headgasket for 86/86.5mm pistons?

[thebrelon]

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

[bmwman91]

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.

[thebrelon]

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.

[bmwman91]

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.

[wazzu70]

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.

[benz-tech]

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.

[MLM]

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.

[thebrelon]

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?

[wazzu70]

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.

[thebrelon]

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!

[wazzu70]

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.

[MLM]

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.

[thebrelon]

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.

[MLM]

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.

[wazzu70]

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