Rod bearing clearance

[Dave Koehler]

I got roped into building this using a myriad of parts.
It is the biggest CI I have messed with and I am having some doubts about clearances.
The patient: 706 bbc, 14.6, BME alum. rods, Pontiac heads, 300 shot of n2o, Dan Olson wet sump, vacuum pump, 2400 lb car, bruno trans.

The rod bearing clearance currently works out to .0032.
Have not measured the mains yet.
I am thinking of going at least .0015 on the pin clearance.
Opinions?

[mt-engines]

.0032" A little loose for what you are doing. But its fine. .990 .250 wall pin? .0018-.002 is what I would shoot for if you plan to spin it over 8k and make 1700+

Do what you already are thinking . Unless you have a flimsy pin and piston. Im assuming you are keeping this below 8000rpm?

[Dave Koehler]

I added Dan Olson wet sump and vacuum pump to the original specs.

[mag2555]

I would possibly have doubts about the block,
8K with that stroke!

[mt-engines]

I would possibly have doubts about the block,
8K with that stroke!

Dude.... STOP RESPONDING TO ANYTHING POSTED ON THIS FORUM.. YOU ARE CLUELESS!!

[Walter R. Malik]

I got roped into building this using a myriad of parts.
It is the biggest CI I have messed with and I am having some doubts about clearances.
The patient: 706 bbc, 14.6, BME alum. rods, Pontiac heads, 300 shot of n2o, Dan Olson wet sump, vacuum pump, 2400 lb car, bruno trans.

The rod bearing clearance currently works out to .0032.
Have not measured the mains yet.
I am thinking of going at least .0015 on the pin clearance.
Opinions?

Actually, with that probable 5.300" stroke, I would run minimum .040" rod side clearance and guide the rods side movement within the piston; (piston guided rods); similar to a 2-stroke.
What diameter is the rod journal on that crank ?

[Dave Koehler]

I would possibly have doubts about the block,
8K with that stroke!

Did I mention a block? Ahh, no because it's not the subject here. FYI the block is correct for this app.

[PackardV8]

Actually, with that probable 5.300" stroke, I would run minimum .040" rod side clearance and guide the rods side movement within the piston; (piston guided rods); similar to a 2-stroke.
What diameter is the rod journal on that crank ?

Walter, I'll never build one of those beasts, but have done a few aluminum rod builds on a smaller scale. Just help me understand the science of your side clearance and piston guided recommendations.

[Dave Koehler]

Actually, with that probable 5.300" stroke, I would run minimum .040" rod side clearance and guide the rods side movement within the piston; (piston guided rods); similar to a 2-stroke.
What diameter is the rod journal on that crank ?

nominal is 2.200

[Walter R. Malik]

Actually, with that probable 5.300" stroke, I would run minimum .040" rod side clearance and guide the rods side movement within the piston; (piston guided rods); similar to a 2-stroke.
What diameter is the rod journal on that crank ?

nominal is 2.200

When running at about 7,000 RPM, a good steel 4340 crankshaft having a 5.300" stroke with that rod journal diameter will get about .035" shorter from end to end simply because of flex; be certain that the involved rods will have no angularity when running.

Then the rod bearing clearance can be about a thou tighter and oil throw-off will be considerably less.

[mt-engines]

Actually, with that probable 5.300" stroke, I would run minimum .040" rod side clearance and guide the rods side movement within the piston; (piston guided rods); similar to a 2-stroke.
What diameter is the rod journal on that crank ?

nominal is 2.200

When running at about 7,000 RPM, a good steel 4340 crankshaft having a 5.300" stroke with that rod journal diameter will get about .035" shorter from end to end simply because of flex; be certain that the involved rods will have no angularity when running.

Then the rod bearing clearance can be about a thou tighter and oil throw-off will be considerably less.

I'd love to see that data.. could you share it?

[Walter R. Malik]

nominal is 2.200

When running at about 7,000 RPM, a good steel 4340 crankshaft having a 5.300" stroke with that rod journal diameter will get about .035" shorter from end to end simply because of flex; be certain that the involved rods will have no angularity when running.

Then the rod bearing clearance can be about a thou tighter and oil throw-off will be considerably less.

I'd love to see that data.. could you share it?

No, I can't share the data because I was only an onlooker to the testing being done. A 706 on a dyno with laser measuring devices on the nose of the damper and the flywheel along with a lot of other places.

If you don't want to believe it, then don't ... I don't really care.

[Baprace]

Why do you recommend .040 side clearance , that seems excessive to me , I would not want to exceed .025 to keep the rods from dancing from side to side , what would you suggest for vertical clearance , with what weight oil , also try the right side tomorro , your a little grumpy. Have a great day

[Walter R. Malik]

Why do you recommend .040 side clearance , that seems excessive to me , I would not want to exceed .025 to keep the rods from dancing from side to side , what would you suggest for vertical clearance , with what weight oil , also try the right side tomorro , your a little grumpy. Have a great day

A "piston guided rod" doesn't dance around like that but, you can believe whatever you want.

No bad mood or anything ... I really don't care what is believed or isn't.

[Dave Koehler]

aluminum rod trivia
I have a customer that was concerned about rod stretch.
I keep notes and made sure the room temp and parts are the same as my initial measurements.
For 3 years we measured at season end. Each year the rods decreased in length 1-2 tenths per year.
I cannot say for certain the rods actually decreased in length due to compression over time or it decreased due to the cap serrations bedding in.
I tend to think it is more the latter.

The important point is that it verified that alum rod stretch is a myth. They do grow with heat but return home when cooled.+

I torque up my own alum rods that I use 5 times. The big end ID always shrinks as the serrations bed in.
I then remove the spin pins and hone to minimum specs. Slightly less sometimes on my own.