AusS2000

I've recently read about grinding a cranks rod journals (where the big end of the rod attaches) offset from center so that the diameter decreases but the outside edge is the same.

Effectively if you take 6mm out of the diameter, offset to the outer edge you are moving the centre 3mm out and hence increasing the stroke by 6mm (3m at the top + 3mm at the bottom).

Of course you will require custom rods with smaller big ends and to suit the stroke.

Is this an acceptable practice on our cranks? Does it weaken the crank substantially?

I know it decereases the rotationa speed of the big end because of the smaller diameter but am wondering what the negatives are? Would it still be possible to balance this crank and get it spinnin got 9K (not taking into account increase piston speed etc which is hotly debated elsewhere).

Greenlight

6mm increase in stroke is a BUNCH.

The engine will go from 2.0 liter (~122 cid) to 2.14 liter (~131 cid).

I don't think you will be able to find rod bearings.

Getting someone to custom make a steel rod will be difficult, but aluminum rods (not suitable for street use) won't be difficult.

You're better off switching to the F22 crank, rods, and pistons.

SpitfireS

This is what you mean right?
Grind away the red area (6 mm) to create more stroke (3mm)?

chris_barry

Aus,

This is what I do when I make my 1900cc 4age engines. Drop around to the factory and I'll show you a crank before/after. I think I even have some rods/pistons so you can see how it works. I can even suggest a machine shop for you.

In terms of what is does. It helps reduce the weight of the rod/pin as the journal is smaller, and the bid end of the rod smaller. This helps the balance at higher rpm. Downside is there is less of the crank, and less bearing area. I reduce the toyota journals from 48mm to 42mm without any problems with either the crankstrength or bearings.

General race motor practice is to get the rod/piston/crank pin as light as practical, and leave the counterweights as heavy as possible. Most production car cranks cannot be lightened to the same extend as a race crank as the material cant take the flex you get if you start cutting back counterweights.

Chris

AusS2000

I think a trip to the shop is in order.

I'll bring some engine bits with me.

Do you also know someone who can inspect/hone the FRM sleeves?

AusS2000

Precisely!

jzz30

you can also have material added to the crank by welding it then grinding, heat treat ,then polish. i think castillo does this. then you dont have to sacrifice bearing size.