woodmaster9

Hi everyone! I'm new to the forum and I need a little advice from you seasoned engine re-builders.

I'm doing a piston replacement in a 20R and I'm looking for a little knowledgeable advice.

I have an 80 pickup with a damaged piston. I decided to replace all four pistons (with rings). I bought a set of Nippon (NPR) pistons and rings. I also purchased a set of remanufactured rods.

The factory service manual states:
"At 80 deg C (176 degrees F) the pin should be able to be pushed into the piston with your thumb. If the pin can be installed at a lower temperature, replace the piston and pin."

Another section states:
"Try to move the piston back and forth on the piston pin. If any movement is felt, replace the piston and pin."

Well, the new pistons and pins do not have the snug fit described in the manual. They fit really close, with no detectible play, but the pins readily slip into the piston at room temperature.

Same goes with the rod to pin fit. It is a precision fit but the rod does move fairly easily on the pin.

Has anyone else run into this? Any opinions or ideas? I'm almost scared to install these in my truck!

slacker

iTrader: (1)

yes .. you should be .. the rod end bushings need to be "sized" by a machine shop



.

woodmaster9

Yes the pin-to-rod fit can be fixed by replacing the bushing and honing. The guy I got the rods from assured me they were ready to go (without honing). The spec tolerance (for new bushings) is .0002 - .0004 clearance.

It is the pin-to-piston fit that concerns me the most. There's no way to fix that except to get new pistons. Heck, these are new pistons.

I've read where others have used the NPR pistons with no problems.... so I'm scratchin' my head here. It's like the pins are undersized yet they're supposed to be a matched set.

I'm going to take a stab at measuring all the pieces but my tools are graduated in thousanths, not ten-thousanths, so there's going to be a bit guess-error involved.

truckman1966

as long as your NPR's cam with locks (like mine) that prevent the pins from sliding out of the pistons, you will be fine. floating pins are nicer for engine assembly anyways.

Yopar

iTrader: (2)

Yeah check the pistons to see if the pin lock grooves are there, and better yet the locks themselves might be in the piston boxes??? These are called full floating pins.

woodmaster9

Yes the pistons came with retaining rings (locks)..... but then the factory ones have locks, too.

I think the way the original ones work is that (at room temp) they are tight. When they heat up from the engine running, they become loose (from expansion) and free-floating. That's why the service manual says that you can push the pins out after heating the assembly to 176 deg F.

So my concern is that the new pistons are loose at room temp....

I returned the NPRs and purchased another set. The are in a box labeled "Rock Pistons" but they have the correct Beck Arnley part # handwritten. Unfortunately the fit is the same as the NPRs.... loose at room temp. The pins on these are hollow tubes whereas the pins on the NPRs were solid bars. MAybe these don't expand when heated like the factory originals?? I'm confused now!!!

Yopar

iTrader: (2)

They are definitely free floating pistons/pin assemblies. I'd run either set of new ones you have. With a lock in place, there is no way for them to come apart, and free floating means just that, they are NOT a pressed fit. If the pins fit nicely in the pin bore, there is nothing to worry about as they were machined for a free floating situation. Your original post states the fit is precision. Nothing seems wrong, just you are taking the manual too literally. It's possible the factory units had .0001-.0002 more tightness than the replacement units. I would oil them up with some assembly lube, and build the engine.

woodmaster9

Thanks for the info... I don't want to be anal about the pistons. This is the first time I have ever been this deep into an engine so all I have to go on is the Toyota factory service manual. I have believed they would be fine all along but the service manual scared me.

woodmaster9

Well I got the pistons installed and the truck seems to be running well. I do have a small timing issue that I'll address in a new thread.

Thanks everyone!