M42club.com - Home of the BMW E30/E36 318i/iS
DISCUSSION => Swaps, Turbos, Buildups => Topic started by: thebrelon on October 10, 2013, 11:15:12 AM
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I did some reading but couldn't find any answer, so for those who used 86mm or 86.5mm pistons to build a M42 stroker, where did you get your headgasket from?
custom MLS or what?
thanks
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On the 2.1L M42 I have from Metric Mechanic, they sent me an M44 head gasket when I was putting the head back on. It works fine with the 87mm bore. As for the thickness, you'll need to do some figuring based on the compression ratio you are targeting though.
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M44 headgasket are 86mm in diameter if I'm right, so you have to make sure that the pistons don't go above the deck, right?
looking at MM website I couldn't find any M42/M44 headgasket for sale, but they talk about cometic MLS gaskets for FI MM engines in the M42 leaflet.
thickness is not an issue so far. :)
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Yeah, it wouldn't work so well if the pistons get within ~1mm of the deck. The PN stamped into it was definitely the BMW PN for the M44 HG. So.....choose your crank/rods/pistons wisely! MM manages to get 2.1L and 11.5:1 compression with the M44 HG, so I'd say that it's a good contender, and economical too.
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As mentioned you can just use the M44 gasket. The other option is to get a Cometic gasket which they can make for any bore size. VAC Motorsports is a distributor for them.
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MM passed along to me that the best headgasket for an M44 gasket needs to be made by Goetze - I know I'm spelling it incorrectly. They told me that this is the only one that will cheat a bit to run the S52 86.4mm piston -assuming you have an M47 crank to make that work, but that is another story.
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Athena do a 87mm gasket, http://www.rallyshop.it/index.php?main_page=product_info&products_id=22065&language=en bit pricey but another option.
If the m44 is 86mm and 1.7mm stock thickness, you would want to ensure the piston is below deck height to go bigger or equal bore.
I suspect the .15mm above deck the m42 uses is to achieve a 1.55mm squish clearance (Which acording to my reading is quite wide already, 1.2 is more ideal). This is somthing you loose if you have to stay below the deck to avoid the HG eg 1.75mm+. Squish/quench helps burn which helps power at lower rpm which is useful power.
Not sure if the numbers there are 100% but be aware choosing a piston to not hit the HG means your trading off another aspect of design which make power too.
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thanks for your replys guys, this is very interresting as usual...
to sum up:
- M44 gaskets are cheap and readily available.
- cometic gaskets are available in different thickness to adjust CR but 86mm only.
- athena gaskets are 87mm but one thickness only.
pick one!
to MLM, I'm not too familiar with quench/squish despite some readings, but to achieve a 1.2mm squish clearance do the pistons have to go 0.5mm above deck?
and what is the relationship between CR and squish? I mean with the same CR and lower squish same engine produce less power (?). does a higher CR compensate for a lower squish?
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You can get the Comotec in any diameter as I understand it.
Squish is the distance from the combustion chamber in the head to the piston crown around the edges of the piston. If you look at the shape of a piston and the combustion chamber you will see the outer edge of the piston crown mimics the shape of the combustion chamber. If the piston is a flat top design, then its harder to notice so you have to look at the combustion chamber and will notice it is also flat at the edges.
The purpose of squish is to push the air/fuel mixture into the center of the chamber for better combustion. If the mixture is allowed to hang out in this area (too big of a squish distance) it tends to not combust, or combust late. Essentially when this happens you are wasting energy potential.
Squish is a function of proper/improved combustion and is for the most part irrelevant of compression ratio. This is why increasing squish distance to get a bogger bore can lead to less power as you may have less complete combustion.
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that's my understanding too.
thanks for the explanations. it will be something to look into when the time comes...
what is the right distance then? I've read 0.05"/1.2mm as acceptable and 0.03"/0.7mm as correct.
0.7mm seems tight!
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Im not sure what the best option is for squish. A lot of it depends on how tight the bore is and if you expect the piston to rock in the bore.
When you have the pistons made, Im sure they will have a good answer that matches the specs they recommend for bore tolerance.
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found a good explanation in this thread http://forum.9000rpm.co.za/viewtopic.php?id=5272
These guys say 0.7mm is good on a 4age.
Worth cosidering when building for sure.
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yes it looks like "free" power when considering stock squish distance... from what you said it can be divided by 2 on M42!
another question regarding squish: what about squish for NA and FI engines? are they the same?
I'm asking because most FI moded engines use tall headgasket and/or different piston/rod combos to lower CR and consequentely increase squish distance (I presume)...
my idea is to design a high CR NA M42 as a first stage, and then lower CR to stock value and go FI at a second stage. so, if squish distance has to stay the same in both case (NA / FI) that means my plans to only change the headgasket between stage 1 and 2 won't work.
