Resurfacing Block Needed?

[TC Baur MD]

My block has pitting between the coolant jacket holes. Do I need to have the block re-surfaced?

[moatilliatta]

If you're completely dissembled it would be silly not to...

A lugged around rebuilt Eta may never know, a high comp / Turbo I motor with MLS would show the weakness from poor tolerances a lot quicker.

[reelizmpro]

Definitely.

[TC Baur MD]

Thanks for the responses. Is there a standard amount that's taken off during re-surfacing (0.005"?) and does this effect any of the re-assembly tolerances?

[bmwman91]

If the block is stripped and on a bench, it's 100% sensible to get it resurfaced whether in any case. Call it extra insurance against issues...it would probably cost you more than resurfacing to have to pull the head and replace everything involved if there was a pinhole coolant leak after final assembly.

Shops shoot for minimal material removal to attain the required flatness across the mating surface. Pitting and stuff does not necessarily need to be completely eliminated, but getting rid of any contiguous pitted regions is usually a goal.

You will be increasing your compression ratios slightly, and if you have super hot cams with long duration & high lift in an interference engine, you would potentially need to carefully check the valve-piston clearances. Most stock HG's have one or two options for slightly increased thicknesses since replacement almost always involves resurfacing the block & head, which can add up over time. Custom gaskets give you more options in that regard.

Anyway, speak with the machine shop. They are the experts and can provide good guidance to you.



Example (late M20B25): https://www.realoem.com/bmw/enUS/sho...diagId=11_1910
The stock HG a 0.3mm / 12mil oversize option for thickness.

[ForcedFirebird]

Be careful with how much you take off with the b25. There's a reason there's indicators on the head in case you shave too much.

Every mm you take from the block has to be subtracted from the head total. The head can only be shaved .4mm. So, if you shave the block .2mm, your head must be 124.9mm tall after shaving (it starts at 125.1mm and has a minimum thickness of 124.7), or the thick gasket won't compensate and you run the risk of PISTON TO HEAD contact, and it WILL ruin your rod bearings in short order.

There's plenty of valve to piston clearance, it's the dome of the pistons that hit the chamber, FAR before the valves hit.

That being said, you can use a precision flat stone and rub the deck surface. It will show you more information than witness marks. Some of those will be removed with a stone, and nothing more.

The stock-style gaskets come in 1.7 and 2mm total compressed thickness. That leaves you with a total of .4mm to play with (0 to -.2mm variance for 1.7mm gasket, .2mm for 2mm gasket). You can probably get away with another .2-.3mm, but at that point you will need to use a sacrificial gasket and solder/clay to test the squish. I have found the m20 responds very well to .037" (.9mm piston to head clearance). Generally with a 272 cam and .037" squish, the stock pistons will see .057" (1.45mm) on the intake valves and .075" (1.9mm) exhaust.

[bmwman91]

Oof, yeah piston to head contact would be bad. ForcedFirebird drops the numbers and facts upon us once again!

[TC Baur MD]

Excellent information. I stand educated. Thanks!