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Discussion Starter #1
my number 4 rod has about a 1.5 to 2mm slide clearance on it (the spec in the BBG is like .5mm I think), ive driven about 2000 miles on the engine and its not giving me any trouble but at an idle once the engine is at regular temp, my oil pressure drops to below ten psi (aftermarket gauge). It runs at about 26psi regularly. The gauge is tapped at the head (front left corner I do believe). Would it be appropriate to run more than stock boost on it, or is that too serious a risk? Also my lifters make a lot of noise, not sure if thats uncommon
thanks
daniel
 

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maybe the rod bearings are the wrong size.....i had to get another crank cause the only size bearings that would fit my stocker were .030 over. i now have .010 over clevites. anywho, maybe the person who put the motor together put the wrong size bearing like a .010 over bearing instead of a .020 over bearing.....

i'm no expert so take my post as a "what if/maybe" but not expert advice or even a correct answer to that matter! good luck...
 
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Discussion Starter #3
my other rods fit fine, when I took my crank to the machine shop they had to weld onto the rod journal and then mill it off to get the correct diameter on it, I assume that they milled the journal too widely, giving the rod more room to slide back and forth
 

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dkent said:
my other rods fit fine, when I took my crank to the machine shop they had to weld onto the rod journal and then mill it off to get the correct diameter on it, I assume that they milled the journal too widely, giving the rod more room to slide back and forth

yuck.

Well Im not an expert but I know a bit... the purpose of the rod side clearance is to allow oil to freely escape without creating backpressure... while not being so large that the rod is able to move around too much.

What you may end up with is excessive wear on the rod bearing and small end bushing / piston pin... as the rod is allowed to move side to side and may end up sitting on slight angles, causing uneven loading, fast wear or even failure.

STD limit is 0.35mm by the way. The wall clearance should not be causing your low oil pressure... I would be considering poor crank machining or something ... giving you excessive bearing oil clearances.

The bgb specifies 5 - 20psi idle oil pressure.. but in reality, you should not be dropping under 10psi on normal oils unless the motor is poorly machined/blueprinted... or the bearings heavily worn (or the oil pump faulty)

In the end.. its probably not boost that will kill your motor, but oil starvation from lack of pressure. Keeping RPMs lower will help longevity.

-Tristan

ps. everything above is theory and suggestion only.. a guess based on your description
 

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2mm rod side clearance is excessive, my concern would be the motor's liability in the long run. Low oil pressure is also not a good sign. Was the oil pump replaced when the motor was rebuilt?
 
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I did replace the oil pump during the rebuild, currently im running 20w50 oil in it as well in attempts to keep higher pressure, if not caused by excessive slide clearance, what are other specific causes for low oil pressure? (the rod bearings, main bearings, thrust washers were all replaced) The motor was rebuilt due to a spun rod bearing (on the number 4 rod). And the added boost theoretically would add any danger to the motor?
thanks for the help
 

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dkent said:
I did replace the oil pump during the rebuild, currently im running 20w50 oil in it as well in attempts to keep higher pressure, if not caused by excessive slide clearance, what are other specific causes for low oil pressure? (the rod bearings, main bearings, thrust washers were all replaced) The motor was rebuilt due to a spun rod bearing (on the number 4 rod). And the added boost theoretically would add any danger to the motor?
thanks for the help
If the machine shop was that careless on the rod thrust clearance... they could have been equally careless when grinding the crank journals and you may be too far over spec causing low pressure via the main and rod bearings.. even if they are new.

I had my motor machined on the loose side of things (0.002" oil clearance) for lots of oil flow through the bearings... and I still maintain 25psi pressure at 1000rpm idle and 5w50 oil.

Sure, its a slightly higher flowing gen III pump but thats not enough to create that much difference.

Personally I would be pulling your motor down and having it checked & redone.. and having a good shout at the machinist if its wrong

-Tristan
 
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Discussion Starter #8
that is true, my thought though is that since the engine was blown due to a spun rod bearing in the first place, could there be something else internally causing a drop in oil pressure? What is there that i can do in attempt at longevity and reliability, and is there anyway to pull the crank without pulling the whole engine from the car?
 

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dkent said:
that is true, my thought though is that since the engine was blown due to a spun rod bearing in the first place, could there be something else internally causing a drop in oil pressure? What is there that i can do in attempt at longevity and reliability, and is there anyway to pull the crank without pulling the whole engine from the car?

longevity = drive very gently!

you can pull the crank without removing the motor but its a lot lot lot easier to just drop the motor

you need to a) remove gearbox.. b) partially remove timing belt & oil pump.. remove exhaust, sump.. then unbolt the rods.. then unbolt main caps and drop the crank..... and do it all whilst laying on your back hehe

-Tristan
 
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