milled head + euro motor = thicker head gasket?

[31Hertz]

I FINALLY got the head to the machine shop. I am getting things done at a snail's pace.

The machine shop took .01" off the bottom of the head. This equates to .254mm. Given that the pistons are the 10 to 1 compression ratio euro-style; would anyone recommend that I install the .3mm thicker head gasket? Or is it safe (interference-wise and/or compression-wise) to run the stock-thickness gasket?

I ran various searches on mye28 and .info. I read that many folks went up to .03" or even .04" milled and still ran the stock thickness without issue, but none of the results were in reference to the 10 to 1 piano-domed piston. Aside from that fact, my ECU is an early fixed-mapping with added O2 sensor; so would I run the risk of pinging with a stock gasket?

I do not care to spend the extra cash for the thicker gasket. I just want to be informed and ecourage discussion.

[Barney Paull-Edwards]

1 thou is irrelevant,But you might want to remember that it shortens the Cam chain,or do you mean 1mm, whole different ball game.If you really want to keep CR the same then machine off the piston heads, get out a calculator to calculate the volume of metal required to take off.

[31Hertz]

definitely just .01 (one-hundredth) inches or .254 mm...

[Ross]

Stock gasket. No worry about chain, the tensioner can cope with .010
EDIT Bently says .012 is the limit for machining and BM recommends.3mm over gasket, who am I to argue. I think the concern is detonation not piston to valve "interferrence.

[whiskychaser]

definitely just .01 (one-hundredth) inches or .254 mm...
I would like someone to say that if you take ten thou off the head and use the standard gasket, then the pistons definitely wont hit the valves. I wont :)

[31Hertz]

I would like someone to say that if you take ten thou off the head and use the standard gasket, then the pistons definitely wont hit the valves. I wont :)

1/4 of 1mm doesn't seem like much, but yeah; I am hoping to hear something like that as well. My gut tells me it won't, but my gut is also the place that makes poop...

It will shorten the chain which means slightly retarded cam timing. I am not so worried about that. Or will this actually work in my favor to avoid detonation?

The compression ratio will be around 10.25 to 1 according to reliable sources.

Apocalypse or Sunshine?

[whiskychaser]

It will shorten the chain which means slightly retarded cam timing. I am not so worried about that. Or will this actually work in my favor to avoid detonation?

Not familiar with your motor but if you shorten the distance between the top and bottom sprockets, surely the chain is longer? :) Mine has got little smilie faces on the tops of the pistons. I guess thats cos there isnt much clearance between the piston head and valves. So are you going to put in the 'thin' gasket and live dangerously?:)

[Turbo Ready]

Not familiar with your motor but if you shorten the distance between the top and bottom sprockets, surely the chain is longer? :) Mine has got little smilie faces on the tops of the pistons. I guess thats cos there isnt much clearance between the piston head and valves. So are you going to put in the 'thin' gasket and live dangerously?:)


Was wondering the same thing, you beat me to it :D

[Jon K]

The stock HG thickness is 0.070" and you took off 0.010"... You should run a 0.080 gasket and yes, 0.010" does make a difference! That would probably raise compression from 9.0 to 9.5

[31Hertz]

Not familiar with your motor but if you shorten the distance between the top and bottom sprockets, surely the chain is longer? :) Mine has got little smilie faces on the tops of the pistons. I guess thats cos there isnt much clearance between the piston head and valves. So are you going to put in the 'thin' gasket and live dangerously?:)

The chain will slacken more, but the tensioner will take care of that. HOWEVER when the cam sprocket and crankshaft are lined up to TDC there will be ever-so-slightly more slack on the guide rail side of the chain. Of course when the motor starts this slack will go to the tensioner side; which means the cam will be slightly behind where it would normally be in the stock setup.

I am still riding the fence on this one...

[whiskychaser]

motor starts this slack will go to the tensioner side; which means the cam will be slightly behind where it would normally be in the stock setup.
I am still riding the fence on this one...
Like I said, I dont know your motor. On an M50 you pre-tension the main chain before you tighten the top sprocket bolts. That gets rid of the slack but doesnt get round the fact your chain is "longer" so your timing will still be behind. Over its life I'm sure a chain stretches more than a couple of thou:)
This may help you decide which side of the fence you want to be on: :)

[beetos]

:)
The stock HG thickness is 0.070" and you took off 0.010"... You should run a 0.080 gasket and yes, 0.010" does make a difference! That would probably raise compression from 9.0 to 9.5

If possible, I am looking for some kind of relationship between head thickness reduction and CR. I hear all kinds of theory on how much CR changes for a different deck height. Care to explain this a little bit more as .5 a point seem like a lot for a typical 10 thou skim.

