I read a lot of people talking about loose clearances when building a race engine. Was hoping people on here had experience and insight into exactly what that means...
Does this mean that you do a little more aggressive micro polish on the crank so that the bearing tolerances are closer to the max OE spec? Or are you going beyond those specs?
Thanks
I used to build my race motors at the high end of the spec but with modern equipment the materials and machining are so good that, except for pistons, I'll usually be happy anywhere in the factory spec.
The cheap way to get race rotor bearings for Mazdas was to use Scotch-Brite to remove all of the bearing overlay, down to the copper.
You need more clearance if your engine components are floppy noodles (like certain old V8s) or you have balance issues with respect to where the weights and counterweights are (Mazda rotaries, some partially-counterweighted crank engines). I've heard of .005" main bearing clearance on some older V8s!
The rotary issue is that at high RPM, the eccentric shaft tries to turn into an S shape. So you need to open up the clearances so nothing important hits anything else important, and then straight 40 or 50 oil so it stays in the bearings long enough to work and run 100+psi pressure to cram it through the system.
A modern engine with a stiff block and well-engineered crankshaft can get by with remarkably low clearances, viscosity, and pressure.
Some race motors will not turn when cold!!!
bentwrench said:Some race motors will not turn when cold!!!
Oh yeah, I remember something about having to preheat a Cosworth DFV by draining the coolant, boiling it on a stove, and reinstalling, before you try to actually start it.
Speaking of Cosworth, I also remember a Famous Engine Builder describing a rig he made up to measure main cap lateral deflection on a YBB engine (Sierra Cosworth/RS500) using studs extended through the oil pan, lasers, and mirrors. He wouldn't divulge how much the main caps were walking around other than it was many times the radial bearing clearances.
Iron and steel are remarkably soft, springy, ropey materials...
We grenaded two Yamaha 1000 engines because the motor ran to cool; the motor would progressively loose oil pressure along the crank between #1 & #4 to the point that the #4 rod would seize to the journal. The clearances on a cold motor were very loose.
I know of some guys using narrower bearings to try and get the last bit out of a motor; note this requires custom rods and machining the crank.
On the type of engines I'm building, reliability notwithstanding, the gains from playing with bearings simply aren't enough to justify the extra effort.
Generally speaking you're shooting for a .001" of clearance per 1" of journal size and then add another .0005". So 3" journal size would have .0035" clearance. This is not always the case and if you are using very good parts you can go tighter.
For setting clearance you can mix and match over and under sized bearings until you find the right combination. Most common is to cut down the bearing caps and then hone them round again. For very high level stuff you can actually machine each journal to the desired size to get your clearance right.
In reply to amg_rx7 (Forum Supporter) :
I went a bit over factory spec on my big turbo build on the rings. Bearings were between .0022-.0025 for my SR20 on rod and mains. I am however running a larger oil pump to help with oil pressure as well.
Tom1200 said:
I know of some guys using narrower bearings to try and get the last bit out of a motor; note this requires custom rods and machining the crank.
I don't see why it should, unless the journal was also being reduced in size (like "Honda rods" in Chevy engines - really, the only Honda part is the journal diameter)
You can reduce rod bearings' width no problem, people do this all the time when they have cranks reground with deep fillets and need to narrow the bearings. There's no reason the bearing has to be the full width of the rod, either. IIRC the Toyota GR engine I was elbow-deep into a while back had rod bearings only 1/2" wide, significantly narrower than the actual rods. No bearing tangs, either 
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