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Need help/advice with bearing taper clearance, eccentricity and arp studs.

Desean Strickland
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Desean Strickland Contributor

Desean Strickland

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Hey lads 

so im finally putting together my rb30 forged bottom end and ran into an issue. I measured my main bearing clearance with arp main studs torqued to 60 ft-lbs using ACL H series STD size bearings and standard, un-ground crank shaft journals and got an oil clearance reading of about 1.3 thou measuring straight up and down and about 2.8 thou measuring at a 45 degree angle (just above and below the parting line). My machine shop said they measured the main tunnel and it was all within spec (they didnt say the actual measurement) and to go with a standard size bearing, which i have done and the clearance is too tight, I'm guessing because of the extra clamping force from the arp studs distorting the main tunnel. I was wanting to run about 2.5 thou main bearing clearance. 

My questions are:

1. could i just use the HX extra 1 thou clearance ACL bearings? that would fix my straight up and down clearance making it about 2.3 thou, but then would the side to side clearance be too big at around 3.8 thou?

2. what actually is the recommended main bearing clearance for measuring near the parting line / side to side. i know its supposed to be bigger as the bearing has some eccentricity built into it but how much more clearance should there be compared to the straight up and down measurement? at the moment there is about 1.5thou difference, is that acceptable or should it be less?

3. If i took the engine block + girdle back to the machine shop and got them to line bore the main tunnel (like i told them to do the first time, but they said it didnt need it) what bearing size would i buy? the STD size bearing shells already slide in fairly easily with no real resistance, some even falling out if i tip the girdle up-side-down. If im taking material out of the main tunnel would i need a bearing with extra material on the back side to make up for it?

this is probably confusing af to read so if something doesn't make sense let me know and ill try explaining in a different way. My machine shop doesn't come back from christmas break until mid January, hence why i'm asking these questions here.

TIA for any help or info 

Duncan Grand Master

Duncan

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Posted

I can't help with the clearance question, I've always taken my machine shop's advice on that. It is worth considering that a shop that does that every day for road cars will be looking for as good as they can get off the shelf, not perfect. If you want more careful race style "blueprinting" then you need a race shop to measure/machine the engine.

I do have an opinion on line boring the block though....don't do it unless you have to. The crank centre moves higher but the oil pump stays in the same place when you line bore the block.

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Desean Strickland Contributor

Desean Strickland

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2 hours ago, Duncan said:

I can't help with the clearance question, I've always taken my machine shop's advice on that. It is worth considering that a shop that does that every day for road cars will be looking for as good as they can get off the shelf, not perfect. If you want more careful race style "blueprinting" then you need a race shop to measure/machine the engine.

I do have an opinion on line boring the block though....don't do it unless you have to. The crank centre moves higher but the oil pump stays in the same place when you line bore the block.

Thanks for the response. I get why line boring the block could cause issues with a spline drive oil pump but I’m just using an extended flat drive collar, u reccon it could still cause issues? 

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