bad crank bearing clearances

[survyor2]

I am rebuilding my 7mgte for 600hp. When at the machine shop I had the crank polished. This crank was probably polished at least once in the past. I installed new main cap bearings in standard size. When I measure the clearance with the plastigage I'm getting a consistent 0.07-0.08mm clearance. TSRM says 0.025-0.050mm is good.

I don't think its good enough for me. What options do I have here? Is it possible to buy 0.04mm oversize bearings to bring my clearance down to 0.03mm? Should I go with 0.25mm oversize bearings and regrind the crank? Whats the cost for that? Either way, I'm going to have the machine shop verify all of my plastigage readings.

Thanks,
Dennis

 

[jdub]

I wouldn't do anything to the crank journals...the surface hardness is not that thick.

0.07mm is the max clearance. Have the machinist resize the main caps and line hone to standard size. If you are using ARP main bolts, he will need to torque them to ARP spec as part of this work. One other benefit is that your main caps will be perfectly aligned

 

[survyor2]

I wouldn't do anything to the crank journals...the surface hardness is not that thick.

0.07mm is the max clearance. Have the machinist resize the main caps and line hone to standard size. If you are using ARP main bolts, he will need to torque them to ARP spec as part of this work. One other benefit is that your main caps will be perfectly aligned

If I did it that way what size bearings would I use? oversize?

 

[jdub]

Standard size
I assume you already have new std size bearings.

Think about it...a small amount of material is removed from the caps where they mate to the block. The problem is it's no longer a perfect circle. It's line honed to bring it back to that circle. If you provide the caps, bearings, and main bolts to the machinist, he can make the clearance exact...in your case, 0.03-0.04mm.

BTW - the rod ends can be resized the same way.

 

[survyor2]

Standard size
a small amount of material is removed from the caps where they mate to the block

I understand you now.

 

[survyor2]

Ok. I thought about this for the past few hours and its not making sense again. If my crank journals are slightly undersized because of the polishing and I use a standard line bore and standard bearings, I'm still going to end up with the same 0.07mm gap. I need something to make the gap 0.04mm smaller...like 0.04mm oversized bearings.

help me understand this.

Thanks

 

[starscream5000]

You are taking material off the main cap to "shrink the size of the bearing needed back to stock specs (this is a very crude definition).

 

[jdub]

Heehee...you're thinking too much. j/k

You are dealing with a circle (crank journal) inside another circle (block/main caps). Including the bearing, there is a 0.07mm difference in size (the gap). If you remove material from the main cap where it bolts on to the block, the outside circle gets smaller decreasing the gap between the two circles since the crank journal diameter and bearing thickness remains the same. Removing material from the main caps also makes the outside circle out-of-round...the line hone restores the outside back to a circle, but maintains the new smaller gap.

Got it?

 

[jdub]

Also keep in mind that the only reason I'm telling you to do it this way is due to the very small change in gap needed. You want to reduce the gap by 0.04mm...that is 0.0016". Not very much.

If the machinist determines that the line hone alone will leave too much of a side gap, you can still line hone to get a perfect circle, but use one or more of the Toyota "standard" bearings. They actually come in 5 sizes...here are the thickness for each in the TRSM:
http://www.cygnusx1.net/supra/Library/TSRM/MK3/manual.aspx?Section=EM&Page=73

You could use the thickness appropriate to give you the clearance needed....just choose the size bearing for each journal that give you a 0.03-0.04mm clearance. Just ignore the Toyota numbering system...I would write down which bearing was used on what journal for future reference.

 

[survyor2]

I think i got it now. Thanks for the help. I'm taking it to the machine shop today. I'll see what he says.

Dennis

 

[jdub]

You may still have to go with 0.25 undersize bearings, depending on the condition of your block/main caps. The machinist can tell you the bearing size needed. The good thing about using a line hone to get the clearance is it aligns the mains perfectly...on a high HP motor, you want that.

Let us know how it turns out.

 

[survyor2]

Follow up: I dropped the block and crank off at the machine shop. They found the problem. I had the main caps backwards. The shop got a 0.025mm clearance all the way across once they fixed them. That explains alot, since i was getting 0.025mm clearance the first time I measured then I started getting larger numbers when I double, tripled, 4x, 5x checked. I must have switched them after the first time. Opps. At least I check my work until its right. Live and learn.

Dennis

 

[starscream5000]

Problem solved .

 

[jdub]

LOL...good for you man. Don't you love it when it turns out to be a simple thing

And brovo for double checking your work. Everybody makes mistakes...you were smart enough to catch it!