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Old 08-24-2012, 03:42 AM   #1
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Default Huge leak down numbers after valve job

Recently had my valves and seats dressed (valve job) after putting the head
back together the leak down on my cylinders were as follows.
#1 - 35%
#2 - 38%
#3 - 32%
#4 - 48%

The engine was already together and I had a baseline dyno session lined up so I took the car down to see what kind of numbers I was making. After getting the fluids warm the 3 pulls came back at 146hp and 106tq, the engine sounded weak and seemed to struggle throughout the rpm range In comparison, after getting the car here from Boston and tuning it to altitude It mads 156/112. Unfortunately, I did not do a leak down test before that dyno session. Engine is a limited prep f-prod for scca competition.

After the dyno session I pulled the head and took in to the shop that my dyno shop recommended. I feel much better about this shop than the one that performed the faulty valve job. One man operation, took time to get to know what I was doing with the engine, and walked me around his shop. He instills confidence in his customers, on a side bar he also builds cylinder heads for T-6 race planes.

On the original job I was pressed for time so the head was not glass beaded, and I did the tear down and build. I wasn't worried about carbon in the runners or combustion chamber my thought was as long as I had good matting surfaces I was going to be ahead of the game.

Here is a pic of one of the intake seats, shiny area is good seal, carbon area is leaking.

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Old 08-24-2012, 09:34 AM   #2
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That usually happens if the seats have been cut too many times. You said you assembled the head- I assume you didn't hand over any of the valve train to the shop so they could check it after a cut?
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Old 08-24-2012, 10:08 AM   #3
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How, does how many times seats have been cut cause this? The machinist had all valves, springs, abundant and retainers. Speaking with the original owner, there was only a single valve job done performed, and the seats only needed to be lightly dressed.
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Old 08-27-2012, 11:30 PM   #4
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Sloppy machinist. Our guy vacuum tests each port to make sure the seats are perfect before the head leaves his shop. There's absolutely no excuse for this - go get your money back.
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Old 08-28-2012, 11:37 AM   #5
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I saw this on a second seat cut. Like with all things to be machined, the starting point should be observed first regardless of what has been done in the past. Similar to wall testing with an overbore- just because it should be X thick, doesn't mean it is.
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