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Lapped in valves?

Submitted by simon_amos on Sat, 06/10/2012
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Having lapped in the valves and reassembled the engine I can hear a tiny bit of hissing from the valves when under compression.

Would I be correct in assuming (hoping) that this is normal and that when in use a small amount of carbon will deposit on the mating surfaces in effect create the airtight seal ... or have I got it wrong?

Regards

Ian

Submitted by simon_amos on Sun, 07/10/2012

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After mulling on this overnight I decided to whip off the head and test the seals by filling the ports with petrol and looking for leakage.

The inlet valve showed a fine film of fuel seeping past the mating services and the exhaust valve had ust the tiniest leak showing.

Looks like I didn't lap the joints in properly.

Mmmmm ...

Any comments?

Submitted by john_shorter on Sun, 07/10/2012

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Yep, you've got it right, wrong first time! Carbon build up will not seal valves, it will make matters worse. John.

Submitted by simon_amos on Sun, 07/10/2012

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So started lapping again, reassembled the inlet valve tested it with petrol again, all fine.

Relapped the exhaust valve, tested it, petrol seepage â?. bother.

Retested (for the hell of it) the inlet valve without altering anything, slight seepage!

What am I doing wrong?

Submitted by Xanna_jeannett… on Sun, 07/10/2012

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Previously wrote:

So started lapping again, reassembled the inlet valve tested it with petrol again, all fine.

Relapped the exhaust valve, tested it, petrol seepage â?. bother.

Retested (for the hell of it) the inlet valve without altering anything, slight seepage!

What am I doing wrong?

hello a valve seat cutter comes to mind hear it will help you to lap in better you should do your testing with paraffin or white spirit . you should do the testing of the valves with no springs fitted and spark plug in place . there should be no leaks from any where .

Submitted by simon_amos on Sun, 07/10/2012

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Previously wrote:

hello a valve seat cutter comes to mind hear it will help you to lap in better you should do your testing with paraffin or white spirit . you should do the testing of the valves with no springs fitted and spark plug in place . there should be no leaks from any where .

So would I be correct in thinking that to test I am to just pop the valves back in, fit a spark plug and then fill the cylinder head with paraffin and hope that the level does not drop?

Since posting my previous email I have lapped the valves again and they both appear to have no leeks, but will retest in the morning.

Thanks

Ian

Submitted by gordon_johnston on Mon, 08/10/2012

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Be circumspect before resorting to valve seat cutters - you can end up ceating more problems if you are not careful. Two things to check. Are the valves very slightly bent? You can get the impression of a nicely lapped in interface but you would still get seepage. Are the guides unworn and absolutely concentric? Wear in the guide could produce slight ovality, allowing an apparent perfect lap but disguising poor sealing. Again, all can look well but the seepage could still appear. For paraffin testing, I fill the exhaust port and inlet tract in turn and examine the underside of the head. You can get a good idea of just how much is seeping by looking directly. Finally, and there is no need to state this, but even a speck of dirt or grinding paste would stop the valve seating properly. But I am sure you have everything spotless. Gordon.

Submitted by simon_amos on Mon, 08/10/2012

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Checked the valves again this morning, this time using paraffin.

No immediate signs of seepage, but after 5 minutes there is just the tiniest bit of evidence of seepage, just a glimmer so to speak) around the lowest edge of the circunference.

Is this acceptable or does it have to be perfect?

Ian

 


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