Rod bearings too tight... what to do ?

[vpittman]

I'm staring to put the bottom end together on my 360. The crank was ground .020, new eagle 6123cp rods.
I've measured with a mic/dial bore gauge and plastiguage and come up with .001 - .0015 clearance. I'd really like to get .0025

What are my options ?

Thanks !

[jimynick]

The first thing I'd do would be to borrow another set of measuring tools and do it all over again, or better yet, take it to a reputable machine shop and have THEM do it. THEN, if it still showed tight, I'd make sure the crank was right on the money, which would put the onus on the rods or bearings. You're only looking for one thou, do ALL the bearings measure the same, in ALL the rods? If it all measures out and is confirmed, I'd approach the crank grinder and tell him your problem. I'd think he could polish the throws down a thou pretty quickly if he wanted to. Just my 

[72cudamaan]

I'm staring to put the bottom end together on my 360. The crank was ground .020, new eagle 6123cp rods.
I've measured with a mic/dial bore gauge and plastiguage and come up with .001 - .0015 clearance. I'd really like to get .0025

What are my options ?

Thanks !

First, are you sure the bearings aren't hitting on the radius of the journal?  If not, then having the crank polished
is about the only option.

[nqkjw]

You can polish the bearings with a scotchbrite pad(or whatever you call them over there) but a whole thou is getting to be a lot to polish off the bearings.

[highberg]

Typical approach is know what you want in thous. Take the rods and bearings into a machine shop and they will resize the rod big end to provide the clearance you want. Are you doing some high rpm because you are within clearance range for engines up to 6000.

[72cudamaan]

Typical approach is know what you want in thous. Take the rods and bearings into a machine shop and they will resize the rod big end to provide the clearance you want. Are you doing some high rpm because you are within clearance range for engines up to 6000.

Sorry, but NO, you do NOT resize the big end of the rod to change your clearances. The bore diameter of the big
end is to make sure you have the right crush on the bearing to keep it from spinning.

[highberg]

Right but the crush is controlled by the big end of the rod. 1 thou is not going to spin a bearing. If they are that loose, then it wouldn't make it anyhow. If the crank is machined correctly, then this is how it is done. Just like main bearings, you align bore for clearance. Any race shop will tell you how they do it.

Sorry, but NO, you do NOT resize the big end of the rod to change your clearances. The bore diameter of the big
end is to make sure you have the right crush on the bearing to keep it from spinning.

[HP2]

Do you know that the factory FSM recommends these clearances to be .0015 to .0025 for street engines? The Engine manual recommends .0025 to .0035 for race applications. Yes, you are on the tight end of things, but odds are your engine will live fine with .0015 rod clearance provided your oil system is all up to snuff with a good pump, pickup, baffled pan, and a high quality oil.

However, if you really want to extra thou of clearance, the crank will need to come out for some polishing. A thou is not a lot, but if you tried doing this type of polishing by hand, it is a huge amount. Have the shop do it so they can ensure no taper is introduced in the process. I would think .0025 clearance with taper would be a worse situation to have than .0015 with a nice flat journal.

[vpittman]

Thanks everyone... That's kinda where I thought I was at. I'll give the shop call today and see when they can check it.

Thanks again for all the help.

[72cudamaan]

Right but the crush is controlled by the big end of the rod. 1 thou is not going to spin a bearing. If they are that loose, then it wouldn't make it anyhow. If the crank is machined correctly, then this is how it is done. Just like main bearings, you align bore for clearance. Any race shop will tell you how they do it.

The crush is controlled by the size of the big end, but the clearance is a different measurement. Your clearance is always the measurement of bearing bore diameter minus crankpin diameter. The bearing bore will change minimally if at all because of rod bore diameter. And NO you do not align bore (or hone for that matter) to give you clearance. You align bore or hone to make the bores the right diameter for bearing crush and straightness. Your clearance is taken off the I.D. of the bearing. I've been a machinist for a lot longer than you might imagine. I'm pretty sure I know what I'm talking about.

[highberg]

Licensed mechanic doing race engines for 40 years. Eagle rods are generally bad needing both machining on the pin and the big end. Just finished a stroker with the rods so tight at the pin that you couldn't slide it in and the big ends out by .003 from largest to smallest. If the crank is right, check the rods. It is likely a rod problem. If you want more clearances, you will need to machine the rods to the large side of the specs to not crush the bearing too much. I do know what I'm talking about.

The crush is controlled by the size of the big end, but the clearance is a different measurement. Your clearance is always the measurement of bearing bore diameter minus crankpin diameter. The bearing bore will change minimally if at all because of rod bore diameter. And NO you do not align bore (or hone for that matter) to give you clearance. You align bore or hone to make the bores the right diameter for bearing crush and straightness. Your clearance is taken off the I.D. of the bearing. I've been a machinist for a lot longer than you might imagine. I'm pretty sure I know what I'm talking about.

[moper]

Licensed mechanic doing race engines for 40 years. Eagle rods are generally bad needing both machining on the pin and the big end. Just finished a stroker with the rods so tight at the pin that you couldn't slide it in and the big ends out by .003 from largest to smallest. If the crank is right, check the rods. It is likely a rod problem. If you want more clearances, you will need to machine the rods to the large side of the specs to not crush the bearing too much. I do know what I'm talking about.

Previously certified ASE Master Tech - building engines for almost 30yrs. All aftermarket rods should be cycled first, then deminsions checked. I agree with the above - it is common for the rods to need the big end resized and the pin ends properly honed to size. Kudos to you for measuring and checking. If it ends up at .0015" without any taper I wouldnt worry too much about it.

[HP2]

FWIW, my last 360 had .0015 for rod clearance. I built it in 1994 and ran it up until 2010. Launched it at 4500, shifted at 6500. Had well over 300 passes on it. When I was checking it out two years ago, I noticed I had to rods that were worn to the copper which necessitated a regrind on everything. Opinions on that damage have ranged from diluted oil, oil starvation, out of round rods, detonation, ignition miss-fire, to not putting the car in neuteral at the finish line. Compared to the oval track chevys I used to run that required new bearings every 45 days, I really can't complain.