Aluminum rod bearing clearance

[boxhead]

I put together a blown gas 392 for a lake boat; we used Venoila alum rods. After running several barrels of av-gas thru it, we pulled it down for a check-up. It lost a full .001 rod bearing clearance, but because it had enough to start with we got away with it. Our own drag motors didn't get these kind of laps, but after 25 hard runs or so they'd lose a 1/2 thou, too. They seemed to hold after that, but we never ran them over 90 runs. Something to keep in mind; be careful setting it up tight with new rods.

[GOAT69]

The "rule of thumb" I use for (billet) alloy rod bearing clearance is a .001" per inch of journal diameter + .001" & round up to the nearest .0005". Works with any oil weight providing there's enough oil pressure for "oil wedge".

[Milan..]

1. Vertical @ 70 deg .0027 This on a 2.2 BBC journal
2. Vertical @ 110 deg .0037
3. Vertical @ 125 deg .0039
4. Vertical @ 150 deg .0042
5. Vertical @ 175 deg .0051

Obviously more clearance is needed if you tow your car to the line , but if you dont the vertical clearance seems to grow quickly. I used a bolted up R & R rod , dial bore gauge and a pan of water , for sure the wife was not home! Milan

[Adger Smith]

Milan,
You did half the test and half the equation for clearance at temp.
Take your crankshaft and heat it to the same temp and see what it grows to. Then subtract the differences in growth for the overall clearance increase. You will be surprised how much some of these factory cast steel cranks we have to run in NHRA Super Stock will grow at operating temp. That is a class the cars have to be driven to the line and back to the pit. Ihave checked many crank and rod assys at room temp, found the clearance. then I put them both in the 180 degree spray washer and let them run a while to normalize. Then check the sizes of each and calculate the HOT clearance. You will find that there are some basics you can follow in setting cold clearances. I have found that real large diameter factory cast cranks grow more per inch than the better aftermarket steel cranks. The more flemsy a bottom end is the more it seems to move around. I see someone else has had the caps move/settle into the serrations on new rods and loose verticle clearance. I like to pull the pins and re round alum rods after a few runs. They don't seem to close up after the inital clearance loss. I have found I can run them more runs after the tune up.

[Milan..]

Thanks Adger , Those rods are off my billet crank , but I had no idea a cast one can grow that much. Super Stock tech is the best!

[Strange Magic]

I work with engine components all day long. When you do quite a bit of pin fit work, you get a true understanding of how certain materials can grow by just laying your hands on them and working with them while in process. Even a dial bore gauge when setting a mic will grow just by heat absorbsion from your hands. For those who work frequently with mics, dial bore gauges and ag300 gauges, you can understand exactly what I speak of. It is understood that when you have large factory oem rod journals such as 2.500, the growth rate could be surprising and very much underestimated. The true meaning of where clearances should be set up is ultimately determined by how that engine looks after a period of use. Why run the ragged edge of whats known as "possibly acceptable", when a little extra clearance will never hurt a thing and it's very doubtful that the loss of about 3-4 psi would be concerning or damaging to the engines well being. Pressures built from flow restriction (tighter clearances) are more of a concern than the pressures that are a slight bit lower from non restriction. There is no layer of oil and no pressure setting that will prevent parts from moving, flexing or twisting, eespecially if an engine is filled with inferior parts, such as oem crankshafts with heavy chinese rods attached to it. Many of times individuals rely on oem crankshafts to really do the work that aftermarket shafts should be doing. It is seen all the time in the industry.