Scary numbers

[Dobber]

I finally tore down the engine that came with the 71 240Z that I bought ( It was mounted to a nice stainless steel rolling stand. ) and started to pull everything apart. The guy said it was just rebuilt and had not been run yet. No way was this going in the car without a thorough tear down and tolerance check. This is an F54 block with std 79 flat tops. I pulled all the pistons and started to plasti gauge the center main bearing. I started with green plastic and it did not even touch the cap. ( these are brand new bearing shells ) I went to the red plastic and torqued the cap down to 40flbs. the reading was approx 3/4 of 0.152. approx 0.114. the next were the same, moving toward the front of the block. The front main came in at exactly 0.152.

What I need to know is, ( because these decimal points make me crazy ) where do these numbers fall into the main bearing running clearances of 0.020 to 0.072?

[240260280z]

0.020 to 0.072 is mm.

Is your 0.152 measurement in mm? If so then you have ~ twice the maximum clearance which means more oil will leak out than specified and the crank can move more than specified.

[Dobber]

That is what I was afraid of. Yes that is in mm. I wonder how he got so far off with the measurements? The crank journals look like new. Someone goofed big time. That would have been nice, install that in my rebuilt car. Black dragon lists 2 over sized bearings .010" .25mm and .020" .50mm but if I am twice over, then even the .020" won't be enough, right? I hope I am reading this right. All I am going by is the plastigauge at this point. I know I can lift up on the snout of the crank and feel it move up and down ( very slight but noticeable )

[240260280z]

I believe 0.25mm means the crank is ground so that 0.25mm is removed from the journal diameter and the corresponding bearing has to form a circle that is 0.25mm less in diameter. This would in turn remove 0.125mm from your (one side) oil gap.... so instead of 0.152mm, you would get ~0.027mm which is within spec.

So it looks like you have the wrong size bearings. You could have std when you need 0.25 or you could have 0.25 when you need 0.50mm.

Your next step is to "mic" the journal diameter to see how much was taken off. It should be:

STD= 54.942 to 54.955mm

0.25mm = 54.692 to 54.705mm

0.50mm = 54.442 to 54.455mm

Edited August 4, 2011 by Blue

[Dobber]

Thats the kind of info. I need. thanks for the help Blue. I will mic the crank tomorrow and let you know whats up.

[Dobber]

Looks like wrong size bearings for sure Blue. Here are the nums.

ALL of the crank bearing ids are at std. spec. 2.162"

ALL of the crank journals are at 2.152-3"

So according to my calcs. ( this is the scary part I was referring to ) that would leave a gap of 0.026"s or .013 that I need to fill. Thus if I go to an over sized bearing of .0040/.0100" or .010/.025mm I should be back in business. Am I in the ball park Blue?

[240260280z]

Your main journals are:

minimum 2.152" is 54.661mm

maximum 2.153" is 54.686mm

This is just slightly smaller than the specified 0.25mm journal size range of:

0.25mm = 54.692 to 54.705mm

So if you used 0.25mm bearings, because of the smaller journals, your oil gap would be bigger for all bearings by a maximum of 0.022mm and a min of 0.003mm

So if your 25mm bearings were manufactured in the middle of their spec of 0.020 to 0.072mm at 0.046mm then your crush results would be from 0.049 to 0.068mm which is fine. However if your 25mm bearings were on the bigger size of 0.072 then your gap would be from 0.075mm to 0.94mm which is out of spec.

You can try a new set of 0.25mm bearings and hope for some statistical luck with manufacturing variances that make them all less than 0.050mm or try another crank or go to the machine shop and have them grind to 0.50mm then buy new 0.50mm bearings.

Edited August 4, 2011 by Blue

[John Coffey]

Plastigauge is an approximate bearing clearance measuring tool. Your clearances might be just fine. Pull the crank and do a proper measurement.

[Dobber]

Thanks Blue. I don't know what they did to that crank. It looks like they polished that much off or something. Oh well, I have the original motor, so I guess I will have to tear it down and have a look at the crank. Trying to avoid machine shop costs if I can. I put allot of money into the head and other related ancillaries

[Dobber]

Well I lucked out. The crank in the L24 that came with the car miced out to perfect spec. Now I am toying with the idea of doing the short stroke setup. F54 block, L24 crank and rods, L28 std. flat tops, N42 head with 450/450 cam, SUs and a header. Late 5 speed with R200 3.7 gearing. I guess this setup would be more of an auto-crosser/canyon carver, than the longer stroked L28 setup, being more of a cruiser, higher speed loving setup. All I would be changing is the stroke. Does this all sound about right? Tak510, are you still out there?