Reason for increasing rod big end side clearances?

[dyeager535]

I have a late 1960's small block chevrolet that I'm building up. Going to be a torqey lower RPM truck motor that I intend to drive often, and want long life out of.

Anyway, due to a machine shop screw up, my stock GM connecting rods were replaced with brand new "Eagle" I beam's.

As I'm going through the build process, methodically checking clearances, I find that I have .024" side clearance on the rods big end (between each pair of rods). Rod width itself does not vary, they are uniformally the same width at the big end.

GM specification is .008"-.016", and obviously what I have is way over that. I called Eagle, and they "assured" me (the rep was a little hesitant if you ask me) that yes, their rods are narrower than stock, and that oil pressure or longevity will not be affected.

I can't help but feel that perhaps Eagle is wrong, and GM was right for the past 30 years or so of making millions of warrantied motors this way, but I can't find any information on what the reasoning is behind certain rod side clearances.

The way I look at it, which of course could be wrong, is that the increased side clearance will only allow the rod to accelerate more before hitting the other rod (or crank journal chamfer), IF the rods actually tend to move front to back. With the piston pin easily moving, and the crank lubricated, the rod of course easily slides along the crank's rod journal.

Can anyone explain why or why not, the side clearance is important?

By the way, I would just like to say that these forums are immensely informative, and I have spent the better part of today reading almost every single post in every thread!

 

[patprimmer]

The side clearance controlls oil flow from thr bearing, and hence is important in cooling the bearings and ensuring all bearings receive their fair share of oil.

The larger the clearance, the higher the flow, but the lower the pressure. This means more cooling, but possible starvation at a bearing further down the gallerie.

Tighter clearance means less flow, but more chance of uniform distibution.

If you have a higher volume pump or more viscous oil, you can run higher clearance. Higher clearance also helps accomodate expansion or the rod, however, I would normally expect the journal to expand more than the rod.

Sorry the answer is not simple.

I think that I normally run 0.022" clearance on my Carrillo rods, but I havn't rebuilt that engine for a few years now, so I would need to look it up. It runs a Mellings High Volume pump and Mobil 1 synthetic oil. Regards
pat

 

[evelrod]

I can't address the issue as it pertains to an SBC but, I increase the side clearance on most of the race engines I build in an attempt to get a little more oil flow across the bearings---of course , inconjunction with a proper high volumn oil system (pump, lines, cooler etc.) and appropriate bearing clearance. Higher oil flow equals cooler bearings in most cases.
IMHO, .024 doesn't sound all that bad. I have on at least one engine used .020 on a single rod with no problems.
Perhaps some of the V8 group will chime in here.

Rod

 

[tstom]

We build a lot of small block Chevys with Eagle rods and .022" to .025" is a pretty normal side clearence you won't have any trouble set your rod brg clearence at .0025" and it will run for a long time The little Chevy has a simple yet very effective oiling system We have had drag engines run to 8500 rpm with a standard volume oil pump with just a high pressure spring added Be careful with the HV pump if you are not increasing the pan capacity
I agree with you about the forums I'm new here and I think their great...keeps you thinking

Good luck
Tom

 

[SUPERKYLE]

Good afternoon guys, Here's my take on side clearance vs. oil flow: The annulus of the rod bearing and the respective bearing clearance dictate the individual rod bearing oil flow, independent of the side clearance, unless the side clearance is nearly as small as the bearing clearance. Calculate the rod bearing clearance around the annulus (twice) and consider it as the orfice that the oil has to leak out, then calculate the orfice size of the side clearance, I think you will find that the side clearance is a considerably larger orfice.
TS Tom, I'm sure you have had some combination with "big" side clearance? (say like BBC rods on a BB Mopar)

Later,
Kyle AKA S/C 3028

 

[JoeyHughJorgan]

An old trick of questionable value, especially when applied to the low-end torque, pickup truck application driven on a daily basis.

Increasing conrod side clearances is employed by certain performance engine builders (drags, short trackers, etc) when oil control & consumption are not an issue. Rod brngs are the terminus of an engine's oil supply - after lubing, it's back to the sump for another round. Extra clearance here will only serve the truck application to artificially increase the amount of airborne oil & therefore (ya probably guessed it) windage. The dreaded W word !!

Spelled out, your oil control rings may have a severe time handling the extra splash volume. Serious oil consumption could be an issue. Would advise against the Eagle's in favor of ANY rod with proper side clearances.

 

[RacerRick]

oem ford v 8 motor have .018 to .022 side clearence Chev sbc can be up.016 oem. I wouldn't about it being .024 at all. At hendrick motorsport,the rods are guide by the pistons not by the crank the side clearence is big big.

 

[patprimmer]

I am sure that a good scraper and sump design will do a lot more to control windage than a few thou of rod side clearance.
Lots of very durable, successfull, high performance SBC's run side clearance in the low 20s with no oil control problems Regards
pat