rod bearing clearance

[h2ojet001]

After measuring the rod bearing clearances I found all to be .002 and one is .0015. Is this too small and should I use an undersize bearing to bring it to the correct reading?
Barry

[shockwaveharry]

Too tight for a BBC. The last motor I built was the first one that the clearences were too tight. .0015 to .002, Eagle crank, Clevite H bearings. I measured and recorded all the rod and main journals and crank thrust. Took the crank back to the machine shop and told him where I wanted my clearences set. For final assy. I was at .003 on the rods and mains, perfect.

[396_WAYS_TO_SPIT]

Thats a little on the tight side. You want .0025-.0035 on the mains and .002-.003 on the rods. have them polish the crank down. Also, was that your first reading? Arp states to torque 4 times. What bolts are in the mains and rods?What did you torque the mains and rods to? If they are arp, did you use molly lube or oil? That plays a nice role in your clearances as well.
396

[396_WAYS_TO_SPIT]

Originally posted by h2ojet001
After measuring the rod bearing clearances I found all to be .002 and one is .0015. Is this too small and should I use an undersize bearing to bring it to the correct reading?
Barry
I misundersood your post so I am going to reply again. The rods should be .002-.003 but like I said you need to retorque several times before you get the right readings..
396

[h2ojet001]

I think I have several issues to contend with here. First I am in the learning stages of using micrometers. Last night I remeasured the crank journals and found them to be .0006 smaller than my original reading. this should give me the necessary .002-.003 clearance for the rods. Second issue is I am using the old rod bolts to check these clearances(I have new arp bolts). I was worried I might stretch the new ones too much. So maybe I should use the new ones with the moly lube on the bolts. I am torqueing them to 73 ftlb. Is this correct? Is it necessary to use a stretch gauge for the rod bolt or is the torque wrench sufficient. thanks for your input
Barry

[396_WAYS_TO_SPIT]

Originally posted by h2ojet001
I think I have several issues to contend with here. First I am in the learning stages of using micrometers. Last night I remeasured the crank journals and found them to be .0006 smaller than my original reading. this should give me the necessary .002-.003 clearance for the rods. Second issue is I am using the old rod bolts to check these clearances(I have new arp bolts). I was worried I might stretch the new ones too much. So maybe I should use the new ones with the moly lube on the bolts. I am torqueing them to 73 ftlb. Is this correct? Is it necessary to use a stretch gauge for the rod bolt or is the torque wrench sufficient. thanks for your input
Barry
First of all, I am glad to hear your learning. We all started here.
Have you resized the rods? Second of all, DO NOT TORQUE YOUR ROD BOLTS TO 73 FT LBS!!!!!! You will kill the motor! What motor are we working on? That will help alot. Usually rod bolt torque specs are around 40-50 ft lbs.
396

[Snowboat]

Just thought I'd throw this out. How does over torquing a rod bolt change the clearances? Doesn't it just treat the fastener unfairly? When you clamp two flat surfaces together, they only get so tight, after that, the bolt suffers.

[h2ojet001]

396- First off I am building a BBC 454 bored .030. the rods are gm hi-performance rods with the dimple on the rod and 7/16 rod bolts. I just called arp, because that is the rod bolts I am using and they told me the torque spec is 75ftlb or .0080 stretch. Please tell me if this is wrong. Also does anyone have an opinion on using a stretch gauge. Thanks
Barry

[396_WAYS_TO_SPIT]

Now were talkin Ok since we know what you have then yes your torque spec is right. 3/8 bolts are 50ft lbs. Since you have much larger bolts then yes they are much higher. Did you use molly lube when you torqued the bolts? Did you mic' all the journals of the crank? If so was the 1 much larger than the others? if so then you just need someone to polish up that journal for you. if you have .002 then its fine. I personally like mine to be .0025 on my rods. Yes, I do use plastiguage. Its not perfect but it works just fine for what you are doing. If you were to go to the track with serious horsepower then you need to be using guages. But like I said before, if you have .002, run that baby and have some fun
Oh and by the way, I used plastiguage on my olds and that baby ran all summer long. I can hold the throttle down for a while and she has no problems.Matter of fact, my father build his flattie in 1980 and he used plastiguage on his 396 bbc and it ran 6500rpm's all day long. he just sold it 4 years ago and my uncle is still running the same motor that was built with plastiguage!!! if you look in a haynes repair manuel they will show plastiguage in the rebuil section. If plastiguage was bad then people wouldnt use it(Right). Thats just my opinion though so you can take it however you want i am sure some people will have another opinion and thats ok too
396

[h2ojet001]

Thanks 396 for the response. I used plastiguage about 15 years ago. But I bought a set of mics and a dial bore guage on ebay for a very reasonalbe price so I want to learn to use them in case I decide to build another engine( which I am already thinking about)
Barry