ACE VenTRa

My feeling is that if a part is within spec and not damaged in any way, it can be reused. For example, I've reused pistons, rods, wristpins, heads, valves and pretty much anything in between. I bring this up because I was having a conversation not too long ago about marking parts on disassembly if they are to be reused so they can go back in the same place. My first reaction is that I mark parts as they come apart so if I find an issue I can identify the root cause more easily. My second reaction is that if a part is within spec, I can use it regardless of its original or new home. Let's take shim buckets for example; they have a spec for OD, if bucket A comes out of position A but is too small for that spot, I wouldn't put it back there, I would see if it fits another spot better, it would not make sense to put a good part that is at the small end of the spec in a place that is at the large end of the spec just because it came from there. Regarding "wear in," wouldn't the part have to wear in whether or not it's new or used? Won't a new part with the same spec as a used part be virtually indistinguishable? Lastly, has anyone ever mismatched used parts and had a failure as a result? I sure haven't.

AV

E.Marquez

Tolerance stack..
That is another reason it is good practice to return parts to the home they came from.
Using the valve lifter (bucket by another name) as an example.
The hole in the head that it fits down into has a tolerance for size....Spec is X.XX but it can be that or + / - some as well and still be in tolerance. same as the valve lifter.

4 holes on intake side of the head fitting 4 lifters.... for ease of discussion.. lets say the hole is spec'd as 30.00 mm + / - .03 mm
and the valve lifter is 29.95 mm + / - .02mm

Hole #1 is 30.00 mm
Hole #2 is 30.00 mm
Hole #3 is 30.00 mm
Hole #4 is 29.97 mm
All "in spec" at 30.00 mm + / - .03 mm

Lifter #1 is 29.95 mm
Lifter #2 is 29.96 mm
Lifter #3 is 29.97 mm
Lifter #4 is 29.97 mm
All "in spec at "29.95 mm + / - .02mm
Lifter 1 & 2 would fit any position
Lifter 3 & 4 would fit in position 1,2,3 but not in #4

Thats a very over simplified example of tolerance stack, but an accurate example non the less.
Sometimes you will have two parts that inter fit..both individually meet the spec dimensions, but at ends of the tolerance range.. so they do not fit together as they should.
Other times its less obvious .... both parts are at opposite ends of the tolerance range and they fit loosely together.

Some may have heard the term "Blueprinted" motor its referring to all the component parts being selected or corrected to "perfect" blueprint spec. No range, no tolerance..or perfect tolerance .say a rod lower side clearance that has a 0.10-0.30 clearance range with a service limit of 0.40 allowed but the engine builder has leaned that a side clearance of 0.22mm if the best number to meet for longevity on the endurance motor he is building ..so that 0.22mm is the blueprint number both rides will have...period.. no tolerance.

ACE VenTRa

Exactly...So in the case of used parts...if the tolerances add up...I can see no reason not to use used parts but with few exceptions, and I see no reason to put parts back in the location they came out of if you are tearing it down and checking every part for wear. Especially in the case where oem or aftermarket new parts are not available. I remember many times scouring junk yard for old parts that are no longer made with the hopes of finding a few serviceable items out of the bunch.

Thoughts?