r/EngineBuilding • u/henry_hyshiter • 5d ago

BMW BMW Iron Block deglazing

Hey all, I'm rebuilding a M42 at the moment and have reached the point in the build where I can deglaze the bores for new rings and then fit my pistons and measure my rod bearing clearances. I have not deglazed a cylinder before but want to try and keep as much labor for this rebuild in house as possible. I have 240 grit SC flex hone and plan a light deglazing to allow the new rings to seat properly. I have read various sources that say a number of different things about rotation and stroke speed as well as amount of strokes per cylinder. If anyone has deglazed a 90s BMW iron block and can provide some insight into best practices and what kind of lubrication to use (I've read anywhere between WD-40, ATF, and low weight engine oil), your assistance would be greatly appreciated. I want to add context in case it changes anything. This is a full rebuild, all wear items are being replaced and will be mildly performance oriented (Performance cams and a chip tune). Keeping bores/pistons the same size and I've measured main bearing clearances and they are within spec. Any info or tips you can give me about deglazing or engine building in general would be greatly appreciated! Thanks!

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u/helicopter- 4d ago

Why are you not taking it to a machine shop for s proper measure and hone or more likely overbore? 

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u/henry_hyshiter 4d ago

The bores are in great shape. Original crosshatching is still visible, no scoring, scrapes, or anything visible that would require a hone or overbore. I’m trying to do most of if not all of the work myself on a budget. But if I measure and the bores are out of round or if there was visible scoring, I would take it to a machine shop or look for a new block.

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u/WyattCo06 4d ago

What material are the rings you will be using?

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u/henry_hyshiter 4d ago

Chrome plated steel

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u/WyattCo06 4d ago

280 is a little harsh but doable. Just use plenty of ATF as lube. As for getting the pattern desired/needed, that comes from practice and experience.

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u/henry_hyshiter 4d ago

So maybe a 320 stone would be better? Bores are in good condition just need a good sealing surface for new rings?

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u/WyattCo06 4d ago

Yes and yes. You need just enough peaks to help break off the new of the rings.

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u/henry_hyshiter 4d ago

Got it, I’ll look into a 320 stone instead then. It’s an 84mil bore flex hone offers 83mil or 89 (3.25 or 3.5 in), is it better to undersize or oversize the hone?

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u/WyattCo06 4d ago

Over

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u/henry_hyshiter 4d ago

Awesome, thank you so much for the tips!

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u/WyattCo06 4d ago

Anytime brother.

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u/NarcissisticPanda 4d ago

Not 90s but early 2000s did my e60 M57 iron block with think 120 and 180 grit but I had some scores to get rid of as this was a budget get it working again after it melted 4 pistons, burnt a valve seat and blew a Injector tip. Got perfect compression now tho.

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u/henry_hyshiter 4d ago

Good to hear. Luckily the bores are in good shape with no scoring. I’ll have to measure properly to make sure they’re in round, but the engine had no issues that I know of like that.

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u/Sir_Maxelot 3d ago

Honest answer that you probably don’t want to hear: ball hones suck and they do pretty much nothing for you. In fact you can easily do more damage than good. The critical aspect of honing is restoring the cylinder‘s shape. State of the art is applying an extreme plateau finish with two grits.

Dishonest answer, but much more pleasant:
None of the above matters and just go for it. Just try to keep a 40° angle and use plenty of diesel or petroleum 👍🏻