M70 Head Gaskets

[tsmcg161]

I finally got around to doing the head gaskets on my 89 750. No problem with the teardown except the drivers side coolant crossover. Ive got 250,000 miles on this car and compression still at 170psi on all 12 cylinders. After getting both heads off i started noticing some unusual things. Somebody has been up in this engine prior to me. Blue silicon on the coolant valley cover. Ive got some issues with the old parts ive removed and Im not quite sure which direction to take. Cylinders 1-6 had a 2 loch head gasket. Cylinder 7-12 has a 1 loch. I can also clearly see Goetz stamped on it. Cylinders 1-6 has some numbers on it, but not very clear. The pistons on 7-12 all have "A" stamped on them and there is a symbol under the "A". On bank 1-6, 2,3,4,5,6 all have an "A" stamped on them, no symbol. #1 has a "B" stamped on it. Im like WTF. So im not at all sure what I got. I don't know if I should move forward or start with another engine>

[Sansouci]

Take some pictures, please.

[tsmcg161]

I don't understand how to post pictures from my iPhone. Ive been trying for 30 minutes

[jclausen]

Well the Valley pan could have been resealed no big deal, all pistons have numerous markings on them from the factory the a or the b could correspond to which side the on and pistons will always have some sort of mark that is there so you install them in the correct direction. You might have to clean the piston tops to see all the numbers then research on line. It would see like a very small chance that the the pistons had been removed and messed with.

[tsmcg161]

IMG_1370.jpgIMG_1371.jpg The picture with 1 hole was bank 7-12. The 2 hole was bank 1-6

[shogun]

it is shown on page 11-12/7.3 about the headgaskets selection and how to measure. https://www.bmwtechinfo.com/repair/main/
a bit difficult to get to that page, select the workshop manual #6, select engine, then is shown 735, 750, M60. Go to the 735 first, as nothing comes up when you select 750 directly. then click the forward button for many pages till the M30 engine ends and then you come to the M70 engine, direct selection of the M70 is not possible, just click many time forward

[unity]

I am pretty sure you can only get one thickness of gaskets now. There was a recent thread about this and even when I looked for mine, I could only find one thickness. As you may also already be planning, get that coolant valley cover replaced. Those bolts will probably break - with any luck at the head. So once the cover is off you can use a vise-grip to slowly work then back and forth to remove the remaining bit. I put new stainless hardware back when I did mine.

[shogun]

accdg to online ETM , up to 04/1990 1-6 only available 11121729893 1 hole, 1.5mm; 7-12: 11121729897 1 hole, 11121729907 1 hole, 11121731654 3 hole, +0.3mm http://bmwfans.info/parts-catalog/E3...rts/#1731654_1
but:
actual checking what is available, for example Autohaus AZ
11121729893 VictorReinz Cylinder Head Gasket; Right (Cylinders 1-6); 1.64mm 1 per car. $33
11121729897 VictorReinz Cylinder Head Gasket; Left (Cylinders 7-12); 1.64mm 1 per car. $33

So there you have it, one size only available.

[tsmcg161]

It looks like the 1 hole replaces the 2 hole per realoem. I guess the acid test will be when I put the heads back on and turn the engine over. All valley cover bolts came out with no problem. The main mystery for me now is this #1 cylinder piston has a "B" stamped on it. All the pistons on 7-12 match "A", 2-6 match "A", #1 no match. Per realoem "B" stamped is larger? Ive had this car 10 plus year and have spent tons of dollars on it. The first time I replace the spark plugs they were in rough shape, especially #1. 3000 mile later I started noticing a miss, it was #1 spark plug fouled out. Replaced plug and misfire was gone. I finally replaced all the lifters and valve stem seals, which helped, yet #1 still burned differently. Now I got this "B" piston issue on #1 and am baffled. I will send pics of pistons tonite. Also on 2-12 pistons carbon build up was bad, real bad. Piston #1 though wasn't that bad.??????

[unity]

If B pistons are larger I would be worried that someone honed the wall. Which if I recall, can not be done because there are no sleeves and doing so damages the ultra think silicon lining.

[tsmcg161]

Per realoem, cylinder 1-6 side "A" IS 84,000mm. "B" IS 84,011mm

[unity]

That difference is pretty minor, but its a difference nonetheless. Im sorta confused as to why there are different sized cylinders in the first place. But I guess thats for another discussion. So you essentially have one extra B piston in that engine. I really wonder if it was factory or not - I doubt it. I recall at least one thread where someone found an entire drill bit in an oil passage. Snapped off during production and made it into an operating car. The oil snaked around the coil of the drill bit so it ran. I guess I can maybe see this engine being a Friday special. Myself, I would be a bit paranoid given the issues you had with #1 and at the very least pull the piston to inspect for cylinder damage. #1 is in front, I wonder if you can drop the lower oil pan and get to it from below without pulling the entire motor. Would be a good idea to rebuild the oil pump and leave it out until the top end is clean. That way you can flush the oil passages.

