Plastiguage proceedure

[jerry598]

I'm now installing pistons and connecting rods on my 6.5 rebuild project. I'm checking the rod bearing clearances with green plastiguage (the really thin stuff) and am getting readings that suggest things are tight. On dry bearings at proper torque, the green smashed strips are elongated but slightly irregular along their length. The narrowest parts of the strip indicates .002" bearing clearance. The wider part indicates a clearance closer to .0015". My book says the clearance should be a minimum of .0018".

This makes me a little uncomfortable. When I switch to the red plastiguage which is a little thicker, I get .002" consistently (the smallest reading the red stuff will give). The narrowest part of the red smashed strip suggests a reading slightly greater than .002" which would be fine. With the red, the readings indicate the clearance is good to go. With the green, they suggest clearances are borderline.

Could this inconsistency between the red and the green be due to temperature? I'm working out in the sun and the metal is very warm to the touch, but not too hot to work with. Or could it just be the plastiguage itself?

These are new standard bearings being installed on a polished crank. This is not supposed to be rocket science, right? Is there any reason for me to be concerned here?

[Robyn]

Plastigauge is a guess stick.
Its pretty reliable though.

The secret is to have both rods on a journal to keep the rod from moving around. Place a tiny, tiny spot of oil (use a Q tip dipped in tranny oil) and smear just a bit of oil on the insert in the cap.
Dont need much, just enough to stick the plastigauge to the insert.

Slide the cap onto the rod then torque down, back off and remove the cap.

This will give you the best reading. Keep all other parts dry.

Once your happy, then lube the bearings with Lubriplate 104 and assemble and retorque.


Missy

[john8662]

Also, which direction are you placing the strip of plastigauge?

It goes parallel to the length of the crankshaft, which means it does not wrap around the journal.

Sounds like the metal is also too hot, this still is a wax, so, the hotter, the less accurate the readings.

Besides your rods, how did the main bearing clearance end up coming out when you torqued the main caps to their proper torque ratings?

Really interesting that we're taking about plastigage, as this was the task over the weekend on my own project engine. Really making sure everything is perfect on this engine, as it's bottom end has been modified with the splayed main caps. .002" clearance all the way around, and perfect thrust on the crank, something's going well.

[jerry598]

Glad things are going well on your engine, John!

Yes, I placed the plastiguage in line with the crankshaft on both the mains and the rod journals. The heat is probably why the green material is spreading out more than maybe it should. The red material gave me .002" (or slightly more) clearance readings on the main journals #1-4, and a little greater clearance on the #5. If I remember right the book says #5 should have more clearance. Only have 4 pistons installed so far, all measured .002" or just a little more with the red plastiguage. I am of the opinion now that the primary reason to use plastiguage is to be sure there is no serious clearance problem.

I missed that about crankshaft thrust clearance. I did smack the crank for and aft several times as Robyn said to do, but failed to do any check on the endplay, thinking I didn't need to worry about it since its the original crank going back into the engine. Guess its not too late to check though. Just requires a feeler guage, right?

Novice that I am, I wasted some time trying to install the first couple pistons with rings before checking the bearing clearance. Can be done both ways I guess, but checking the rod bearing clearance seems to be alot less trouble by just sticking the ringless piston and rod into the cylinder, checking the clearance, and then pulling them out to clean the plastiguage off the bearings and journals, etc.

Also makes it easier to have the rod out of the way when lubriplating the journal.

My machine shop supplied me with rings already installed on the pistons but they did not check ring gap or anything. So I removed the rings to check ring gap and piston clearance, etc. Not so hard to do if you have 4 hands and don't put the rings back on upside down.

And, it would help to memorize the specs, because going back to the book all the time is a pain. I even read the wrong line in the book when checking the gap on one ring and filed it down when I didn't have too. Sure glad I don't have a life right now. This part of the rebuild seems to be taking the most time.

[john8662]

Don't fret too much over the thrust, as long as there is some. Sounds like you did the important part and got the two bearing pieces aligned (the part about smacking from the front).

You measure the thrust the same way you do a camshaft, with a telescoping magnetic base with a dial gauge mounted to it.

It can get real picky can't it?

Most rings the gap is pre set, and you can check for peace of mind. Gapless rings are another story though, required filing. File only one way, pushing into the file, so no burrs on the outside of the ring, btw. A ring file fixture is also nice...

Carry on.

[Robyn]

Unless I have reason to believe that there are issues, I just use a large screwdriver and pry (lightly) the crank back and forth and look at the shaft move. About .010" is fine, if you can see it "JUST MOVE" your set.

Spec on the end play is .003" to .010" from the GM book. even a couple more thou wont hurt but if it gets to .020" plus then there would be an issue with the shaft.

Like John said a mag base dial indicator is the ticket to get this measurement. Just stick it to the front of the block and align the indicator stem on the crank snout or ballancer and then pry the crank back and forth.


Good luck on the build.

Missy

[jerry598]

Thanks again, Robyn.

Don't have a mag base dial caliper, but with the screwdriver I can feel and hear the endplay. With a feeler guage, I can measure about .005" gap between the thrust bearing and the crank.

[Big Green]

One of the biggest mistakes I see with plastigauge is after the cap is torqued down and then the nuts are removed is how the cap is removed. Most people (myself included) just tap the ends of the rod bolts with a hammer to break it loose. Normally no problem. However it's been my experience that doing this with plastigauge in place will result in inconsistent readings.

[Robyn]

Here is a Pix I shot during the DaHooooley build.
There are many pix in the 6.2/6.5 tech forum in the thread marked build tips.

Notice that the gage piece does not have perfectly straight sides after being compressed. The guess stick :) shows the clearance and this is quite acceptable.

If you can, snap a piccy of what you have with the guess stick showing as in the pix here and post it so we can see what ya got.

Robyn

[jerry598]

Had lots of trouble getting photos that were half way decent with my camera. Below are photos of the green plastiguage on the #7 & #8 rod journals.

I had similar results with all the main bearings and all of the other rod bearings. The green indicated a tight clearance (smashed strip barely wider than the .002" green guage mark). The red indicated .002" or slightly greater clearance (smashed strip just a bit narrower than the .002" red guage mark).

All shells were marked "std." The Clevite website states that a person could use std. halfshells in the upper half and oversize shells in the lower half to get a little more clearance - if its necessary. That would save some $$$.

But I still have not found a supplier for oversize shells (larger diameter than standard). All I see on the web are undersize shells for crankshafts that have been ground.

The local mechanics that I talk to about this think I'm overly concerned about this clearance issue. They say that if the plastiguage is showing clearance near the book minimum (.0018") or .002" then I should just slap it together and go.

I've got it all put together now except for the last two pistons, but not so far along that I couldn't take it all apart again to insert oversize bearings.

[jerry598]

Green plastiguage photos posted in the last reply. Here are the pics of the red strips.