Shims
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Shims
So the machine shop called me telling me the head work is done, but when I went to pick it up they told I need to buy a shim because the head has been previously worked on. Only thing I want to know is if anyone is boosted and running a shim? Thanks
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You will also need to get the head and block surfaced according to if you're running a MLS (cometic or cosworth) or an oem type.
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you dont need a thicker h/g....only thing is does is very very slightly raise your comp ratio which wont hurt anything being so miniscule....
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This.500hpKA240sx wrote:you dont need a thicker h/g....only thing is does is very very slightly raise your comp ratio which wont hurt anything being so miniscule....
The machine shop should have no reason to take enough material off the head to require a 'shim'. If they did it was for one of two reasons (that I can think of). First, you had a deep scratch or other surface imperfection. Second, your head was warped and took a significant amount of material to get it flat.
The problem is that if they took material off and the head wasn't flat to begin with, you will still run the risk of your cam journals not being concentric even if the bottom of the head is now 'flat'.
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Yea the head had some deep scratches that's why they had to machine it further, I understand it well raise the compression but I also understand the more they machine the closer the valve can come into contact with the piston, hence why I wuld need a thicker hg, correct? Thanks for the help and advice guys.
You can specify thickness with Cometic gaskets, there are a few sizes that are regularly stocked. You can get almost any thickness you want.
After decking the block and head my pistons were .007" out of the hole. While tuning I used a stock Felpro gasket and didn't have any issues until I popped it by detonating.
Piston to head clearance should be one of your main concerns if the block was decked a lot. With stock redline and upgraded rods and pistons Arias recommended I use minimum .028" clearance. So maybe .032" for stock rods/pistons? Anyone else have input on this?
Just be careful when adding HG height, don't try to change the compression ratio much with it, you will make detonation worse.
EDIT: Spelling
After decking the block and head my pistons were .007" out of the hole. While tuning I used a stock Felpro gasket and didn't have any issues until I popped it by detonating.
Piston to head clearance should be one of your main concerns if the block was decked a lot. With stock redline and upgraded rods and pistons Arias recommended I use minimum .028" clearance. So maybe .032" for stock rods/pistons? Anyone else have input on this?
Just be careful when adding HG height, don't try to change the compression ratio much with it, you will make detonation worse.
EDIT: Spelling
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By piston to head clearance, are you referring to piston to valve clearance? or total volume of the combustion chamber?p00t wrote:You can specify thickness with Cometic gaskets, there are a few sizes that are regularly stocked. You can get almost any thickness you want.
After decking the block and head my pistons were .007" out of the hole. While tuning I used a stock Felpro gasket and didn't have any issues until I popped it by detonating.
Piston to head clearance should be one of your main concerns if the block was decked a lot. With stock redline and upgraded rods and pistons Arias recommended I use minimum .028" clearance. So maybe .032" for stock rods/pistons? Anyone else have input on this?
Just be careful when adding HG height, don't try to change the compression ratio much with it, you will make detonation worse.
EDIT: Spelling
For piston to valve clearance you can dance on either end of the valve lash adjustment to compensate for head resurfacing I'd imagine. You can run on the looser end of the spec to accomodate for the lost material on the head. Right?
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Hmmm no I was referring to the distance between the top of the piston and the bottom surface of the head. The rods stretch with RPM, just something to watch if you are banging off the limiter or raising the redline.TryingToTurbo wrote: By piston to head clearance, are you referring to piston to valve clearance? or total volume of the combustion chamber?
For piston to valve clearance you can dance on either end of the valve lash adjustment to compensate for head resurfacing I'd imagine. You can run on the looser end of the spec to accomodate for the lost material on the head. Right?
I'm not sure how much we have to worry about valve to piston clearance, at least with stock cams. I mean you read about all the people installing their cam shafts wrong all the time. One tooth off is 18 degrees or something, if it was critical I would think that would cause some interference.
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That is definitely not what you should take away from this discussion. Assuming you set your timing correctly, no there shouldn't be any worry.signals13 wrote:so because im running a stock head i shouldnt worry too much about valves coming in contact with the piston?
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I had to sand a couple of shims down on a belt sander and installed the unmollested side towards the cam... had a mic to measure them so I took a little bit off at a time til it was perfect.
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Picture a new vs old head with 1/8 inch differenceRick wrote:schmauster920 wrote:Having the head over machined throws off the cam timing but not enough to really matter
What???
The idler sprocket sits lower after the head has been machined, the main chain has more slack for the tensioner to take up but we set our marks in the same place.
