When you're repairing a car, do the parts need to be clean?

Dear Car Talk

Dear Car Talk | May 01, 2008

Dear Tom and Ray:

My husband is an excellent mechanic and can put together just about any vehicle made. Trouble is, he never cleans any of the parts off before he puts them back in. It is MY opinion that putting in dirty parts can mess with the parts' tolerances and cause problems later on. I know it sounds weird, but isn't it possible that grime can change the performance and/or weaken the rebuild? If not, why do they sell clean parts to begin with? Please do not publish my name and embarrass my husband; he thinks I'm crazy enough already. Thanks. -- Claire

RAY: Don't worry. This'll be just between us, Claire. Frank won't ever know you wrote to us about it.

TOM: The answer is, it depends. Generally speaking, internal parts of the engine, transmission, rack and pinion or any parts that are not normally exposed to the elements need to be perfectly clean before they're reinstalled. But external parts -- parts that go on the outside of the engine or underneath the car -- can be covered with years of grime without any adverse consequences.

RAY: My brother is covered with years of grime, and he's had no adverse consequences. Unless you count the three or four wives who ran away screaming.

TOM: That was because of the grime? And all this time I thought it was my personality!

RAY: So, if I'm putting a set of valves into an engine, they have to be clean enough to eat off of. Things like valves, piston rings and connecting rods all are parts that go inside the engine, where gunk and grime can plug up oil passages and lead to engine failure.

TOM: There are also internal parts of "external" parts that need to be clean. Like when you're rebuilding a water pump -- which goes outside the engine -- the mating surface, where the gasket attaches, has to perfectly clean. Otherwise, the seal may fail, taking a brand-new pump with it.

RAY: But if I took out an alternator or a starter, I could put it back in without worrying about the outside of the casing being filthy. That makes no difference mechanically.

TOM: So, as long as your husband is cleaning the internal parts, he's not causing any damage to anything.

RAY: On the other hand, we generally do clean all of our parts, because it makes it much easier to spot a leak. After a repair, you start up the engine, and if there's any internal fluid or oil on a part that you just cleaned, you know you screwed up. Which is bad news. But it's always better if we discover it BEFORE the customer comes back with a baseball bat.

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