2 stages because $$$$$$ if you ask... and it may be years between each stage.
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I think the benifit of squish applies to any induction type maybe more so in a turbo where detonation is a real problem. :-\
Modded turbo engines with thicker gaskets would indeed upset squich to achieve a lower CR. Just happens to be the easiest way to reduce cr is a thick head gasket.
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Squish works the same for FI and atmo engines. This is a big part of the reason thicker head gaskets are frowned upon and not considered a proper solution. Ironically you are loweting CR but you are increasing the chance for detonation through improper combustion.
The reason really to lower the CR in a stock motor is due to the strength of the rods as they cannot handle major torque increases. Its not unusual to have a 10:1 turbo build, you just need to appropriately tune for it. Also this style build puts low end torque and response over peak power. Lots of people just want highway queens and high power dyno charts though :(
Due to financial costs, if you arent building the bottom end, a thicker gasket is a decent option if using pump petrol. Just realize what you are giving up, but like anything its a cost benefit analysis :)
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allright! very interresting once again!
then I will have to change my plan.
plan A was to use a crank from a M44, honda conrod and euro S50B32 pistons. around 12:1 CR NA. the whole idea was to keep machining to the minimum (boring) for cost reasons.
plan B will be M44 crank, M42/44 rods and S52B32 pistons and aim for a squish of 0.8/0.9 depending on pistons rocking. don't know yet how much CR to look for with that combo but I'd definitely want to be around 12:1 (E85)
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Just a thought. If you skimmed the block 0.65mm and kept everything else stock. You would get a squish of 0.8mm and 10.7:1 CR...
If my maths is right, it would be a relativly cheap rebuild option assuming you could get the chip to suit. Would require piston to valve checks but may yeild some HP otherwise missed out on.
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or use a 1mm thick cometic headgasket. that would be interresting to see how power we can get out of a M42 with its stock crank/rods/pistons...
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or use M50TUB25 pistons which have 0.65mm higher compression height and are flat top...
I will calculate new CR with that combo later.
I've to re-think the whole project and decide which way to go...
EDIT: M50TUB25 pistons have 0.9mm taller KH. sorry... but tehy can still be shaved to the correct value...
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According to the excell spread sheet i just whipped up...
If .65 above deck and flat top you get ~ 12.1:1. Squish distance on 1.7mm HG = 1.05mm
If .9mm above deck and flat top you get ~12.5:1. Squish distance on 1.7mm HG = 0.8mm
Machining to reduce compression would take a fair bit of meat off the piston.
Im was looking at a Nissan piston which gives 12:1. A squish distance on 1.3mm HG of .8mm (86mm bore). To get 11:1, I need to gain 4cc in the chamber as i dont want to machine the piston.
Or
Stick with 1.7mm HG and resultant 1.2mm squish and 11:1.
Do I shoot for a better squish of .8 but have to machine somthing to drop CR to say 11:1 OR go the easy way of 11:1 CR and wide squish gap with no machining. What i have learnt so far im not sure if i want to trade off squish gap...
Takes alot of math and playing with ideas to get numbers which sound right...
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thank you very much for your reply MLM. I use the same kind of spreadsheet but it seems I don't use the right numbers for the M42...
from calculation the dead volume of a stock m42 is 49.8cc, of which the chamber itself is 32cc, HG 9cc and piston dish around 11cc. is this right?
it seems that most BMW engines from that era (M50/52/42/44 and even S50/52) use a 32.5cc combustion chambers...
then I tried different combo of piston/rod/crank aiming for a squish somewhere between 0.8 and 1mm and a 12:1 CR -ish...
added to that I tried to get wrist pin diameter and rod small end diameter identical to avoid machining or bushing.
I also want to limit piston machining or deck machining to the minimum.
taking all that in account i ended up with a M44 crank, honda 138mm rod, euro S50B32 piston and a 1mm headgasket for a 1mm squish and a little over 12 CR.
I was very interrested by the M42 crank/rod with flat top pistons but my calculations showed a much high CR than yours. I must be wrong somewhere...
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From measurement I recorded.
Piston 7.5cc
Head 34.5cc
CR comes out at 9.87:1
Make sure your adding and subtracting volumes appropriatly for deck heights and dishes etc. Messed me around for a while. For my theoretical combo I use stock rods, pin diameter, HG, Crank, bored but not decked block and get 11.5:1 with the nissan piston and 1.2mm squish. Just trying to decide if I decrease the squish to .8mm or not with some block machining (or thin HG) and go for 12:1.
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thank you for the numbers. I will see what I get with that.
for your engine, you may also have a look at your camshaft profile before deciding which way to go, as you may have 12:1 static but less dynamic.
you may also decide which petrol to use and which engine management (knock sensing and advance management associated) before cutting between the 2 options you have.
