1st undersize on a single rod journal
 

Hi All

I know the prefered method to fix a journal which has spun a bearing is to find a good used 964/3.6l std/std crank but my machinest insists machining the journal to accept a 1st undersize bearing is acceptable (i.e. he maybe a bit prejuduced) . His argument is that the hardening goes deeper than the 10 thou he will machine off and if Porsche makes the undersize bearings available, it must be an acceptable method by Porsche. From my point of view, buying a good used std/std crank for $1500.00 vs. the 2 or 3 hundred dollars to have it machined, polished and magnufluxed is tempting. Also, if I go the new used route, what is the best way to ship one across the country. It seems like the perfect weight and size for shippers to toss and cause real damage.

All responses welcome

Gas nitriding is typically 0.020", so 1st under should be just dandy. Salt bath nitriding is thinner.

t

Thanks t

So if I understand correctly, the hardening from the factory is a gas nitride process which penetrates the crankshaft to a depth of 0.020". In order to cleanup my rod journal, it will require grinding it to the 1st undersize or 0.010" leaving 0.010" of hardening on the journal. Will it be necessary to perform any hardening after the journal is ground the 10 thou required to clean it up?

All response welcomed

the problem with a journal that had a spun bearing is that the wear is usually not evenly distributed around the journal. it often takes two undersizes to clean.

If the machinist says he can get it in .010 he probably can.

Would removing 0.010" of material off of one journal so that 1st undersize bearings can be fitted, cause enough of an imbalance to be concerned?

The permiabilty of 4340 in this case the material for the crank and salt nitrite (which is what they use) is only .004. Check this out before you proceed with someone like Armando up north and if you are going to heat treat weld it up harden it and straighten it it will be like new. Another alternative is Mike Reed at Electronic Chroming in Santa Fe springs he can fix it like new.

regards

if you see blued metal in with the scoring marks on the journal from the spun bearing it got very very hot while spinning... and the crank may not be straight now and the intense heat could soften the surface metal hardening.

obviously no one can judge that without seeing it but if there is heat blueing mixed in with the spun bearing scoring marks the crank is probably junk and could make an interesting heavy table lamp if you put a base on it.

The reason it spun is still a bit of a mystery. No blueing from heat so it wasn't starved for oil (i.e. #3 bearing) and the crank checks out straight still. Exactly what is the process in "heat treat welding" or that done at Electronic Chroming? Another suggestion from my machinest is to tig weld and harden in some sort of bath. Would the bath be the same as salt nitrite hardening as mentioned above? Is the process at Electronic Chroming, as the name implies a chroming process which I've been told to avoid?,