This will throw the centerline of the cams off very very slightly unless you mess with your cam timing, shouldnt be enough to notice or matter
AKA there is less room between the head and block but the chain is the same length so the idler will sit in a slightly skewed position
Last edited by schmauster920 on Sat Mar 23, 2013 10:20 am, edited 1 time in total.
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While this is true... the older the chain is the longer it gets which would also throw off timing by a similar amount. IMO almost negligible.schmauster920 wrote:The idler sprocket sits lower after the head has been machined, the main chain has more slack for the tensioner to take up and therefore will throw the centerline of the cams off very very slightlyRick wrote:schmauster920 wrote:Having the head over machined throws off the cam timing but not enough to really matter
What???
AKA there is less room between the painted links when this happens
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And when I throw a rock into the ocean it theoretically goes up just a little bit.wannabethestig wrote:While this is true... the older the chain is the longer it gets which would also throw off timing by a similar amount. IMO almost negligible.schmauster920 wrote:The idler sprocket sits lower after the head has been machined, the main chain has more slack for the tensioner to take up and therefore will throw the centerline of the cams off very very slightlyRick wrote:schmauster920 wrote:Having the head over machined throws off the cam timing but not enough to really matter
What???
AKA there is less room between the painted links when this happens
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http://www.google.com/#hl=en&output=sea ... 95&safe=onsignals13 wrote:Ok so I got my head back from the machine shop and I was told I need an hg 10 thousands thicker than stock, can anyone help me findout what size that is?
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Do a search for head gasket thickness or something similar. For example: viewtopic.php?t=52526&highlight=head+gasket+thicknesssignals13 wrote:Yes I did google that, thank you. I also know that the stock hg size is about .050+ .010 that i'm over should equal .060 which I end up with 1.5mm correct?
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This conversation got me thinking - I wouldn't think it would make much of a difference either but I did a quick math problem.Rick wrote:And when I throw a rock into the ocean it theoretically goes up just a little bit.wannabethestig wrote:While this is true... the older the chain is the longer it gets which would also throw off timing by a similar amount. IMO almost negligible.schmauster920 wrote:The idler sprocket sits lower after the head has been machined, the main chain has more slack for the tensioner to take up and therefore will throw the centerline of the cams off very very slightlyRick wrote:
What???
AKA there is less room between the painted links when this happens
Okay let's saying 0.010" decking would mean 0.020" slack in the chain on one side (because one side stays tight). Assuming about a 2.5" diameter cam gear (don't have one to measure), the chain only has to move about 0.022" to rotate the cams by a degree. So a 0.020" slack next to a 0.022"/degree might be noticeable.
Again this has the assumption that the slack isn't taken up by the tensioner; I don't know how much extra it picks up - 0.020" doesn't sound like a lot but if it's already maxed out in normal operation it could make a difference.
0.010" is a lot for a simple decking; in most cases we're taking off something like 0.004-0.008" right?
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Yea did that already lol that's why i'm stuck just want to be sure what hg size I would need.p00t wrote:Do a search for head gasket thickness or something similar. For example: viewtopic.php?t=52526&highlight=head+gasket+thicknesssignals13 wrote:Yes I did google that, thank you. I also know that the stock hg size is about .050+ .010 that i'm over should equal .060 which I end up with 1.5mm correct?
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Yea that sounds about right, i'm not sure if that is the amount that was removed but the machine shop did say the hg needs to be that much thicker.airman wrote:This conversation got me thinking - I wouldn't think it would make much of a difference either but I did a quick math problem.Rick wrote:And when I throw a rock into the ocean it theoretically goes up just a little bit.wannabethestig wrote:While this is true... the older the chain is the longer it gets which would also throw off timing by a similar amount. IMO almost negligible.schmauster920 wrote: The idler sprocket sits lower after the head has been machined, the main chain has more slack for the tensioner to take up and therefore will throw the centerline of the cams off very very slightly
AKA there is less room between the painted links when this happens
Okay let's saying 0.010" decking would mean 0.020" slack in the chain on one side (because one side stays tight). Assuming about a 2.5" diameter cam gear (don't have one to measure), the chain only has to move about 0.022" to rotate the cams by a degree. So a 0.020" slack next to a 0.022"/degree might be noticeable.
Again this has the assumption that the slack isn't taken up by the tensioner; I don't know how much extra it picks up - 0.020" doesn't sound like a lot but if it's already maxed out in normal operation it could make a difference.
0.010" is a lot for a simple decking; in most cases we're taking off something like 0.004-0.008" right?
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