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 Tempest / Lemans / GTOs
 if u have too much main & rod clearance
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beertracker
Cochise

452 Posts

Posted - 20 Jun 2019 :  8:26:13 PM  Show Profile  Reply with Quote
I know how much main and rod bearing clearance is required for Pontiacs. My question is how much clearance does it take to hear engine rattle? My clearance is fine this is a what if question.

bt

Blued and Painted
Chief PONTIAC

USA
3520 Posts

Posted - 21 Jun 2019 :  12:14:49 PM  Show Profile  Reply with Quote
Factory speck can be a minimum of .0005 on the mains . Super tight clearance will increase the oil temp. You would notice super black oil after only a few hundred miles.
More clearance like .0025 to .003 on the mains is preferred for performance apps. This will provide greater oil flow and cooling between bearing and crank.
So, as long as the oil pump can keep up with the extra clearance, durability should not be a problem.
Basic rule of thumb minimum is 10 psi per 1K of rpm's

461/TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 21 Jun 2019 12:16:49 PM
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beertracker
Cochise

452 Posts

Posted - 21 Jun 2019 :  12:47:49 PM  Show Profile  Reply with Quote
doesn't answer my question in post 1
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Corncob2061
Crazy Horse

USA
589 Posts

Posted - 22 Jun 2019 :  11:35:41 AM  Show Profile  Reply with Quote
Just a guess here, we have built some Pontiac engines that had a ton of clearence for endurance type circle track racing. I think one had .0030-.0035 on the rods and .0035-.004 on the mains. Honestly I do not recall having the engine rattle from to much clearance until a bearing fails. Main bearings can have a ton of clearance and make no noise. You just loose oil pressure, mainly at low rpm, but if loose enough at high rpm will have oil pressure loss.

Wrist pins on the other hand rattle very quickly when loose.

I am probably not answering the question either. As crazy as it sounds my guess would be someplace north of .004 to .005 on a rod, and I have no idea what it would take to make a main rattle. I have not seen many main bearing failures accross brands, the main failures I have seen were either from to tight of clearances, loss of lubrication or from cap walk and lastly broken cranks. The 500557 pontiacs blocks seem to be more prone to main problems, the others Pontiacs are pretty darn tough there. Jay

Edited by - Corncob2061 on 22 Jun 2019 11:58:14 AM
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Corncob2061
Crazy Horse

USA
589 Posts

Posted - 22 Jun 2019 :  12:51:20 PM  Show Profile  Reply with Quote
This is certainly not a recommendation, but If I recall correctly, the one of guys that run the Boss Bird, the nitro Pontiac power ra5 funny car run over .005, may have been closer to .007 and well past 100 psi oil pressure. Maybe 250-300? That is as big of numbers as I have seen in a Pontiac. I do not recall what they said they ran for main clearance. I do remember they run 3 inch mains. Just throwing that out there, thought that was interesting. Some Pontiac pioneers there. It appears if you can force enough oil to the bearings you can run a lot of clearance.

Edited by - Corncob2061 on 22 Jun 2019 1:09:27 PM
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beertracker
Cochise

452 Posts

Posted - 24 Jun 2019 :  4:03:38 PM  Show Profile  Reply with Quote
Thanks. That's the best explanation I have heard and it makes sense.
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Blued and Painted
Chief PONTIAC

USA
3520 Posts

Posted - 25 Jun 2019 :  3:14:06 PM  Show Profile  Reply with Quote
When does the rattle occure? Hot, cold, rpm, load.

461/TH400/ 3.08 8.5 / R44TS.
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beertracker
Cochise

452 Posts

Posted - 25 Jun 2019 :  9:43:31 PM  Show Profile  Reply with Quote
It doesn't on my engine. As I stated in post one this is a what if question. My clearances are good.
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