Ok, maybe I am getting somewhere (maybe).
I have been having a loss of coolant issue you can read about in my other threads.
I pulled the head again and measured it cross-wise with a straight edge and feeler gauges. The only straight edge I had was 6in long. I ordered an 18 that should be here soon so I can measure length-wise and diagonally. The head seems flat. I couldn't get any gauges in.
However, as you can see in the pic below, I was able to get a .0025mm feeler crosswise on the block on both the middle cylinders. Anyone know what the tolerances are for the block? Does this need to be surfaced?
thanks

 

well I have some questions,


are you sure its a .0025mm? that's basically .0001'' which is the smallest feeler gage I would have heard of, and it definently wouldn't look like that picture =D. is it a .025mm? (.001)? if that's the case I would say that's fine


ive never had to surface a head, but .001 can easily be made up with the gasket, when you start getting to .010. I looked at some guidelines, with aluminum heads, the head needs to be flat within .002, but cast iron flat within .006? but flatter the better. (those specs aren't our engine) I would imagine the Bently Manual has this tolerance. also have you checked if your straight edge is flat? the one you ordered would be fine, but its always worth a check.


it may also be a good idea to get some copper sealant and put that on the gasket when reassembling and give that deck a good cleaning

 

Sorry, that should read 0.0025 inch, not mm. Wasn't paying attention.

 

I believe the mini spec is 0.004 inch. When I've had mine off, I hand lap it flat within 0.001. Very hard to do better than 0.001 inch by hand. I found the block and head were both warped a little, and in the same places, so the gap was double what I measured on the head.

 

David.r53, how are you hand laping the block? That seems like it would take ages.

 

You don't need the fancy tooling if you are careful. It doesn't take that long. Cleaning up may be the slowest part. Stuff some paper towels and/or rags into the cylinders to catch grit and protect your rings/bearings. Try to remove coolant from the block so it isn't up to the top edge of the block and getting dragged into the cylinders. I pull out the two alignment dowel pins if they don't come out on their own so you have no obstacles to work around - they are pressed in. They will get damaged - get new ones from Mini (their manual says you need 3, but there are only 2...).

You want a real rigid sanding block - not a foam or rubber pad - that will just distort and make things worse. I have a piece of glass I use (once was a bathroom shelf - about 5 inches by 12 inches and 3/8 thick - glass is ideal as it is very flat and rigid - and you want it fairly large so it doesn't follow the ups and downs of whatever warp you have) - use 3M spray adhesive (art supply, low tack version for paper - Home Depot sells it also) to stick some 180 grit paper (metal cutting) to it without wrinkles or bubbles. In the machine shop, we would paint the surface with something called dyekem (very thin blue paint in a fast-drying solvent). At home, I just use a sharpy marker and scribble all over the surface I want to work on. Start sanding, long smooth strokes. You will quickly remove the marker (and some metal) from the high spots. Keep working on the high spots until you get them down, and keep going until you just clean up the low spots (you will see the marker gets worn away). Shouldn't take too much. Remember you are only going to remove .002 inch. You can check your work with the feeler gage and straight edge. Apply fresh marker if you want to confirm the high and low spots. Change the paper when it stops cutting. Then switch to 280 grit paper and work 90-degrees to the coarser scratches until you have only the finer scratches. Same approach for the head and the block, I also did the exhaust manifold and intake manifold the same way. Goes pretty quick.

Cleaning the head is easy since it is off the car. You want to get any residual grit off, and leave a very clean surface for the gasket. I wash it with dish soap and water (cam and rockers are removed before I start) and then dry it with compressed air. This part can be a little messy. Don't let it sit wet and have the steel parts rust. Wipe the sealing surfaces with good quality solvent (like automotive paint grade) that won't leave oil behind.

Cleaning the block is a bit harder as some of the grit may be in the cooling jacket. Clean up the cylinder bores - remove your towels and wipe it out with a slightly oily towel several times until it comes out clean. I have one of those liquid sucking machines (not really a machine, a hand pump and a 2 gallon tank). So I use that to suck out any remaining coolant (and any abrasive in there), then fill the block with a little more coolant, suck it out, fill it again, suck it out, etc. Then wipe the top surface carefully to dry it, then wipe again several times with the solvent.

You want an immaculate metal surface for the MLS gasket to seal to. What I said here will get you that, and the 280-grit scratches are within the roughness spec for a MLS gasket. Most MLS gaskets come treated with a coating of viton polymer to fill small gaps and scratches.

 

This is a photo of the head. You can see the center was low compared to the edges so it would have not sealed well between the cylinders. This was after just a little work. I kept going, focusing on the high spots (where the marker was removed).

P.s. I would only try this if the initial inspection with the feeler gage said it was a little out. If it's way out, probably should go to a proper shop.

 

Thanks David.R53, I am think about trying this...However...

I have been trying to find this coolant leak for a few months now. It looks like the warpage on the block may possibly be the cause (i wish i could be more certain) the warp is moderate and not really a "smoking gun". So the possibility still exists that there is a crack in the block somewhere.
Is there a way to test for this with the block in the car? If I can rule out a cracked block, then I am going to lap the block and reassemble. If not, I think I will just finish pulling it out and run it to a machine shop to check for cracks.

 

A pressure test is the best way which would need a machine shop. They make dye you can use to check for cracks. I have not used it on an engine block.

Look around for a kit. Not the cheapest stuff.

The idea is some dye in a very thin solvent. Clean the surface first to get rid of rust or dirt. Wipe it on, let it sit a little, then wipe it off and watch to see if any dye oozes out - that would be your crack.

 

I had a cyllinder head warped in a similar location. I could fit 2 thousandths of an inch under a straight edge. I had it resurfaced for 75 bucks. The only thing id be worried about is the camshaft, if the warpage was worse than i thought. Ive driven around 2000 miles pretty hard, and i push my car hard after its warmed up. I think youd be okay getting it resurfaced if you need it.