Setting Crank Thrust Clearance?
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Setting Crank Thrust Clearance?
Hey guys, just looking for a little confirmation here on setting the crank thrust bearing clearance on a SBO 350. I had been lead to believe one should center the journal, and that a good set of mains would prevent lateral movement. However, the shop manual says to wedge the shaft towards the front of the engine while torquing the caps (after bumping forward/rearward to align the #3 bearings).
Can someone explain the reasoning behind this? I did such, and obviously can't get a feeler gauge in the tight side; loose side checks out A-OK (0.0035"-0.0135"). Fresh bearings, fresh grind on the crank. Thanks.
Can someone explain the reasoning behind this? I did such, and obviously can't get a feeler gauge in the tight side; loose side checks out A-OK (0.0035"-0.0135"). Fresh bearings, fresh grind on the crank. Thanks.
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Joined: Aug 2011
Posts: 4,120
The specification/clearance is for the total combined clearance of both sides. When the crankshaft is wedged all the way in one direction the clearance on one side is zero, the other side is the combined/total clearance. You should be able to wedge the crank in the opposite direction and get the same clearance.
IMO, it was a good question.
Good luck!!!
IMO, it was a good question.
Good luck!!!
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Joined: Dec 2016
Posts: 1,986
From: Beautiful Southern Maryland
Eldo,
The reason is that the thrust force on the crank is forward so you want to make sure that the crank is hitting the back side of both halfs of the thrust bearing evenly. You dont care about the front surface.
For instance if the main cap was pushed all the way to the rear, the crank would probably only be hitting the half of the bearing in the cap and not hitting the part in the block. Kind of hard to describe.
The reason is that the thrust force on the crank is forward so you want to make sure that the crank is hitting the back side of both halfs of the thrust bearing evenly. You dont care about the front surface.
For instance if the main cap was pushed all the way to the rear, the crank would probably only be hitting the half of the bearing in the cap and not hitting the part in the block. Kind of hard to describe.
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Joined: Sep 2011
Posts: 2,148
From: Plano, TX
The shoulder on the block for the main caps will locate the cap from side to side. It doesn't locate the cap from front to back. The holes in the caps are larger than the bolts, so there's quite a bit of room for the caps to shimmy forward or backward while being installed. For the #1/#2/#4 this doesn't matter - lineup isn't critical. #5 has a dowel.
So the smacking-while-just-seated routine moves the #3 cap so it's aligned vertically with the block. Now, assuming that the block or the cap isn't causing a bearing shell to spread, and assuming the shells are in good shape, you should have two nice flat and straight surfaces presented by the thrust surface.
I only use oil at this stage so the crank can move freely. Also usually easiest to check thrust with, say, #1 and #4 also installed just to keep the crank in line.
Torque down #3, and you should be able to move the crank forward and backward with a nice little "thunk".
For precise measurements I use a dial gauge on the nose of the crank. I *also* use a feeler gauge to make sure the clearance is uniform around the thrust surface. Push the crank forward and you should have, say, 8 thou clearance on the back side. Push the crank back and you should have the same 8 thou on the front. And the clearance at the top of the cap should be about the same as at the parting line and the same down as far as you can reach into the block.
That method helped me find one where the cap was fubar'd, causing the bearing to spread. The top edge of the thrust surface got pushed out, leading to something like 2 thou clearance, but there was over 8 thou clearance around the rest of the thrust surface.
If the clearance is too tight, then use a flat surface (e.g. glass pane) to sand the thrust side of the shell with a fine grit, like 320 or 400. I try to only sand the "front" side - the back side is what does the hard work when the trans tries to push the engine's guts out the front. Do figure 8 swirls, same count on each shell. Recently what I've done is one shell at a time - installing each shell in the block, not even bothering with the cap - and working it until each shell gives the same clearance. Then fully assemble and re-check. A little slower but easier to make sure the width of each shell is the same. And, again, this assumes the block and cap aren't messed up and causing problems. I also had one where the thrust surface on the bearing was taller than the relief machined into the cap. But just barely, so didn't notice while installing the shell.
Just slapping down #3 like the other caps can easily lead to it being offset enough that you have no thrust clearance. The engine will run just fine, and for an automatic it might even be perfectly happy for its entire life.
So the smacking-while-just-seated routine moves the #3 cap so it's aligned vertically with the block. Now, assuming that the block or the cap isn't causing a bearing shell to spread, and assuming the shells are in good shape, you should have two nice flat and straight surfaces presented by the thrust surface.
I only use oil at this stage so the crank can move freely. Also usually easiest to check thrust with, say, #1 and #4 also installed just to keep the crank in line.
Torque down #3, and you should be able to move the crank forward and backward with a nice little "thunk".
For precise measurements I use a dial gauge on the nose of the crank. I *also* use a feeler gauge to make sure the clearance is uniform around the thrust surface. Push the crank forward and you should have, say, 8 thou clearance on the back side. Push the crank back and you should have the same 8 thou on the front. And the clearance at the top of the cap should be about the same as at the parting line and the same down as far as you can reach into the block.
That method helped me find one where the cap was fubar'd, causing the bearing to spread. The top edge of the thrust surface got pushed out, leading to something like 2 thou clearance, but there was over 8 thou clearance around the rest of the thrust surface.
If the clearance is too tight, then use a flat surface (e.g. glass pane) to sand the thrust side of the shell with a fine grit, like 320 or 400. I try to only sand the "front" side - the back side is what does the hard work when the trans tries to push the engine's guts out the front. Do figure 8 swirls, same count on each shell. Recently what I've done is one shell at a time - installing each shell in the block, not even bothering with the cap - and working it until each shell gives the same clearance. Then fully assemble and re-check. A little slower but easier to make sure the width of each shell is the same. And, again, this assumes the block and cap aren't messed up and causing problems. I also had one where the thrust surface on the bearing was taller than the relief machined into the cap. But just barely, so didn't notice while installing the shell.
Just slapping down #3 like the other caps can easily lead to it being offset enough that you have no thrust clearance. The engine will run just fine, and for an automatic it might even be perfectly happy for its entire life.
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Joined: Nov 2008
Posts: 8,495
From: Central Fl
To add to that, check it without the thrust cap installed. See what you have there first. Then you can adjust the amount from there. But it’ll never exceed the amount you have without the cap, unless you alter that half as well.
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