It's Really Simple.....was: Re: [Shotimes] rod bearings

[George Fourchy]

On Tue, 12 Aug 2003 13:03:54 -0400, Leigh Smith wrote:

>The mains are before 
>the rods, so if that one wears, it does double damage by starving the 
>rods. The SHOs seem to wear out rod bearings first. Why? Who knows? 
>Highly stressed :-), starved at RPM, or starved on start-up?, take your 
>pick. 

Why does a SHO engine wear out rod bearings first, rather than main bearings?

Because the load on the smaller rod bearings is extremely variable, where the load
on the larger mains is constant.  One second the rod is pushing down on the crank
journal....the next minute it is being shoved back up into the cylinder by the back
side of the journal.  The main journals just go round and round, never really taking
or applying any heavy loads in one given spot over and over.  

As we have discovered with the timing sprocket and the crankshaft nose, this engine
is not a perfect design, no matter how much we revere it.  It's a WHOLE LOT BETTER
than anything else out there that performs like it does, and way ahead in technology
a long time ago (relatively), but it isn't perfect.  One of the few places where it
is weak is the size of the big ends of the connecting rods.  They should have been
wider, and the bearings  might want to have been a bit thicker.  But the technology
of the day said they would work the way they are.  They needed to be light, and
there isn't a lot of room....same for the clutch.  We are making these cars last
many times longer than they were ever designed to, and so the weak points are
becoming noticeable.  

It's a fact of life.

Regarding older engines and their bearing life.....two things.....higher oil
pressure at idle, and less power delivered per rod to the crankshaft.  Ford small
block oil pressure specs say 35-55 pounds at 2000 RPM.  Having rebuilt my Boss
engine (and lots of 289s) countless times, and never having had any bearing
troubles, I can tell you that it idled (still does) hot (with a S/W gauge) at 21
pounds and has 45 pounds at 2000 rpm.  Always.*  

A 225 HP 5.0 makes about 28 hp per cylinder, a 290 (yeah, right) hp Boss 302 makes
36+ hp per cylinder, while a much smaller SHO engine makes 220 hp out of 6
cylinders, which is 36.67 hp per cylinder.  (All of these are obviously at max. hp
speed)  The Boss has HUGE rods and crank journals, plus a high volume pump.  The SHO
is tiny compared to this.  It's simple physics....more load over less area equals
more wear.

Don't lug a SHO.  The '2000 RPM' rule, or the 'vehicle speed equals 10 times the
gear it's in' rule, are good rules.

George

*(Well....except when it snapped the oil pump drive shaft after a roller rocker arm
needle bearing got in the gears.....!  No damage, just got the needle out and got a
new shaft (same pump)....that happened while I was passing someone going 90 on a
local freeway.  Shut it down and towed it home.  ..kinda embarrasing!!  LOL!  But no
problem.  Show me a SHO that would do that!)