Piston + Valve: How Much Damage Is Too Much?

[bmwman91]

OK, so some of you are aware of my valve spring problems. When I pulled the head, this was the damage that I found on piston #3.



So, what are my chances that I can just put the head back on after it is repaired and drive it? If a wrist pin or rod was bent, I assume that I could check it relatively easily with a depth gauge by comparing piston #3's height below the deck to piston #2's. Thoughts?

The smaller burr is not really a concern to me since it doesn't take a lot of force to do that, and I can remove the burr myself to prevent a hot-spot. It's the larger indentation that worries me since it probably takes considerably more punch to do that.

Thanks.

[DesktopDave]

To be totally honest, I'd buff it out.  But I'm about as far from a pro engine builder as you can get...

[bmwman91]

Haha, "it'll buff right out!"

For some reason, I get the feeling that this is one of those, "if you have to ask..." scenarios, but then again I am also irrationally paranoid as of now.

[keflaman]

I might be talking out my butt, but I think the valve stem would more likely be bent than the rod and I wouldn't worry about the wrist pin.

I'd blend the damaged areas of the piston and carry on smartly.

[bmwman91]

Thanks, that makes me feel better. I am hearing similar things from r3v ("dude, I have seen way worse put back into service, you will be fine.") One person there just said to make sure that the top ring is not pinched there...how would I check that?

[bmwman91]

Oh, and here's the valve that did this...the valve looks like it "lost" the fight lol.

[keflaman]

...One person there just said to make sure that the top ring is not pinched there...how would I check that?

He's talking about side clearance between the ring groove and ring itself. You'll have to remove the piston to check it.

Refer to page 5, (H): http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CGsQFjAE&url=http%3A%2F%2Fwww.elstars.com.au%2Fpdf%2Fpiston_rings_guide.pdf&ei=Fp8sUfLxGo-w0AGGpoDIBA&usg=AFQjCNEISsv-cDR0gHouBW1VkNmND5mXlA&bvm=bv.42965579,d.dmQ

If you decide to do that, I'd suggest also checking piston diameter for any possible deformation. I think it'd be unlikely to see any damage, but prudent to do so.

Disclaimer: My only qualifications to give engine advice is based on my formal training and certification as an A&P mechanic. Damage such as yours would render the piston and valve parts unserviceable and unairworthy for aircraft use. Inspection for "sudden stoppage" of the rods, wrist pins and crank would be required. The information I provide is for amusement and off-road use only.

[romkasponka]

sclaimer: My only qualifications to give engine advice is based on my formal training and certification as an A&P mechanic. Damage such as yours would render the piston and valve parts unserviceable and unairworthy for aircraft use. Inspection for "sudden stoppage" of the rods, wrist pins and crank would be required. The information I provide is for amusement and off-road use only.

First you should check repair manual for piston damage limits or repair limits with procedure. Valves for sure are U/S and form "Unsalvageable" should be filled. Later should be destroyed preventing anyone use them again.

What was done with the valves head? Why you made these grooves?

[bmwman91]

Thanks for all the info. If I put the head back and do a leak-down test, would that be sufficient to determine if things got messed up? I really want to avoid pulling the piston because that means re-honing the bore, at which point I may as well send the block to the builder to deal with.

romka,
A valve spring broke. The valve hit the piston, bent the stem causing it to get jammed in the guide, and then it probably hit the piston a few more times until it got hammered out of the way. The head is back at the engine builder where all of the springs are being replaced, as well as this valve & its guide.

[romkasponka]

I was meaning the grooves on the valve which probably for some kind of magic turbulence

I suppose you cannot so easily bent con rod. Just take a file and remove all the sharp edges and put the cylinder head with new gaskets and so on. I am sure all other things will be fine.

[bmwman91]

Aaah yeah, those are Metric Mechanic's special "Surface Turbulence" valves.

I had a very productive phone conversation with them today and it sounds like everything will get worked out to my satisfaction.

[keflaman]

I had a very productive phone conversation with them today and it sounds like everything will get worked out to my satisfaction.

Did Metric have any recommendations or advice regarding the piston damage and further inspection?

[bmwman91]

I didn't have a chance to ask about it...had a work conference call. I will be calling them again tomorrow to finish up my questions.

[sfinsf]

I had a recent question/experience with my m20... here's the story.

Bought non-running 90 325i with what was siad to be broken TB, after diving in, found that TB was intact, but other mayhem occurred with timing components. The head was taken to a very reputable machine shop in walnut creek. all exhaust valve replaced (bent) and new guides and seals installed. Head completely checked and given clean bill.
Meanwhile I worked on the block. This is the best one.

This is the worst one.

As you can see, these were classic "interference" issues brought to life.
I posted my images and questions on R3V and spoke to the machinist and other BMW mechanics who all said...
deburr
clean throughly
dont worry

I have been daily driving this 325/M20 for 2 weeks now. It is very solid, no issues to date, runs great, passed smog, pulls very well.
Clearly, I am not a pro mechanic, nor have I worked on more than my m42 and m20. Just thought you could use a bit of perspective on your project. Glad to hear things are looking positive for you and the m42.

[bmwman91]

Thank you sfinsf! That makes me feel a lot more confident. That M20 looks like it got it a bit worse than my M42!