[unity]

Can you post a pic of the piston tops when you get a chance? Both sides. Something just occurred to me but I need some pics first.

[tsmcg161]

Lower oil pan is off because im replacing the timing chain and the tensioner rail. Someone had to have put that piston in somewhere along the line. I found one on ecs tuning. The cylinder walls haven't been honed on #1. As a matter of fact they all look very good. Im betting someone put a "B" piston in with the old "A" rings which is causing my fouled plug issue.

[unity]

Thats makes sense. If you do not pull the pump, check its mount points anyway. Not super common on the M70 but they have been known to come loose. Had one on mine.

[tsmcg161]

I'll send some pics tonite.

[tsmcg161]

IMG_1379.jpgIMG_1380.jpgIMG_1381.jpgIMG_1382.jpgThe first pic is #7, the 2nd is #12, the 3rd is #6 and the 4th is #1

[tsmcg161]

This is my tensioner. Cracks everywhere!IMG_1385.jpg

[unity]

I would be looking at getting a different engine unless you plan to 100% rebuild that thing crank on up.

[ross1]

I would be looking at getting a different engine unless you plan to 100% rebuild that thing crank on up.

What?
It is entirely possible the engine was assembled this way at the factory. The difference is .004", four one thousandths of an inch.
Not sure what piston/wall clearance spec is but perhaps there was an issue when first honed and needed "B" pistons to have correct piston to wall clearance.
It's not a perfect process. If too much material is removed when machining this is what happens. I'd rather have two different sized(.004", really?)pistons than sloppy clearances.
Plenty of engines get built with undersize crank journals too.
Nothing to be concerned with.

[tsmcg161]

Again the engine runs great. Compression is good on all 12. That "b" piston just freaked me out. I mentally still am trying to figure this out. Im ready to put this back together. Got both heads reconditioned and I have all the new parts. Part of me want to find a good donor and the other want to put everything back together.

[tsmcg161]

Im back to report that my 750il is back on the road. The valve job and the new timing chain and tensioner was time and money well spent. The acceleration and idle is smooth as silk. Thanks to all who chimed in and assisted me.

[shogun]

Just to make sure, you installed the following gaskets last year? Unity mentioned, no others gaskets available anymore.
11121729893 VictorReinz Cylinder Head Gasket; Right (Cylinders 1-6); 1.64mm
11121729897 VictorReinz Cylinder Head Gasket; Left (Cylinders 7-12); 1.64mm

The >30 years old workshop manual says:
M70 cylinder head gaskets are available in 2 thicknesses. The gasket must be selected to conform with the maximum piston protrusion of a bank of cylinders. Gaskets are marked with 1 or 2 holes in the rear edge.
max piston protrusion of all 6 pistons mm (inch)...................................identific ation of cylinder head gasket holes
0.388........0.610mm (0.015 - 0.024)............................................ ...................1.............................. ...
0.610.......0.831mm (0.024 - 0.033)............................................ ....................2............................. ......

[tsmcg161]

Correct gaskets were installed and she ran very well. The problem is the oil consumption. I was adding a quart every 300 miles and smokin blue. Found a donor engine with 63k on it, as a matter of fact, I got the engine, transmission, driveshaft, throttle bodies and instrument cluster for 2000 dollars. Everything is installed, runs great and vehicle is at the paint shop getting another respray of bronzit beige.

[shogun]

Just for info: I contacted someone in Germany who professionally overhauls M70 engines, his comment to thickness of cylinder head gaskets was (google translated):
Accdg to the repair instructions the thickness of the head gasket must be determined using the piston protrusion to be measured. The 1-hole head gasket is to be installed up to a protrusion of 0.61 mm, everything that exceeds this gets the 2-hole gasket. In general, however, if the head or the block was not reworked, the same thickness can be used again. Experience has shown that 3 / 10mm can easily be removed without installing a thicker gasket. In 99% of all series M70s, the aforementioned 1-hole seal is installed ex works. BMW only offers 1 or 2 hole cylinder head gaskets. For the gasket sets I only use original BMW sets, even if they mostly consist of Elring gaskets. The risk of later sealing problems with cheaper / unknown manufacturers is too risky for me. In general, especially with the low compression M70, you should aim to give up as good as nothing in terms of compression, for example with an unnecessarily thick head gasket. Every 1 / 10mm counts and contains a certain amount of extra power.
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the above mentioned Victor Reinz mention 1.64mm, but the ETK from a BMW dealer says for the same part number different, see below:
11121729893 VictorReinz Cylinder Head Gasket; Right (Cylinders 1-6); 1.64mm >>>>ETK 1-hole 1,5MM , available
11121729897 VictorReinz Cylinder Head Gasket; Left (Cylinders 7-12); 1.64mm >>>>ETK 1-hole 1.5mm, NLA (?)