Cylinder block surface preparation fro new head gasket

[scipio84]

Hello everyone,
I'm working to replace the head gasket on my M20B20.
I sent the head out to a machining shop and prepared by my self the surface of the cylinder block.
Now, I removed all the residuals of the old gasket and dirt, passed with fine sand-paper, then cleaned everything up.
Below I post the photos before and after the work.
My question is if those semi-circular lines on the surface are due to the original machining of the block surface or are residuals of the old gasket; with a very low pression, I can hardy feel them with the nail of my finger; I made several measurements with a metal ruler and a feeler gauge; the minimum thickness I have is 0,05mm and never passed under the ruler (for the head, the manuals recommend a limit to warpage of 0,10 mm).
What do you think? Can I keep it as it is now and proceed to complete the job or do I have to remove other material (I'm scared of this honestly...).

[zaq123]

those are definitively original machined marks, not the head gasket residue. If your deck is straight, I wouldn't mess with it any further.
I would personally be more worried about the use of sandpaper. If its microscopic abrasive residue got inside the engine (and it most likely did), it will cause all kinds of damage to the engine long term. IMHO, unless one is completely disassembling and washing every little thing in the engine, things such a sand paper or any abrasive pads should be kept far away. Good old scraper, solvent soaked lint free rag and some elbow grease would have been plenty to prep the surface for OEM style head gasket. Would it look shiny like in your pic? No. but as long as the surface is clean, that is all that matters.

[scipio84]

Well, thanks a lot for your opinion.
For the sandpaper residuals, I tried to be as much careful as I could, by closing the openings and covering the cylinders; I worked each time around the pistons at their TDC, so that I could vacuum clean them first, gently clean with a wet rag, bring the piston down and clean the cylinder wall, repeating this some times and sprying time by time with air, to avoid any particle to scrape the cylinder wall.
However, to clean the surface, the sandpaper was the only able to work on certain points. I was afraid too, of course.

[scipio84]

I have a doubt about the head gasket.
The one in the Victor Reinz kit is 1,75mm thick.
The original was 1,88mm thick.
In the shop, they told me before and after the work on the head that it was not necessary the gasket with increased thickness (i.e. 2,05mm - so +0,3mm), since the head was in very good shape and they would have removed few material.
However, my fear is to reduce too much the volumetric ratio with that 1,75mm gasket and to risk the unwanted ignition of the gazoline: the engine has originally 8.8 ratio, and is made to work with unleaded gazoline (even if my model doesn't have any catalitic converter). The ratio with that gasket would grow up to 9.06, considering they removed 0,15mm in the shop (not sure about this value, I'm estimating).
Ok, the engine has more than 150000km, so the piston rings could leak a bit, I could recover something, but as well the rings may suffer for the higher compression (I don't think so, since the first M20 with carburettor has a ratio of 9.9).

[TobyB]

I think you'll be OK. 9:1 is pretty safe on these engines.

However, the Reinz gasket has been known to have problems-
in your shoes, I'd source a different brand (Elring, Corteco, etc)

t

[scipio84]

I was looking for Elring and Ajusa, but the problem is that they're all running out of stock and only Reinz had the complete kit. It was my last choise, also because there is a discrepancy on the codes in their catalogue...

[digger]

I always though the thicker gasket was more for clearance than keeping the CR down. I also doubt it's an issue

[Andre3127]

I used a razor blade to scrape crud off mine, and then followed up with an oilstone and oil to shave off any micro burrs or raised spots from rust or any minute pitting. If using a hard stone, no abrasive bits get left behind. Make sure your stone is flat, though. Dont use your grandfathers knife sharpening stone with a hollow.

[mr.]

I used a razor blade to scrape crud off mine, and then followed up with an oilstone and oil to shave off any micro burrs or raised spots from rust or any minute pitting. If using a hard stone, no abrasive bits get left behind. Make sure your stone is flat, though. Dont use your grandfathers knife sharpening stone with a hollow.

I was going to do this exact thing, recommended by Bimmerheads in the instructions they send you with their rebuilt heads
Use a flat sharpening stone, keep it lubricated with WD40 and just go in a front to back motion, and wiping the surface clean with blue shop towels every now and then
Anything else to look out for?

(sorry to thread jack OP)

[D.Martijn]

I was looking for Elring and Ajusa, but the problem is that they're all running out of stock and only Reinz had the complete kit. It was my last choise, also because there is a discrepancy on the codes in their catalogue...

I went with an Elring kit and got a separate Goetze head gasket.
i also went over the deck with a whetstone