F6

I noticed that I don't have anymore plastic gauge and it seems very hard to come by these days.
Well, I measured rod and main bearing clearance with a mic and a dial bore gauge. Since I'm measuring the complete bore and not just half like with a plastic gauge, wouldn't I have to divide the measured clearance by 2 to get the real clearance??

seawolf06

That sounds right.

MachinistOne

no


You measure the crank journal with a mic, then set the dial bore gauge to read "0" at that number. Then what ever (+) measurement is shown on the gauge when checking the bearing bore is the true measurement. That's just the easiest way until you have more practice - a simple comparison measurement for each journal. I typically set the dial bore gauge to the low spec and write down true numbers for each part, then subtract to show the clearance, that way I have all the true numbers + the clearance numbers written down, just makes it easier when you need to decide about polishing a journal a little more or using .001 bearings to set the clearance where you want it.

F6

I have done this, but I don't think this is the correct clearance!! Your trying to tell me that this tiny little bit of plastic (when crushed) is pushing the crank journal all the way down into it's seat, no way?? I will get a hold of some plastic gauge and compare the two readings. If this would be true then I must have been running the last 6k miles with about twice the allowed clearance!!
Bearing #1,2,3,4 have 0.0043" clearance, bearing #5 has 0.0063" My engine has about 540 chp and my oil pressure is absolutely perfect.

MachinistOne

The plastic doesn't push the crank down onto the bearing, gravity does that for you

There is no air gap under the crank so when you use plastigauge the numbers are only 1/2 of the true number - this only happens when the crank is bent which will give you false numbers.

How about posting up the exact method and numbers you are getting with the mic and bore gauge - what brand of gauges are you using?

F6

I'd like to see what gravity does when the engine hasn't been removed but just the oil pan????

MachinistOne

You're only going to get somewhat accurate numbers with the engine right side up and the crank set down on clean bearings.

F6

I'll be doing the comparison test tomorrow morning and will post the results.

F6

Okay here is the comparison data I got from measuring both ways. It didn't turn out as I thought. There is definitely a difference though, of about .001". Maybe this will give some people guidance of what to expect when checking with a dial bore gauge.

The measurements were taken with new std bearings.
dial bore gauge......plastic gauge
.0059 ....................0050
.0057 ....................0050
.0063 ....................0054
.0061 ....................0050
.0059 ....................0050

marshall93z

Well, it's good to know that we can run HUGE clearances without problems!

F6

I must add, I have been running .001 undersize main bearings all the time and will now even use .002 for the upper bearing shell. As soon as I get the bearings I will see what clearances I actually have. I also ordered new plastigauge, so no one can say I used old and dried out plastigauge.