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Author Topic: Spun a rod bearing, what did i do wrong?  (Read 3182 times)
scienceguy
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Location: West Virginia

« Reply #15 on: July 22, 2017, 04:12:43 pm »

Bearings usually spin because of clearances that are too TIGHT...  not too loose.  Too loose is better than too tight ANY DAY.  With the big bearings of the 351W, you'll want 0.0030 to 0.0035" on the mains, and 0.0025" on the rods.

The QUALITY of the crank and rod machining also makes ALL the difference.  There is no such thing as 'bargain shopping' when it comes to machine work.  There are more shops out there that put out BAD work than good.  When it comes to machining a crank.  Let's say there is 0.0020" clearance.  That is only 0.001" on each side.  Yes, ONE thousandth of an inch.  It takes a TALENTED crank grinder to put out GOOD precision on a daily basis.  Most of those guys have died out or retired. 

The tighter the clearances, THE LESS ROOM THERE IS FOR ERROR.  Let's say there is 0.0020" clearance, which is 0.0010" on each side, and the crank grinder is 0.0010" off...  or even a half thou...  0.0005" off...  You're in trouble!  With wider clearances, there's a lot more wiggle room to play with. 

Buy some mics that will measure crank journal diameters, and learn how to use them.  It takes PRACTICE to measure these values consistently and accurately.  Also learn to measure rod big ends.  I always measure all that stuff on the counter before paying for it.  It annoyed them at first, but then they knew it had better be right before I went to come pick it up. 

Good Luck!
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« Reply #16 on: July 23, 2017, 09:03:02 am »

Bearings usually spin because of clearances that are too TIGHT...  not too loose.  Too loose is better than too tight ANY DAY.  With the big bearings of the 351W, you'll want 0.0030 to 0.0035" on the mains, and 0.0025" on the rods.

The QUALITY of the crank and rod machining also makes ALL the difference.  There is no such thing as 'bargain shopping' when it comes to machine work.  There are more shops out there that put out BAD work than good.  When it comes to machining a crank.  Let's say there is 0.0020" clearance.  That is only 0.001" on each side.  Yes, ONE thousandth of an inch.  It takes a TALENTED crank grinder to put out GOOD precision on a daily basis.  Most of those guys have died out or retired. 

The tighter the clearances, THE LESS ROOM THERE IS FOR ERROR.  Let's say there is 0.0020" clearance, which is 0.0010" on each side, and the crank grinder is 0.0010" off...  or even a half thou...  0.0005" off...  You're in trouble!  With wider clearances, there's a lot more wiggle room to play with. 

Buy some mics that will measure crank journal diameters, and learn how to use them.  It takes PRACTICE to measure these values consistently and accurately.  Also learn to measure rod big ends.  I always measure all that stuff on the counter before paying for it.  It annoyed them at first, but then they knew it had better be right before I went to come pick it up. 

Good Luck!

I definitely agree with everything you've said!
I use mics and t gauges on a daily basis at work so I'm confident in my tools and that the bearing clearances were ok. I used a mic and t gauge on everything then double checked with a bore gauge. The mains ranged from .003-.0033, the rods I posted up already but they ranged from .002 being the tightest which was number 8 rod and it did not spin even though it was the tightest and .0023 being the loosest for rod oil clearance.

I checked the crank and as I posted all the main and rod journals were good. My machinist is very good ive actually never had a problem with him and like you I always check it while I'm still there and I haven't had a issue yet.

What I was saying what I think was to large as far as size goes was the rod big end, I did not check this as they were new and I assumed they were good. The only logical explanation I have as to the bearing spinning is the big end diameter was over size not providing sufficient bearing crush. The oil ports were all still open and clear as well I checked that yesterday. That's all just a theory though as I cant really confirm that.

Thanks for your guys input!

Unfortunately this block is junk now though, number 6 and 7 piston must have picked up some trash from the oil and it put some very heavy gouges in the walls. I'm not sure it would even clean up if I took it and had it .060 over.

So now my plans are to contact woody and probably get a 408 short block or a rotating assy and put my AFR 205's on that with another custom ground cam for the setup.
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