The Stovebolt Site Forums The Shop Area Engine Shop New question on my 270 engine build - piston orientation

My 270 has flat top pistons with a slight dish on them. is there a right and wrong way they should go on the rods?? the only indication of any difference in the piston is that they have a small notch at the bottom of the skirt on one side only.
Jay D.

jay d - Ya gotta start posting some pictures to go along with your questions, please. Also, you neglect to mention what year 270 you're working on. It can make a difference, so would help immensely if you would provide as much information and detail as you can to help us help you.

Look at the side of a piston, where the wrist pin goes through. If the pin hole is offset from center, install the pistons on the rods so the top of the rod will be closest to the camshaft at the top of the stroke. The offset is usually quite small- - - -about 1/16", so you might need to do some measuring to be sure. The pin offset is there to prevent piston slap at low speeds such as idle and part throttle. The oil spit hole in the upper half of the connecting rod big end also points toward the camshaft. Sometimes a piston will have the letter "F" cast into the wrist pin boss, indicating "front".
Jerry

Here's a picture. Pin looks centered and no F anywhere. I think they're going in as they fit. NOTE: the notch at the bottom of the skirt and ONLY on one side ????

Unless you've measured the pin location, you can't be sure. That notch at the bottom of the skirt must mean something. How about the sides not shown. Any relief cuts for expansion on one side?

To measure, stick a pin in each side so they stick out, lay the piston on it's side on a flat surface and measure up to the pins without moving the piston. Add those measurements together. Then do the same with the piston laying on the other side. Like Jerry said, the pin should be closest to the camshaft.

Yes, the pin is offset- - - - -after looking at these things for 65+ years, it's very obvious. Use a machinist's square and a sharp pencil, and mark a vertical line centered on the slot in the skirt with the piston crown as a reference point. Then measure from the line to the edge of the wrist pin hole in both directions. Like the Sun test equipment slogan used to say- - - - -"WE TEST- - - -NOT GUESS!"
Jerry

The offset is not very much about .046 or so memory????? And per Jerry's instructions it looks to me the notch marks the front, because it looks shifted to the Left as shown in pic.

Most offset pins are around 1/16" (.0625) but some could be less. The idea is to make the piston tilt sideways just before the crankshaft reaches TDC on the compression stroke, preloading the piston skirt against the cylinder wall before the spark plug fires, to minimize piston slap. A backwards piston can make a noise resembling a rod knock, especially if the piston is fitted a bit loose in the cylinder.
Jerry

Take three pistons and lay on their sides, place a wrist pin half way in each outer piston and the other half in the center piston. The center piston notch up, the two outers with notch is down. If the hole is offset, the center piston will be up off the table. If not, turn the whole mess over and recheck. No measuring needed this way !

We used to install our Pontiac pistons on the wrong side ( backwards ) when racing, this makes the piston dwell longer at TDC which makes more power. Never had any problem with them. Not sure it really worked, but the Chrysler guys did it, so we followed them.

LOL "Not sure it really worked, but the Chrysler guys did it, so we followed them." That old trick was straight from the "Mopar Drag Racing Bible", put out by a group of Mopar engineers. I still have my copy and yes I always put the pistons in backwards, never really noticed any piston slap but we always used a torque plate when boring and finishing a new engine build, fitting each piston to a specific hole. I've still got specs on engines from the 70's and early 80's ( I think). In the world of fast, my car wasn't fast, just running in E Stock, never had the money to really threaten the national record but had a lot of fun.

RonR

That's a good trick. A lot easier than measuring the way I suggested.

SO, i'm reading this early this morning after hoorawing LATE last night. so which piston pin is offset? the one in the center that is lifted up slightly with the notch up, or is it the ones on the outside with the notches down??? AND which way is the offset there are several combinations here! or i'm i just overthinking this! i'm on my way out to test this maybe i'll have a better understanding.
Jay D.

Funny thing is my 52 GMC Maintenance Manual and my 55.2-59 Manual do not mention anything about pin offset in the piston. They do talk about proper rod orientation so the oil hole would lube the cam lobe to lifter interface.