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Thanks, cams are another set of tradeoffs.. but eyeing up VAC 280/247 "hot" cams. Management is sorted so will be fine there and good fuel readily available here.
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I just spent 2 hours re-doing my math with the actual volume figures from MLM...
the M42 crank/rods + M50TUB25 pistons combo requires some pistons shaving to get a CR in the low 12.
the M44 crank + honda rods + S50B32 euro piston combo has a 11.3:1 CR with a 1.25 HG to achieve a 1.2mm squish. looking for a lower squish would require even thinner HG... (0.7 squish, 0.75 HG and 12:1 CR)
so my last "idea":
M42 crank/rods + M52B28 pistons. no shaving required but a thinner HG (1.3mm) to achieve a squish of 1mm and 12.2:1 CR. this isn't a stroker, but it should make a nice, cheap hi-perf M42B18.
no machining is needed, only a thinner HG (cometic) and M52B28 pistons. looks easy!
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How about plan C?
M52B25Tu (euro) flat top pistons. Piston height is little bit bigger, 32,7mm vs. M50b25 32,55mm vs. M52B28 ~31,8mm. These pistons already have more then 3mm deep valve pockets intake and exhaust side. With thicker 2mm (stock)HG you get 0,8mm squish. I can't calculate exact CR but deep valve pockets will lower it. You can also shave the pistons 0,2mm to get 1,0mm squish.
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it should be in the mid/hi 12:1. actually I did calculcations with M50TUB25 flat top pistons, they have a piston height of 31.55mm. you have a 0.7mm squish without any mods but a 12.7:1 CR (without taking account of valve pocket volume). same pistons with 0.3mm piston shaving give a 12.2:1 CR with a 1mm squish.
so M52TUB25 pistons with a 32.7 comp height should give a higher CR. I will check that option tomorrow...
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I did a quick CR calculation using stock bottom end and M52TUB25 piston: 12.5:1 CR. this is without valve pocket volume as I don't have any volume measurement.
so this is the max theoritical CR with that combo, considering that each additional cc to the dead volume has a big impact on the final CR...
so yes, it may also be a good combo as it doesn't need any machining but a 2mm HG to achieve a 0.8mm squish.
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I did a quick CR calculation using stock bottom end and M52TUB25 piston: 12.5:1 CR. this is without valve pocket volume as I don't have any volume measurement.
so this is the max theoritical CR with that combo, considering that each additional cc to the dead volume has a big impact on the final CR...
so yes, it may also be a good combo as it doesn't need any machining but a 2mm HG to achieve a 0.8mm squish.
And you can use quite wild cam with high valve lift without machining pistons.
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interesting... do you have any experience with such a setup?
what kind of power/torque should be expected with this setup and mild cams (between 260 and 280°) with E85 and MS3X standalone? 150hp at the wheels?
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interesting... do you have any experience with such a setup?
what kind of power/torque should be expected with this setup and mild cams (between 260 and 280°) with E85 and MS3X standalone? 150hp at the wheels?
Nope, I don't have. My engine is still under construction and I found that piston setup after I bought M52B28 pistons. I decided to use them. Maybe next time...
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do you guys confirm that M44 rods are lighter than M42's?
I can have a set of M44 rods for a good price and I would like to be sure they are ligther first. that would be my first step torward a high CR M42...
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They are. I scrapped an m42 a while back and those rods are quite heavy. M44 rods are essentially longer s50 M3 rods. I read 100 grams lighter and even though I didn't weigh them I believe it.
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thank you!
they must be the same as one of the M50s and you shouldn't be far away from truth with 100g from what I've read.
next is a set of M52B28's pistons for the same kind of build as you, the stroke apart...
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It looks like the M42 rods have been superseded with 11 24 1 437 617 (I think this is the M44 rod).
This part is listed for the M40, M42, M43, M44 engines.
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good to know! thank you colin.
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I weighed a rod from my late (Z3) M42 block - it came in at 547.1gm. This is around the same weight at the Eagle H beam rod (as stated by Eagle).
The Cometic head gasket part number for the 87mm bore is H2197SP2040S, I've been advised to use a marginally larger gasket than the bore hence the 87mm gasket for an intended 86.5mm overbore.
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thanks for all the details!
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FYI if you are still looking for a gasket. I spoke with Mike at VAC motorsports yesterday and they can make an MLS 86.5mm m42 gasket. Or any custom bore for that matter.
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actually I gave up the stroker idea so for financial reasons, the latest project is M44 rods and M52B28 pistons to build a cheap high CR NA M42.
but! that's still good to know, either for anybody who would want to stroke his M42, or in case of yet another project change... ;-)