Scalpel

iTrader: (4)

I have great oil pressure in my iron block 414 with 0.0025" main bearing clearance and 0.002" on the rods.

Steve Bryant

iTrader: (1)

It seems like we are getting some good information here and I appreciate everyone's input. I wonder what the difference should be for long term durability versus an engine that's going to be refreshed about once a season. It seems to me that the tighter the clearances (within reason) the bearing life will be longer but the friction will be greater. On the other hand, looser clearances (again within reason) will result in less frictional loss (especially at higher RPM's) but the extra clearance could cause the bearing to become eccentric or oval shaped due to the metal to metal contact that occurs at TDC and BDC.

Steve

Redline-Motorsports

iTrader: (3)

Steve, this is an odd view but; how many backyard wrenches are building motors and not even checking the clearances!! Dirty motors just thrown together.....tons and they keep living! Of course this topic is VERY specific to exact tolerances but it just goes to show you that some of these answers are preference. I typically drive my self nuts over a halfa' thou, but thats me and most the guys in this section. We want things perfect. Whether it's piece of mind or not....I don't know!

I am not sure if we'll ever know which way is "better". Thats why the main and rod numbers I use are right in the middle of GM's spec. This kind of blends all the factors in one clearance!

Seems to be working great so I stopped think about it! LOL!!!

Steve Bryant

iTrader: (1)

9D9LS,
I'm kind of **** about best practices myself. I used to be a Detroit Diesel mechanic and my dad was very much a master mechanic back in his time. He had his own independent garage and in time became a new car dealer. He even taught depot overhaul of engines and drivetrain components in the Army. He was a stickler for detail and he was my first teacher.

Now, at age 56 I'm an aircraft electronics engineer (been in the industry for 30 years) but I never put up my wrenches. I used to do it out of economic necessity. Now I just do it for fun and am proud to consider myself a gear-head.

Steve

P Mack

iTrader: (6)

You guys keep saying that the expansion of the aluminum block means different main clearances. What about the fact that the main caps aren't aluminum? Doesn't that reduce the effect?

FWIW my new aluminum shortblock has .0025" main, .0022" rod clearance. Oil pressure is 60 psi at startup when cold and idling around 1000 rpm. By the time the motor warms up the combination of the expansion, the thinning of the oil, and the rpms dropping to 750 leaves me with only about 25 psi oil pressure.

Redline-Motorsports

iTrader: (3)

Since all the caps are steel, both iron and aluminum blocks, it is the actual block material that is in question.

P Mack

iTrader: (6)

I realize that, i'm saying won't the steel caps resist the expansion of the aluminum, at least from side to side? Or would you end up having a different clearance on the top half of the bearing than the bottom half with an aluminum block?

Steve Bryant

iTrader: (1)

P Mack,
What happens (in my opinion) is that since the two halves of the bearing are installed in the cap (lower) and block (upper), the aluminum block grows away from the center of the crank journal with increasing operating temperature. Since the block is backing-up the upper bearing half, the bearing clearances for all of the main bearings increase more than in comparison to a cast iron block. This increased clearance at operating temperature with the aluminum block can cause relatively lower oil pressure when compared to the iron block. This is true even if both blocks have the same main bearing clearances at a given room temperature.

You are right that the rate of expansion of the caps should be the same. It's the expansion of the block that makes the difference.

Steve

jyeager

I think he meant that since the block and the caps expand at different rates, that you end up with eccentric main journals when at operating temperature.

Steve Bryant

iTrader: (1)

jyeager,

You summarized the intent of my thoughts exactly! Thanks. I'ts good to see another Tennessean chiming in on this thread. I live in Kansas now, but I am from Cleveland, TN.

Steve

jyeager

Well, obviously it's not a problem or these engines just wouldn't work so well...but I'm sure that under the right conditions this could be measured to be happening. Perhaps over time as the engine breaks in, the main bearings are losing material in the right places and ultimately become round at operating temp.

In theory doing an align hone could be improved if done with the block heated to operating temp.