Some of the pistons have an asymmetrical dome on the top which would dictate which direction they went on the rod, but others have flat or slightly dished tops.

hear is my findings. you can't use the notches up laying down as they run parallel to the pins. THIS still work rather good by standing the pistons up on their heads. with 3 pistons standing on their heads connected by the pins I used a straight edge the outside of the piston bottom and top, top being the top of the skirt. i tried every combination i could get, outside pistons notches facing right, center facing right, and left. i rotated pistons from outside to the center and so on. i found no discernible difference, but what i did find was some were about .006 high on the skirt on one side and straight on the other side.( this would change depending on what combination of pistons you were useing) so piston is oval by .006. ANOTHER thing i have noticed is that the pistons look to be low hour and show more wear on one side this would be the thrust side against the cyl. wall on the side OPPOISITE the lifters? IF i use this line of thought i should be golden?
Jay D.

MY manuals say nothing also BUT the notch on the skirt had me wondering.
Jay D.

Do you have a surface plate and a height gauge?

NO, on the surface plate. all pistons were straight across the tops ( 3 at a time) where i had them was straight with the straight edge. i have an old machinist friend i'm going to try and take them to him. i'm thinking the .006 is from where i was working, it would only take a microscopic misalignment and you'll get a funny reading.
Jay D.

There is a very simple way to get an accurate offset measurement. Put two pistons on the same wrist pin, lay them on a flat surface with the piston tops facing you (surface plate is best), but even a piece of glass will do. Use a height gauge to measure the distance from the reference surface to the top side of the pin. Turn the piston tops away from you and measure the height of the pin again. The difference in the two measurements (if any) is the offset. So simple even a yankee can do it!
Jerry

This is what GMC had to say in 1958.

did the depth mic thing and found no discernible difference, maybe a couple tho., I'm going to get a piece of glass and try agian.
Jay D.

thanks Jerry i never really had a good understanding why this was done and what it did.
Jay D.

When I was teaching auto mechanics at the high school and vocational trade school level, we would usually spend several hours of classroom instruction time dedicated just to basic piston design. There's an incredible amount of engineering involved in making pistons perform properly and last millions of up/down cycles in an average engine's lifespan. At 2K RPM, every piston in the engine is reversing its direction of travel over 33 times per second!
Jerry

I was just looking at the engine specs for a Model A engine (researching for a friend) and found something I never knew before. It seems the cylinder bores are offset by 1/8" from the crankshaft centerline. That would seem to deal with the same issue as offset piston pins would. I recall that Model A pistons, as well as 216 and 235 stovebolt pistons don't have offset piston pins.

"More than one way to skin a cat!" I've had an old Model A block in the shop for several years- - - - -I'm working on figuring out a way to install a cross flow PT Cruiser dual overhead cam 16 valve cylinder head on it, along with a few other modifications. My Silvolite piston specification catalog lists their #1413 piston as a replacement for the GM #3707453 and #3693894 pistons for a 235 as having an offset pin- - - - -no listing for a 216. One of my friends installed the pistons and rods backwards in a 235 several years ago, and it rattled like a Diesel!
Jerry

last night i was looking at my pistons and noticed there is a wear pattern on the skirt on ONE SIDE only. so i lined all 5 ( i have one in the block) up on my bench with the wear mark area facing out towards me and guess what, the notch on all of them is on the left side. something is trying to tell us something. this wear spot is being thrust to the side of the cly. BUT which side? Jay D.

OK- - - - - -think about the angle the rod makes with the crankshaft journal as the piston moves down the cylinder on the power stroke. The scuff marks on the skirt MUST be facing the passenger side of the block. At least where people drive vehicles on the right side of the road! I believe it was Mark Twain who said "Common sense is an uncommon thing!"
Jerry

GREAT, i think we can put this story to bed now. i
m going to install pistons tonight.. THANKs TO ALL!!!!!!
Jay D.