[Skip Fix]

SO how close is Plastigauge to miking crank clearances? Anyone done comparisons as a double check?

[Jay S]

Plastiguage was .001 tighter than actual here pretty consistently lately. Was at least consistently off. Been off enough we don’t make any changes until after we mic it.

[shaker455]

Hey Skip
I measure it all with bore and outside mikes.
I like to check it with Plastigage afterwards so I have a second method to confirm.

[Skip Fix]

I'm getting 0.001 tighter on the Plastigage also compared to my mike and bore dial indicator. Mains miked 0.004 rods 0.002/0.003 and 0l001 Plastigauge respectively.

[Jay S]

I think our high humidity swells it up some, tends to measure tight.

[Skip Fix]

And 0.001 on bore is 0.001 on mike.

[mback12000]

Plastigage cannot distinguish between the actual clearance and the centering of the shaft within the hole. For example, if your crank is bent a half thou and you use plastigage, the reading will vary if you check with the bend of the crank up towards the cap vs the bend down towards the block. That will not happen with a mic. Similarly with rod journals, when your torque the cap if that pulls the piston against ring friction, or tugs the crank, your squish of the plastigage will vary depending on what's pulling/pushing what.

[steve25]

I don't mean to be a wise ass here by asking this question, but can someone explain to me how it is that plastic is capable of absorbing moisture?

The above explanation is post 7 is what is going on when using Platigauge.

It was explained to me 38 years ago now by a Speed Pro engineer that plastigauge will always report the absolute minimum that you might, have and the average clearance when checked by mic usage will always be atleast .0005" bigger.

I always double check with plastigauge, and if for example for the mains on a stret / strip motor with a cast Crank shows the plastigauge reporting back at .002" then I am perfectly happy!

[67Fbird]

Originally Posted by mback12000

Plastigage cannot distinguish between the actual clearance and the centering of the shaft within the hole. For example, if your crank is bent a half thou and you use plastigage, the reading will vary if you check with the bend of the crank up towards the cap vs the bend down towards the block. That will not happen with a mic. Similarly with rod journals, when your torque the cap if that pulls the piston against ring friction, or tugs the crank, your squish of the plastigage will vary depending on what's pulling/pushing what.

winner winner

mic's and dial bore gauge do NOT lie (provided it's a ROUND hole)

[Jay S]

Yes, moisture is a blind theory, I don’t know what else it could be though. You would think if it set on the self it would be get hard and be more difficult to squish, and give a bigger reading, instead we consistently see it do the opposite. Reads tight, older it is the tighter it reads. Never have seen it give a reading bigger than the mic, it can read close.

Seen it change .001” though on the same engine just changing to a fresher batch after everything is bolted together and checked once. If your pulling parts together enough to swish it more you used it wrong. We sometimes position it around the crank to check the line bore or bent cranks, handy for finding bell mouthing. We use it as a secondary check, same as what 455shaker does. We depend on it pretty exclusively working on inframe large Deisel engines. Can’t get the mic up in there. I get the feeling some people that are quick to point out the short falls of plastiguage do not have much experience with it.

A winter ago I helped another guy on this board that used a mic and plastiguage down close to Houston not to far Skip. Same thing, was .001 tight. At the same time he was complaining about rust being caused by humidity. Have problems with humidity here too. Sometimes it is so bad that that the concrete sweats and it looks like it rained inside the shop. Ugh!

[padgett]

So you mike the plastigauge ? Makes more sense than dropping the crank unless going to anyway.

[Skip Fix]

I just put dehumidifiers in my shop, home garage not as bad. I had water dripping into my eyes off the engine in the Camaro under it! The undercarriage on my 78 TA that was show clean in the 90s looks like it was left in a field form all the humidity!

So you guys change clearances on coated bearings knowing some will wear off? My rods are coated-originally for the 470/409 but that 4" Eagle crank had so much taper it had to go 0.010.