Bearing clearances on street/strip occassional nitrous 555

[1nicenova]

What is the general rule of thumb for bearing clearances for a big block with occassional nitrous use? I'm used to clearances in the .0025 - .0029 range. I just recieved my new Musi 555 short block & they set it up for .0037 on both rod & main. Told me thats how they set up all there motors, so that when you hit the nitrous it doesn't grab the bearing and spin it. I would think this would cause excess oil flow past the bearing increasing windage and lower oil pressure. I guess this is how they set up all the edelbrock 555's too.

[John_Heard]

Steel Rods or Aluminum?

[1nicenova]

Steel Rods or Aluminum?

It has steel 6.385 scat rods. 4340 scat crank & mahle coated pistons. Dart block. I'm waiting for a call back from Pat himself about it. Talked to his builder yesterday, but I just want to hear it from Pat that it's ok. Doesn't seem right to me.

[John_Heard]

That seems just a bit loose for a steel rod engine and limited nitrous, however the use of bigger clearances on mains and rods is very common now with most engine builders. You'll find that as hp goes up so do the clearances as well. Might be well advised to follow what Musi wants on the clearances, I'd be he will say that's where he wants it at.

[1nicenova]

Pat called me back this afternoon and did indeed say that .0037 is fine. They target all they're engines at .0035 but go with anything from .0032 to .0039. I would be much happier with .0032. Said anything tighter would be apt to grab the bearing. I think I'll pop a few caps & just check for myself to make sure they aren't any looser. Seems like you ask 10 different people and get 10 different answers. But it does seem like the majority are going looser with larger hp and journal diameters. For the heck of it I'm going to check with Clevite & Scat see what they say. The heads should be here tomorrow. Went with fully cnc'd afr 335's. Bullet is grinding up the cam. spec'd by Straub Technologies. Hopefully it'll be close to 800.

[aitch o]

I have run and assembled several steel rod 400 SBC's.
The bearing clearances that seem to be happiest are .003-.004 on the rods and mains.
And these are healthy 23* small blocks over 600 horse on the motor and 1000 on the spray.

Be sure to run a good oil pump, new race melling HV or better.

As a matter of fact, my last couple of assemblies were .0025 and I was not happy because they were so tight. So I put in the X bearings which is supposed to provide another .001 clearance. Actually it is more like .0007 more. So bearing clearances are now .0032-.0033 and that is on the tight side of where I like them. Of course running good synthetic race oil 20w50.

[1nicenova]

Sounds good. Thanks for the reply. Musi mentioned having to mix and match with the x bearings to get the clearances where they like em. I'm going to use the moroso 20403 pan with scraper & screen built in. Also got the race pump & pickup to go with it. Hopefully the heads, morel lifters and cam get here by tomorrow.

[jmarkaudio]

Read the following, Darin was with Reher Morrison for about 10 years in charge of cylinder heads.
http://speedtalk.com/forum/viewtopic.php?t=2517