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Author Topic: 86mm Head gasket  (Read 9403 times)

MLM

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86mm Head gasket
« on: November 08, 2012, 06:01:50 PM »
Hi Guys,

I have some questions for those who have overbored to 86mm (or larger) for stroker projects or those in the know.

Once overbored to 86mm what head gasket do you use?
Now I know the M44 overbored head gasket has a 86mm diameter BUT...

-Does a 86mm head gasket on a 86mm bore not present the gasket edge to the combustion in the cylinder potentially leading to hot spot and detonation?

-Should the gasket or bores be misaligned even slightly is there the risk of piston to gasket colision and a greater risk of hot spots

-I assume there is a very good reason for the gasket to be slightly larger than the true bore size.

Can custom gaskets be made economically compared to stock pricing or is there even a off the shelf item available? MM engines at 87mm must do somthing?

There are 86mm pistons begging to be used out there but the rest of the items have to work and be available.

***** Have purchased a standard oem m44 1.74mm head gasket. Bore is 86.1mm. Ok for 86mm piston.
« Last Edit: September 03, 2014, 03:10:51 PM by MLM »
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deansweet

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86mm Head gasket
« Reply #1 on: November 08, 2012, 06:48:15 PM »
I did a search on this and it appears that a std 86mm head gasket will work on over-sized(84mm to 86mm) pistons...
From several different bmw enthusiasts sites, even this one had an answer from 2009.
Here's one of them:
http://forums.bimmerforums.com/forum/showthread.php?t=433471&page=3

-Dean
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MLM

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« Reply #2 on: November 08, 2012, 08:16:07 PM »
Thanks Dean,

I have seen the thread linked before, It does seem to be the common answer.

This still bothers me however as the common design intent of manufacturers is to have a larger gasket bore to the cylinder bore.

Since my first post I have at least partially answered my own question by measuring a 86mm piston. At the top its 85.45mm at the bottom of the skirt its 85.99+mm. This leaves some clearance as the piston passes through TDC and into the gasket. (nigh on same compression height as M42).

Im getting more comfortable with the idea but would still worry about the gasket causing a hot spot..
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accota

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« Reply #3 on: November 09, 2012, 02:31:18 PM »
For peace of mind, you could shave the pistons -0.15mm below deck.
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MLM

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« Reply #4 on: November 10, 2012, 01:23:03 AM »
True, shaving the piston (0.2mm for the one im looking at) would eliminate the risk of piston to gasket collision but may stuff the squish zone by opening the gap up too much.

A small chamfer on the piston edge would be another option tho :).
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Warsteiner

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« Reply #5 on: November 11, 2012, 07:33:41 AM »
Cheers Mate,

FYI...........
Stroker Crank 88mm
Combination for Rod and Piston with (M44 1.9L Head Gasket)

Compression Height of Stock S50 Piston is 32.8mm with Honda 1.8L VTEC B18C (GSR) 138mm rod

Desired Compression Height = Block height - (Stroke/2) - rod length
Desired Compression Height =*   ** * * **212mm - (88mm/2) - ((138mm[Honda rod] - .15mm{protruding out of block}))
Desired Compression Height =*      *212mm - 44mm - 137.85mm
Desired Compression Height =*30.15mm
The DCH is a desirable height.
30.15mm = 212mm - (88mm/2) - 138mm -.15mm*
This tells you that your target compression height is 30.15mm and we have enough meat to shave off 2.65mm from the 32.8mm :-P
Rod needs small end bushing to be drilled to fit 22mm S50 pin.

To get the right compression of about 10.9:1, I dished the piston about 7cc's and used the M44 HG that was thicker 2.04mm not the 1.74mm.

After building mine I thought that the S52 piston would work as well without shaving the piston as much since it only has a CH of 31mm. You'd only have to take off .85mm to get to your 30.15mm DCH.

The S50 piston is 85.96mm so your bore is 86mm with .04mm clearance.
The same goes for the S52 piston at 86.36mm and the bore at 86.4mm.

Cheers,
~Ralph
« Last Edit: November 11, 2012, 06:35:37 PM by Warsteiner »
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ispierrot

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« Reply #6 on: November 11, 2012, 11:39:51 AM »
Hi, I put a metal gasket of S42.

about $ 500 .... ouch.
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MLM

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« Reply #7 on: November 11, 2012, 06:20:42 PM »
Thanks Ralph,

Im still missing some info (stock block height) but the piston im looking to use has a CH of 32mm, 22mm pin and flat top. I believe stock is 31.65mm with a 9cc dish. This is intended to go with a otherwise stock dimensionally m42 (140mm rod 84mm stroke).

So with minimal machining (.45mm off the top plus dish to suit) a high compression piston should be achievable, depending on a bunch of other factors or course.

A $500 head gasket made my wallet shudder!
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Warsteiner

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« Reply #8 on: November 11, 2012, 06:34:17 PM »
Yeah I should have mentioned that was for the 88mm stroker crank! I'll edit that.

The block height is 212mm

You have to shave off .35mm not .45mm right?:rolleyes:
32-.35=31.65

Then you can dish the piston to whatever compression.

Just make sure to cut the reliefs in nicely if you plan to change cams and in turn that will change the cc of your piston as well.

Cheers,
~Ralph
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romkasponka

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« Reply #9 on: November 11, 2012, 06:53:06 PM »
Quote from: Warsteiner;117594

Then you can dish the piston to whatever compression.


Dont you think that you can get a hole in the piston after that? :)
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E36 318is M44

Warsteiner

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« Reply #10 on: November 11, 2012, 09:28:57 PM »
Yes!! Hopefully he won't go that deep! LOL

~R
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MLM

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« Reply #11 on: November 11, 2012, 11:14:00 PM »
Sorry bad math there.

Piston dish would be quite shallow but still yeild the correct amount of cc and then there is head gasket options as well. The pistons Im looking at are sr20de ones. low comp ones have a slight dish already.

Valve reliefs on the other hand may spoil my plans as these may need to be quite deep to accomodate different cams.
« Last Edit: November 12, 2012, 01:42:36 AM by MLM »
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MLM

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« Reply #12 on: November 25, 2012, 02:11:41 AM »
« Last Edit: November 25, 2012, 02:43:55 PM by MLM »
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romkasponka

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« Reply #13 on: November 25, 2012, 04:53:42 AM »
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MLM

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Re: 86mm Head gasket
« Reply #14 on: September 03, 2014, 01:56:49 AM »
Replying to my own thread to close the topic as a reference for us all


Have purchased a m44 head gasket. Bore is 86.1mm.
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