Rod Big End roundness - Help

[mmasse]

I am in the middle of the rebuild of my 993 engine. Most parts are still at the machine shop, where they are taking forever to be done. I was told it would take two weeks and there has been 6 already!!

I sent to them the heads for rebuild; and pistons, cylinders and rods to be inspected. I ask several times if the rod had been inspected and they say: yes and that tere was no problem with them.

Today I received a package from the machine shop with the rods. To my surprise it seems that the bearings have not been even taken out of the big ends. When I took them out there was still some old oil there. So I doubted that the guy had even measured them. To double check I decided to measure them myself using a bore gauge. Below is what I found (I have done only one rod so far).

Can someone let me know whats the specification/tolerance for roundness? I think I read somewhere that the factory recommends no more than 0.005 mm for the rod big ends, in which case this rod would be out of specs, right?

[mmasse]

I want to make sure that what I did is correct. I removed the old rod bearings and torque the rods using the old bolts. I used 44 ftp ( 22ftp first round) , which is mentioned on the service manual as the torque to check that the bolt are stting well.

[mmasse]

Anyone, please!! I would like to confirm if I can assemble as is or send back to machine job. I am not sure about thdvtolerance figure for rod big end roundness

Thanks

[Flat6pac]

set your new bearings in place with the old rod bolts and lube. install on the crank and see if, as you spin the rod on the crank if there is a spot that the rod hangs up on. If not, build the crank up.
Bruce

[matt demaria]

Get new rods.

[mmasse]

Originally Posted by matt demaria

Get new rods.

That's very creative. I could get a new engine, as well


Can anyone confirm what's the tolerance for roundness for the big ends? Come one someone should know this one!!!

[304065]

There is no ovality spec or tolerance listed in the 993 workshop manual.

[chris_seven]

There is nothing in the 993 Spec Book either !

[john walker's workshop]

research the bearing specs. they usually state the bore they are designed to fit.

[matt demaria]

Earlier rods are 58.000 to 58.019. So this is your tolerance. Crank dimensions are the same. The part numbers are different for the 964 and 993 (the prefix). I would bet the shell thicknesses are the same.
But I thought that the 993 started with the 'cracked' form of manufacturing process of the mating surfaces of the rod caps. If these are the 'cracked' rods you cannot resize them; you throw them away.

[Steve@Rennsport]

Originally Posted by matt demaria

Earlier rods are 58.000 to 58.019. So this is your tolerance. Crank dimensions are the same. The part numbers are different for the 964 and 993 (the prefix). I would bet the shell thicknesses are the same.
But I thought that the 993 started with the 'cracked' form of manufacturing process of the mating surfaces of the rod caps. If these are the 'cracked' rods you cannot resize them; you throw them away.

Hi Matt,

993 rods are forged,.....its the M96/M97 rods that are cracked, sintered metal.

[AlfonsoR]

Bruce Anderson's book on pg 171 lists engines 3.3 - 3.8L to have rod big end diameter of 58mm, Length 127.0mm, wrist pin 23mm.

Wayne's book covers engines from 1965 -1989, lists 3.2 and 3.3 liter engines to have a big end diameter of 58.000-58.019mm.

So, from that I would conclude that 58-58.019 is correct, but I would probably get one more source to verify, but I'm anal when it comes those type of things.

So two things:
I am not familiar with the 993 rod bolts and whether they are like the 3.2's which are use once and throw-away after you have loosened them. If so, you need to do this check with a fastener that will impart the correct stresses into the rod big end. This is part of getting the correct form (perfectly round circle). If you are not imparting the correct stresses, you may not get to that desired shape.

Second, your measurements are so far off, that I believe you are not getting a good measurement. Did you calibrate the bore gage to a known standard? Try double checking with an ID mic, then measure the ID mic with an OD mic. I would not use snap gages.

[Sboxin]

My 0.02 ---

We are rebuilding a 964 -- our Porsche Indi tech says get the rods reconditioned no matter what...that way when you put the engine back together you know the rods are "like new".

He explained that they shave off a bit on the rod end and bore the big hole to the exact round size -- this way you know the rod hole is round and not oval.

You might look for a different machine shop...just sayin'

Regards,
Roy T