Cheers.

[31Hertz]

WOW whiskychaser!!! That is a damn scary looking photo. How did the pistons fare in that battle? Was a stretched chain or misaligned cam due to blame for that carnage? Are those from an M30? Just curious.

I should explain why I am still on the fence: I am asking this same question on bigcoupe.com. This site is very helpful for me since there are many members from England who by default deal with Euros all the time. A Mechanical Engineer (Sixerkid) is the one who came up with the new compression ratio of btwn 10.2 and 10.3 to 1 albeit a scratch calculation (from a stock baseline of 10 to 1). Another member there who really knows his stuff is Brucey. He is squarely from the UK. Neither of them seem to think the stock gasket will do any harm. http://bigcoupe.com/phpBB2/viewtopic.php?t=4635

So I have conflicting answers from two boards, both of which I really really trust and love.

I think I will have the machine shop measure head thickness at each combustion chamber before I make my decision. I think this is the one factor missing in the equation.

Again, I thank you all for your help in this, and I will report back when I have this new information.

[whiskychaser]

WOW whiskychaser!!! That is a damn scary looking photo. How did the pistons fare in that battle? Was a stretched chain or misaligned cam due to blame for that carnage? Are those from an M30? Just curious.

M50. Stretched chains did it. Main one slipped. No damage to head or pistons but I was in a car park so no speed at all. Good idea to have the head measured-especially as you may not be able to tell if it was skimmed once before you had it done. I'm in the UK too but given the pick trust you appreciate my reservations :)

[Bellicose Right Winger]

You can't make a exact calc, but you can estimate the worst case if you assume the CC+DH (combustion chamber plus deck height volume) is reduced by a volume equivalent to the diameter of the cyl bore with a height equal to amount machined off head. This results in a higher then actual CR increase because a .254mm slice off the combustion chamber is usually a smaller volume change.

On a 2494cc M50 engine with 84mm bore, 10:1 CR
CSV = Cylinder swept volume = 2494/6 = 415.6CC

CR = (CSV + (CC+DH))/(CC+DH) = 10
Solve for CC+DH = 46.18CC
84mm dia cyl, .254mm tall = 1.41CC

New CR = (415.6 + (46.18-1.41)) / (46.18 - 1.41) = 10.3

Paul Shovestul





:)

If possible, I am looking for some kind of relationship between head thickness reduction and CR. I hear all kinds of theory on how much CR changes for a different deck height. Care to explain this a little bit more as .5 a point seem like a lot for a typical 10 thou skim.

Cheers.

[beetos]

You can't make a exact calc, but you can estimate the worst case if you assume the CC+DH (combustion chamber plus deck height volume) is reduced by a volume equivalent to the diameter of the cyl bore with a height equal to amount machined off head. This results in a higher then actual CR increase because a .254mm slice off the combustion chamber is usually a smaller volume change.

On a 2494cc M50 engine with 84mm bore, 10:1 CR
CSV = Cylinder swept volume = 2494/6 = 415.6CC

CR = (CSV + (CC+DH))/(CC+DH) = 10
Solve for CC+DH = 46.18CC
84mm dia cyl, .254mm tall = 1.41CC

New CR = (415.6 + (46.18-1.41)) / (46.18 - 1.41) = 10.3

Paul Shovestul

Thanks, here is a spreadsheet for others to play with.

[Jon K]

You can't make a exact calc, but you can estimate the worst case if you assume the CC+DH (combustion chamber plus deck height volume) is reduced by a volume equivalent to the diameter of the cyl bore with a height equal to amount machined off head. This results in a higher then actual CR increase because a .254mm slice off the combustion chamber is usually a smaller volume change.

On a 2494cc M50 engine with 84mm bore, 10:1 CR
CSV = Cylinder swept volume = 2494/6 = 415.6CC

CR = (CSV + (CC+DH))/(CC+DH) = 10
Solve for CC+DH = 46.18CC
84mm dia cyl, .254mm tall = 1.41CC

New CR = (415.6 + (46.18-1.41)) / (46.18 - 1.41) = 10.3

Paul Shovestul

I am not sure that is really fair in terms of M30 head volume. An M50 has a greater bowl and can afford some removal. I am still int he train of thought that the removed squish area is going to considerably alter the compression ratio.

If we were talking 0.003 or 0.005 we'd be fine. But 0.010" is 15% of your headgasket thickness removed. I know its not a reasonable calculation but removing 15% from your head gasket thickness would yield a considerable bump in compression. I'd be concerned about valve clearances for one, but tuning for two.

