Valve-lapping underway

I'm hand-lapping valves on the LS1.

This is probably a silly question, but I just wanted to confirm that I have to wipe off the lapping compound prior to the hydro test.

4 Answers 4


Yes, remove all traces of the lapping compound - it is the metal to metal joint between the mating faces that is being tested.


Not only should it be cleaned off, you want to ensure you don't get/leave any on the valve stems. You'll wear out the valve guide in short order if you don't. Remember in engine building, cleanliness is next to godliness. You want to ensure everything remains as clean as possible and this means removing all of the lapping compound. Clean it all and don't spare the towels!! :o)

  • Yeah, it's funny how some of the residue ended up on the stems despite my best efforts. A quick wipe down with some disposable rags rectified the situation.
    – Zaid
    Commented May 5, 2017 at 20:57
  • If you got some on the valve stems and up into the guides, run a brush down the guides with some solvent and clean them up. You CANNOT be too clean here. Commented May 5, 2017 at 20:58
  • Sure thing, will do
    – Zaid
    Commented May 5, 2017 at 20:59

Yes it must absolutely cleaned off. The compound is an abrasive paste, as such on initial start up it will be ingested into the engine. The outcome would be the same as leaving some fine sand on the top off the piston while assembling the motor.


People usually think that the cylinder head, turbos, or transmissions will be the most expensive part of a complete rebuild, but if you're doing it right... the required number of $7-8 cans of carb/brake clean may actually rival any of those component's overall cost lol.

  • Welcome to the site. Appreciate the comment but it doesn't really answer the question.
    – Zaid
    Commented May 10, 2017 at 11:37
  • My bad, I guess I viewed the statement as part of a fluid conversation working off of the previous answer. Definitely clean it off meticulously. It's essentially liquid sandpaper, and should be thought of as a cutting tool. You don't want it touching anything you don't intend to grind down/reshape/polish. Commented May 10, 2017 at 16:42
  • You might want to read this: mechanics.meta.stackexchange.com/q/1460/675
    – Zaid
    Commented May 10, 2017 at 16:44
  • Thank you, I just kind of happened upon this site, and have found it to be an interesting use of my down time the last couple days. One that also stimulates some of the mental exercises and research I've missed since moving from a state where my mechanical expertise could actually pay the bills. So am I correct in my understanding that the only problem with my answer is that it should have been posted in comments rather than as an answer? Commented May 10, 2017 at 16:55
  • That's right. Welcome to the site once again
    – Zaid
    Commented May 10, 2017 at 16:56

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