determining whether a set of pistons and connecting rods are reusable?
 

i have a set of om616 pistons and connecting rods that came from an engine which was supposedly ran out of oil. the rod bearings are messed up, but the pistons and connecting rods themselves appear to be fine other than being dirty. what method can i use to determine whether these are worth holding onto for a later rebuild or whether they are trash at this point?

First thing I would do is clean them up and post some photographs here!

Measuring them and comparing their dimensions to the specifications in the FSM is always a good start - hopefully you'll find some data that matches your pistons. Pay attention to the ring grooves.

You can check the stretch on the big end bolts by measuring the width and the caps need to stay in position and not fall due to gravity when you attach them one side at a time to the big end bolt. (I'll dig out a photograph of this check if you don't know what I mean)

The Pistons can be Measured with Micrometers and Feeler Gauges (for the Ring Grooves).

The Cranshaft end of the Con Rods can be measured for out of round after the Bearing Caps are put back on and they are Torqued.

However, to determine if the Rods are bent would have to be done at a Automotive type Machine Shop becasue there is special equipment to do that.

Good answers !
Plus... on the ring grooves.. if they are too wide...and particularly if that is the only thing wrong with the pistons )...... they can be cleaned up ( on a lathe or there may be a hand tool which resembles the hand tool used to clean the bottom of the ring grooves ) so as to be even and an aftermarket wider ring installed.
You may need the actual 616,617 NA FSM to find the specs for checking.

use a long feeler guage
 

If you have a good standard bore block you can measure the skirt clearance with a 12" feeler guage set by inserting the piston and feeler into the unworn section of the hole at 90 degrees to the piston pin. ie the bottom and judging the pull necessary to remove the guage from beside the piston. You have to start small and work up in small increments. When you get it right there will be an increase in the pull necessary just before you reach one that is too thick. This is in addition to the previous comments regarding ring groove attention. Some people have a harder time reading a caliper or micrometer so the feeler guage is helpful. The bottom skirt of the piston is the area of major wear since the rocking motion of the piston as it reaches TDC transfers from one side of the cylinder bore to the other. Pistons are manufactured to be slightly oval at the bottom so a measurement in line with the piston pin is not an accurate indication of wear they are also smaller at the top than the bottom to compensate for the higher temperature at the top. Cheers Dan

While you MIGHT be able to make that measurement that way... it is not one the FSM describes... and will not give you the other relative measurements which are needed for the decision he needs to make ... having to do with the condition of the bore. Off hand things like ovality, concentricity,etc.. typically 90 degree measurements are needed at three heights in the bore..which are carefully described locations...
Not only are pistons often ground oval at the bottom... but at the top as well since the area of the piston pin allows more expansion one direction than another.. they try to have the piston round in its working temperature.
Speaking of standard bore.... the sleeves for a 616 are cheap... and make getting to a good bore a fairly simple process ( pull and replace)...

You will probably need the services of an automotive machine shop to measure them. Rather than spend the money now just add them to the accumulating hoard of junk until the need to attempt to use them becomes a requirement. Grease them up well to prevent oxidation and store.

Chances are if your 616 engine is now in good shape you will not need them. anyways. Buying the equipment to measure them yourself for one time use basically is not cost effective.

I totally agree with this conclusion.
Many of the measurement instruments require " Touch " ... meaning practice at getting an accurate measurement repeatedly... something a good machinist has had years to develop.

616 pistons are reasonably priced anyway. a new set would not break the bank, and should come with new rings. (I'd change the 2nd ring for a zero gap ring though) the sleeves should certainly be measured, and checked for scoring/oval to determine if the block needs new ones installed/honed/bored...

full set with rings on ebay

i'm pulling a block out of my roller tomorrow. I've got the pistons and rods from it (crank is probably trash). I have a brand new set of rings and rod bearings already, not wanting to buy new pistons at this point, just debating on whether to keep these.

and $290 isnt breaking the bank for some, but thats fully 1/3 of everything i will have spent on the 240D to date!

It is an option to try to find another good used engine as well remember. Suprisingly there are some still out there. If your crank is damaged may be cheaper as well to go the replacement route.

already doing that. the shell im putting my nice engine into came with its old engine - had been run out of oil. i just am trying to figure out what parts to keep and what parts to throw away.

Keep all parts.. never throw any away....

when you are old enough to have your own house and your own places for these things sure. i have to at least justify to my mom why i need to keep more greasy junk in the garage :P

As soon as you get rid of it you will need it....
get pictures of what you throw away... then as soon as you need it... explain how Parts Karhma works to your mom .. then she will understand in the future...