There is a reason BMW offers two sizes on the headgasket for an M30:

02 CYLINDER HEAD GASKET ASBESTOS-FREE 1,72MM 1 11121730223 $101.56
02 CYLINDER HEAD GASKET ASBESTOS-FREE 2,07MM(+0,3) 1 11121730224 $101.56

0,3mm is about 0.011"... which is damn close to exactly what you removed from the head. Maximum removal from the head is 0.012" and the +0,3 mm HG is like 0.0119 or 0.0118 or something (tired). There's no reason for this coincidence.

[beetos]

My head has been hacked to ****. It is WELL under what the minimum recommended deck height. I have no issues what so ever. I run a custom cam anyway to I have a modied cam sprocket to time the cam so that that looks after that issue and my piston are fly cut for intake valves. As far as decking the **** out of a stock head, I can not comment but have heard of other who are well under on head height also and have no issues.

[Bellicose Right Winger]

This calculation will work for M30 engines also. I didn't have the Euro M30 data so I used the M50. The calculation assumes that the combustion chamber is circular and the same diameter as the bore. Therefore the change in combustion chamber volume is bore area x amount machined. Your photo shows that combustion chambers often are not circular due to squish area.

http://blowneuroz.com/mygallery/E34%20525i%20Motor%20Build/head4.jpg

My scrap M30 head has only (1) squish area but the M50 has (2). The flat squish area (below the exhaust valves and above the intakes in photo) is the main reason the actual CR increase is LESS then the calculation predicts. Since the squish area doesn't add to the combustion chamber volume, it won't reduce the combustion chamber volume when machined away.

Paul Shovestul





I am not sure that is really fair in terms of M30 head volume. An M50 has a greater bowl and can afford some removal. I am still int he train of thought that the removed squish area is going to considerably alter the compression ratio.

If we were talking 0.003 or 0.005 we'd be fine. But 0.010" is 15% of your headgasket thickness removed. I know its not a reasonable calculation but removing 15% from your head gasket thickness would yield a considerable bump in compression. I'd be concerned about valve clearances for one, but tuning for two.

There is a reason BMW offers two sizes on the headgasket for an M30:

02 CYLINDER HEAD GASKET ASBESTOS-FREE 1,72MM 1 11121730223 $101.56
02 CYLINDER HEAD GASKET ASBESTOS-FREE 2,07MM(+0,3) 1 11121730224 $101.56

0,3mm is about 0.011"... which is damn close to exactly what you removed from the head. Maximum removal from the head is 0.012" and the +0,3 mm HG is like 0.0119 or 0.0118 or something (tired). There's no reason for this coincidence.

[Jon K]

This calculation will work for M30 engines also. I didn't have the Euro M30 data so I used the M50. The calculation assumes that the combustion chamber is circular and the same diameter as the bore. Therefore the change in combustion chamber volume is bore area x amount machined. Your photo shows that combustion chambers often are not circular due to squish area.

http://blowneuroz.com/mygallery/E34%20525i%20Motor%20Build/head4.jpg

My scrap M30 head has only (1) squish area but the M50 has (2). The flat squish area (below the exhaust valves and above the intakes in photo) is the main reason the actual CR increase is LESS then the calculation predicts. Since the squish area doesn't add to the combustion chamber volume, it won't reduce the combustion chamber volume when machined away.

Paul Shovestul


Well until he actually takes the head and CC's it to find out the displacement all the theory of squish area not affecting it "as much" is just that, theory. I have been tuning a bunch of different engines of different flavors and I can tell you, I created a fuel map for a M50TU that was only 10.8:1 from the stock 10.5:1 and it needed about 8 - 10% more fuel across the board for it to be happy.

When raising compression, seek tuning from someone who knows what they're doing. There is nothing like running too little fuel and too much advance on what is "not that much" of a compression bump. Take it 0.1:1 higher, and you are no longer running a properly tuned system.

[31Hertz]

The machine shop measured total thickness for my cylinder head...
The final measurement is 5.075 inches.

The stock specification is 5.079" +/- .004". The range is 5.075" ~ 5.083"... 0.004" = 0.1016mm.

So the shop cat overestimated on material removed unless I started out with a "thicker than stock" head.

I am going to borrow some calipers and verify this measurement. If it holds true, I think I will go with the stock head gasket.

Feel free to opine; given this new information. My mind is still open.

I hope to have some pictures of the cleaned up head soon. It looks quite nice.

[Evan]

my machine shop took off .006 inches and I'm going with the stock head gasket...

if i were you, i'd order a couple thicker head gaskets and use them... it may take a week to get them in your hands