the end it`s your call on what to do.......sounds like it will work either way at the exspense of a little more wear on parts.

Thanks for the comments guys. I look forward to all comments, opinions, and experiences. That is why I am here.

I am hoping to get an opinion from an engine builder as to the risk associated with counter rotation piston rattle. A little start up rattle ~ Like the 200k miles 4-cylinder cars make on cold start when they have excessive skirt wear should be acceptable. Detonation style hammering clatter would not be acceptable.

It's a 25 yr old boat ~ a little healthy v-8 rattle isn't gonna raise any eyebrows. The strait thru at the water line exhaust will mask most of it.
It should see 20 - 40 hrs use per summer ~ generally you don't wear engines out like this.

the dent mark that would go to the front of your engine if std direction rod tag to out side of the oilpan rail.to get the dent to the back of the engine you would flip bank 1.3.5.7.there new home would be 2.4.6.8 now your pistons then will have the right off set .your crank oiling will be fine .but your rear main seal needs to be for a rev direction or the oil will rope right out of the seal. i would fix this your rods are running around the crank the other way around the off set helps on the side loading

Is flipping the piston / rod assembly the easy way out or are the rods machined to turn / load for a given direction.

If I go that far then I might should ball hone / new std. pistons / rings / and std. bearings ~ flipping 125k pistons from hole to hole scares me.

Looks like either way ~ as long as I'm playing with junk it is like playing russian roulette.
Adventually somthing is gonna go B A N G !!!!!!

Originally Posted by Whitfield

Is flipping the piston / rod assembly the easy way out or are the rods machined to turn / load for a given direction.

If I go that far then I might should ball hone / new std. pistons / rings / and std. bearings ~ flipping 125k pistons from hole to hole scares me.

Looks like either way ~ as long as I'm playing with junk it is like playing russian roulette.
Adventually somthing is gonna go B A N G !!!!!!

the rods can go any hole the tang needs to go to the out side oil pan rail... they are dumb they do not know or care .i would ball hone it flip the banks less you want to pay to have some one press of your old pistons just to flip them and there a good chance of getting them screwed up.nothing going bang less you screw some thing up .i rebuilt my share of rods and used other rods to make a full set if one was hurt or missing .were let s say a #1 was replace with a used rod were in its old life before recon was a #8 after re stamping it it now a #1 and new set of rods can go any were as long as the brearing tang is to the out side of the oil pan rail the heavy chamfer go s to the crank radii as long as you use this rule you will be ok

Sourcing a small block ball hone to pull / filp / dance re-ring...

Flipping a coin on the Marine head gaskets @ $40 each~ when I have a set of standards here ready to go.

8 |

Probally gonna drop another $100 on the right the right head gaskets too.