Rocker shaft selection

[tharbert]

I was going through my project list adding things to the cart for purchase. I noticed a few entries that had links to "Chose Alternate Brands". On the rocker shafts, there were choices between Febi, Porsche or Mahle-Wisemann (OEM).

The two OEM versions run $5-6 each more than the Febi.

What's the difference and will it make a difference in a non-race 2.4t modded to E?

I'm not trying to cheap out here. But, I also don't have unlimited funds and the $70 could go someplace else where it's needed more.

[cgarr]

I was sent "new" shafts with some rockers, 6 were oem and 6 were a different mfg I don't know which mfg but they did not spec out, they were .001 undersize not such a big deal fitting the rocker but could cause problems locking down in the cam carrier.

[Mitch1]

Originally Posted by cgarr

I was sent "new" shafts with some rockers, 6 were oem and 6 were a different mfg I don't know which mfg but they did not spec out, they were .001 undersize not such a big deal fitting the rocker but could cause problems locking down in the cam carrier.

By the way, thanks for that Craig! Probably one of the easiest and most pleasant dealing I've had so far have been with Craig and timmy2.

[chris_seven]

Originally Posted by cgarr

I was sent "new" shafts with some rockers, 6 were oem and 6 were a different mfg I don't know which mfg but they did not spec out, they were .001 undersize not such a big deal fitting the rocker but could cause problems locking down in the cam carrier.

I assume you mean 0.001". Was the shaft 0.001" smaller than the minimum specified size?

The clearance specified in the rocker arm/shaft is between 0.016 and 0.035mm and this calculates as 0.0006" to 0.0014" with the wear limit at 0.003".

If the shaft were 0.001" down on the minimum specified diameter it would still be within the wear limit but the useful life of the rocker/shaft combination would be reduced.

Unfortunately there is no clearance specified between the shaft and the housing.

If we take the smallest shaft and the largest housing then the maximum specified clearance is 0.026mm which is 0.001".

I believe we can assume that the shaft should still lock with this clearance but I am not sure about how much this could be increased.

If the replacement shafts are 0.001" smaller than the minimum there could be issues and it would be worthwhile making measurements of the deformation on the end of an unconstrained shaft to see just how much the end deforms.

Surface finish would also be an issue in terms of rocker arm bush life and there will be an optimum Ra value associated with oil film retention.

[cgarr]

I was only looking at the shaft ends where they expand in the carrier. Factory at rest check at .7084" tight at 18ft lbs expand to .7105" and some still have problems with them walking out of the bores. Its just an assumption but I would think an aftermarket at .707" would be at even a greater risk of walking, but I did not check the aftermarket shaft to see what the expanded measurement would be. Again the condition of the carrier should also be considered.

[chris_seven]

Cam housings are specified with a bore of 18.000 to 18.018mm

This converts to 0.7087" to 0.7093".

At 0.7084" the shaft is 17.993mm which is almost bottom tolerance. (17.992mm)

If the bore is a maximum size at 18.018mm the expanded shaft is 18.047mm (0.7105")

so the 'fit is an interference of 0.0287mm or 0.0013" when cold.

When the engine reaches running temperature (assuming 120degC) this fit reduces to about 0.0007".

I would think that 0.001" per 1" of diameter is the minimum acceptable and for an 18mm dia shaft this works out to 0.0007".

May be just enough to prevent walking.

This is the worst case assuming all parts are within factory tolerances.

If the bore have been damaged and has expanded due to shafts being pressed out then there may be some issues.

Certainly shafts with a smaller diameter than the minimum size specified by Porsche will be a problem.