After getting the community's help on my previous question, I sent my cylinder heads to a local machine shop to get a valve job done (valve seats cut, valves reseated, and valve clearances redone). After I got the heads back, I installed the heads onto my short block (which is on an engine stand) without the camshafts and decided to run a leakdown test. This was a warm and fuzzy test. My expectation is that there should be enough pressure building up where I can no longer hold the crankshaft stationary as I expect a well sealed system to rotate the crankshaft.

However, I found that Cylinders 2,3,4 all leaked a good amount of pressure out the exhaust/intake valves. I was putting in about 70 psi and couldn't get more than 40 psi out of the cylinders. With Cylinder 1, there was enough pressure in the system where I could no longer hold the crankshaft stationary with a wrench.

My expectation is that every cylinder should have behaved like Cylinder 1 so I called the machine shop and the machinist asked me to bring the heads back in. After several days, the machinist is claiming that he can't find anything wrong. When lapping the valves back in, they lapped right away.

The machinist's claim is that there may be some lapping compound on the valve and/or valve seat that's preventing a full seal and states that after running the engine 100-200 miles, all the remaining lapping compound will either dissolve from the gasoline or burn out from the exhaust and a good seal should be made.

Obviously, I'm wary for this claim as I won't be able to know until after I put the engine back in the car. What do you all think?

Edit: I looked at the disassembled head at the machine shop. There's no excess compound anywhere, just the dull grey marks left from a lap job. The machinist is claiming that whatever small amounts of grit left on the lap job can cause pressure to leak. We looked at the lap ring and we both observed a nice thick, even ring around both the valves and valve seats. That was nice to see, but there's still a part of me that would like a good leakdown test before putting everything together.

  • I'm a bit suspicious, but you could try dissolving the lapping compound using a solvent and see if that improves the seal at all.
    – GdD
    Commented Aug 20, 2020 at 16:42
  • Did you rotate thee piston to TDC firing stroke when doing that cylinder's test? Not clear from your story...
    – Solar Mike
    Commented Aug 20, 2020 at 16:54
  • @Solar Mike, yes cylinders were TDC and camshafts were not installed so no issue on the valves being pushed down.
    – Joe
    Commented Aug 20, 2020 at 17:14
  • If this is on the exhaust side (lapping compound interference) causing this, then it's a good way to get a burnt valve. If burning exhaust gasses are being expelled past the valve, it can surely happen. I'm not sure why the machine shop didn't clean the compound in the first place. If it's up on the stem of the valve, it'll wear out a valve guide really quick and no amount of running will cure this (or clean it). If the compound is their excuse, I'd take it back to them and tell them to clean it as it should be. Commented Aug 20, 2020 at 17:16
  • The right way to clean off lapping compound is 5 or 10 minutes in an ultrasonic cleaner filled with hot detergent solution. But if your machine shop didn't do that as part of the original job, good luck getting them do it now!
    – alephzero
    Commented Aug 20, 2020 at 18:04


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