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Question about head machining service marks

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Question about head machining service marks

 
Old 09-09-2018, 07:44 AM
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Default Question about head machining service marks

I am doing some engine work on my AP1 that overheated. I got the head resurfaced, but I am not sure how much. All I know is that it has not hit the limit mark.
This is what I am talking about when I say limit mark. The head and the block have these marks that tell how much they can be machined.

I hear all of this talk about resurfacing messing up the timing chain, tensioner, valve to piston clearance etc. So my question is, do I need to worry about this at all, knowing that the limit mark has not been hit? Is the limit mark a reliable way to know you are within spec? On a brand new head, would the height of the head from the limit mark be exactly equal to .008” (the head resurfacing limit stated in the manual)? Thank you in advance for your help.
Hoping to use an oem headgasket since I already have it in hand.
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Old 09-09-2018, 08:02 AM
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Just used a caliper to measure the limit marks on my engine block to be 0.8mm. But the engine has been rebuilt before, not sure if they resurfaced it or not.
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Old 09-09-2018, 12:05 PM
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Im not aware of any measurable point that the service manual references. Or a point that anyone here references on the gasket surface area to measure other than measuring valve cover gasket surface to headgasket surface.

however, any competent machine shop should tell you how much they took off the head. If they can't tell you, never go back there.
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Old 09-09-2018, 01:49 PM
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If you are running standard components you should be OK as factory clearances aren't super tight. Its only when you start changing bore/cams/valves that things get funky.

I've found the clearance issues are mostly around where the edge of the piston crown that sticks up out of the block fits inside the combustion chamber. That area is cast into the head and has an uneven radius and sometimes isn't exactly concentric with the bores. Fortunately, if it does touch here its only slight and tends to make clearance on startup , sounds brutal, but when I've seen it its only a tiny edge on the casting.

What I do is drop the head onto the block with no head gasket. Don't worry about bolting it down. Make sure the head touches the block all the way around, this makes sure that the dowels aren't propping the head up. ( only an issue when a lot has been taken off the block/head. Next, turn the crank over so the pistons swing past TDC. Rock it back/forth while you are holding down the head and listen/feel for any touching. You will need to turn the engine half a turn to pickup the other pair of pistons. Hold the timing chain tight so it doesnt get tangled around the crank or oil pump chain.Turn the workshop radio off so you can hear any taps or clunks.

If you get no touching you will be good to go. The additional thickness of the head gasket is the right minimum clearance you need to run safely. If its not touching without a head gasket you shouldn't have any issues.

Good luck with the built, good to see guys getting dirty and learning about engines/
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Old 09-10-2018, 01:28 PM
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Chris, when I do the test you mentioned, should the timing chain should be connected, right? Wouldn't the valve timing be retarded due to the head being lower with no head gasket? Thanks for your reply. I already bought some playdough, so I might just clay it tbh.
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Old 09-10-2018, 01:53 PM
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According to this post, the head resurface service limit is .008” & that new cylinder head height is 4.132" - 4.136" (104.95 - 105.05mm). So if we subtract .008” , the head must measure at least 4.124" - 4.128" (104.75mm - 104.85mm) to still be useable.

mine is 4.112", which is .012" too much taken off.
What do you guys think? Get a thicker head gasket, or just run the factory one?

Last edited by janders111; 09-11-2018 at 06:10 AM. Reason: grammar error
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Old 09-10-2018, 04:42 PM
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Chris, I did your test and the pistons do touch the head. . This project is becoming a money pit. Also it seems that the block may be warped about .06mm
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Old 09-10-2018, 05:15 PM
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I have a good low mileage AP2 head if you need one. Just send me a PM
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Old 09-10-2018, 06:35 PM
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My pistons are Honda pistons, but I believe they are oversized, because they are marked with a 25 (instead of an A or B).
Stock pistons come in 0.25mm oversize and stick out of the hole 1mm.
This may be the part of my problem, But I imagine if the machine shop did not machine my head past spec I would not be having this issue.
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Old 09-11-2018, 01:25 PM
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Might just be talking to myself now, but I am getting a .046" extra thick head gasket from Cometic. Hoping this will solve the issue.

CONCLUSION: the mark I pictured in the first post is not a reliable way to measure the tolerances as far as decking and head milling. You can have major problems before hitting any of the marks. Instead, use the thickness of the head from the manual, and the test that Chris told us.

Last edited by janders111; 09-11-2018 at 01:27 PM.
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