lapping valves after a valve job

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lapping valves after a valve job

Post  514Fox87 on March 2nd 2010, 7:09 pm

I was always taught that after a three angle valve job that you need to lap in the valves before putting the heads together. The guy who flowed the heads for me told me NOT to lap them because it will cut out the sharp ridge from the valve job and hurt flow. He said after starting the engine the valves will seat on their own. Engine masters, educate me please! You guys are the best!

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Re: lapping valves after a valve job

Post  69F100 on March 2nd 2010, 8:09 pm

I have a machine shop tell me not to lap a 3 angle valve job that there is no used to unless you want to mess the 3 angles up he said like you said it will cut the the angle edge off if it true or not I don't know I took him at his word and just put the heads on and never had a problem until I dropped a valve about 35,000 miles later

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Re: lapping valves after a valve job

Post  c.evans on March 2nd 2010, 10:50 pm

I do a drop test to check for concentricity and I LIGHTLY lap the valves to do a visual check of the seats. At the end of the day, the quality of your valve job is no better than the condition of your guides. They have to be straight, round and have the proper stem to guide clearance.

Yes, I also have the high dollar Serdi, Sunnen and Newan carbide cutters and we do five angle valve jobs and full radius valve jobs.

Charlie

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Re: lapping valves after a valve job

Post  nuclearcobra on March 3rd 2010, 7:38 am

What about when you buy a set of heads minus valves then get the valves later do you just lap them in or do they need to go to the machine shop for a valve job ? also i had a buddy that took all the valves out of a pair of heads for cleaning and never marked where they go, can they just be lapped in and be ok again ?

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Re: lapping valves after a valve job

Post  the Coug on March 3rd 2010, 7:52 am

the problems I see with most people lapping valves is they use enough lapping compound to do 20 sets of heads on one valve... Shocked if the valve job is done properly they will run without lapping, I agree with Charlie about the drop test though....


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Re: lapping valves after a valve job

Post  nuclearcobra on March 3rd 2010, 8:02 am

What about what my buddy did took out all the valves but didnt mark where they go do you just lap them then ? and then the new heads with new valves purchased elsewhere do you just lap those also or do both need to go to the machine for a valve job ?

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Re: lapping valves after a valve job

Post  DaveMcLain on March 3rd 2010, 8:12 am

I can't believe any reasonable person would ever take off enough material with a small amount of lapping compound to make any difference, nonsense. If that were the case the ace valve job would be good for about 30 seconds of runtime with typical guide clearances and fuels.

With that said I think it can be a good way to double check things.

Your buddy needs to have the heads valve jobbed, lapping is not going to help the heads work properly.

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Re: lapping valves after a valve job

Post  nuclearcobra on March 3rd 2010, 8:54 am

Is that answer for both of my ? the valves not put back in the right place ? and the new set of heads with new valves purchased separately?


Last edited by nuclearcobra on March 3rd 2010, 1:28 pm; edited 1 time in total

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Re: lapping valves after a valve job

Post  rmcomprandy on March 3rd 2010, 9:48 am

IF the valves are round and IF the seats are round and concenrtic with the guides then they do not need any kind of lapping.
However, how do you know...???

Lapping the valve is a fast and easy way to be sure, (if you are just looking to see witness marks and not FIX an unseating valve).

NOW, a seat micrometer will do the same thing but, it takes a "feel" for using one and it certainly is not as inexpensive.

A "drop" test will tell you that the seat is at least as concentric within the parameters of the valve guide to stem clearance dimesion but, won't distinguish a slightly "potato chiped" seat or valve.

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Re: lapping valves after a valve job

Post  chuck stevens on March 3rd 2010, 11:36 am

Ah' use some 1200 grit aluminum oxide, water based compound to check things. Fortunately, I've never had a problem with the guy that does my work. Very Happy Very Happy Very Happy

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Re: lapping valves after a valve job

Post  nuclearcobra on March 3rd 2010, 1:01 pm

rmcomprandy wrote:IF the valves are round and IF the seats are round and concenrtic with the guides then they do not need any kind of lapping.
However, how do you know...???

Lapping the valve is a fast and easy way to be sure, (if you are just looking to see witness marks and not FIX an unseating valve).

NOW, a seat micrometer will do the same thing but, it takes a "feel" for using one and it certainly is not as inexpensive.

A "drop" test will tell you that the seat is at least as concentric within the parameters of the valve guide to stem clearance dimesion but, won't distinguish a slightly "potato chiped" seat or valve.
Thank you that was very well explaned i love learning new stuff and thanks to Melvin for starting this post. I have one more ? tho last year when i blew my 302 to pieces i put a small mark like a fingernail mark would leave in clay in two of the valves in my world products iron heads how can i tell if i bent a valve or is that something that a machine shop has to do ?

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Re: lapping valves after a valve job

Post  514Fox87 on March 3rd 2010, 1:34 pm

Could someone explain the "drop test" and how it is done and how to look for problems. Thanks everyone for the replies. This has been educational.

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Re: lapping valves after a valve job

Post  c.evans on March 3rd 2010, 4:28 pm

514Fox87 wrote:Could someone explain the "drop test" and how it is done and how to look for problems. Thanks everyone for the replies. This has been educational.

1. With the guides straight, round, and snug,,,, in other words in good condition, not lose and worn out.

2. After you have accurately dressed your stone and ground the valves.

3. After you have done a careful valve job.

4. With the head upside down and the seats facing up, barely insert the valve in the guide.

5. With the valve held about 1.5 inches above the seat, drop the valve and it should bounce 2 or 3 times and ring like a church bell. That seat is concentric to the guide centerline.

6. If you drop the valve and it goes "thunk" and stays on the seat, then one side or the other is wedging against the seat. If your lightly raise the valve off the seat with your fingertip, from the stem end, you can feel some light resistance, that's further proof that you need to recut the seats.

Hope this helps,

Charlie

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Re: lapping valves after a valve job

Post  The Pope on March 3rd 2010, 5:10 pm

Thanks for the "Drop Test" info Charlie! cheers

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Re: lapping valves after a valve job

Post  514Fox87 on March 3rd 2010, 5:12 pm

That helps a lot! Thanks Uncle Charlie! cheers

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