Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 2:38 p.m.

I finally got the head pulled of the replacement engine last night. The head its self doesn't look bad - the cam & journals are nicer than the ones in my original engine, and other than possibly a slightly long OCI everything was clean. 
 

So I was surprised when I pulled it off & discovered what I think are the results of valve-to-piston contact?

 

Interestingly there's no sign of contact on the valves though. 
 

 

If I run these pistons as-is at 20psi of boost, will the contact marks cause hotspots that could lead to detonation?

Is tapping/filing down the high spots an option? If I do, will the weight difference cause problems at 7000rpm?

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) SuperDork
11/22/21 2:41 p.m.

I have definitely buffed down pistons after contact and "sent it" with no obvious downsides (in subsequent 1/4mi passes until things met their natural end). 

Did only intake or exhaust hit?I generally see intake hit due to high boost and not enough spring, exhaust hit with timing off or revs so high that the exhaust couldn't be controlled.

There is a chance your valves are slightly bent, worth checking via leakdown (but maybe too late, will have to do on assembly). Even with some pretty big marks on the pistons, the valves don't always show it.

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 2:56 p.m.

In reply to Paul_VR6 (Forum Supporter) :

These are all on the exhaust side. I'm not sure yet whether I'll use this head, or the one off the original engine I had rebuilt. If there's no damage to that head it's definitely the better one, but at that point it comes down to Challenge budget & if I have room for it. 

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 3:03 p.m.

In reply to Paul_VR6 (Forum Supporter) :

Also did you just use a wire wheel on the pistons, or something that removed more metal?

frenchyd
frenchyd UltimaDork
11/22/21 3:20 p.m.
Paul_VR6 (Forum Supporter) said:

I have definitely buffed down pistons after contact and "sent it" with no obvious downsides (in subsequent 1/4mi passes until things met their natural end). 

Did only intake or exhaust hit?I generally see intake hit due to high boost and not enough spring, exhaust hit with timing off or revs so high that the exhaust couldn't be controlled.

There is a chance your valves are slightly bent, worth checking via leakdown (but maybe too late, will have to do on assembly). Even with some pretty big marks on the pistons, the valves don't always show it.

Simple way to check for bent, with spark plugs in fill each cylinder with gasoline? ( if concerned about flammable make it something thin like gas but non flammable ). Let it sit.  
    Valves OK? All cylinders stay filled. Bent? They will leak. 

alfadriver
alfadriver MegaDork
11/22/21 3:23 p.m.
Paul_VR6 (Forum Supporter) said:

I have definitely buffed down pistons after contact and "sent it" with no obvious downsides (in subsequent 1/4mi passes until things met their natural end). 

Did only intake or exhaust hit?I generally see intake hit due to high boost and not enough spring, exhaust hit with timing off or revs so high that the exhaust couldn't be controlled.

There is a chance your valves are slightly bent, worth checking via leakdown (but maybe too late, will have to do on assembly). Even with some pretty big marks on the pistons, the valves don't always show it.

To add to this- Alfa's had a tendency to break the guides.  So if you observe high oil consumption and/or new blue smoke- there's another thing you need to replace.

Hope not, though.  PITA changing guides.

Tom1200
Tom1200 UltraDork
11/22/21 3:25 p.m.

Definitely valve contact.

I had this happen with the Datsun. The only way I knew about it was because the rings had finally given up and I discovered it when I pulled the head off. The valves were still sealing fine, my guess is the valves being harder then the pistons kept them from bending.

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 3:26 p.m.

Fortunately I'm not too worried about the head - at least at this point. But I hope to use the bottom end(after replacing the bearings), as long as these marks on the piston won't cause an unscheduled disassembly event at 20psi. 

RacetruckRon
RacetruckRon Dork
11/22/21 4:15 p.m.

I have a set of brand new pistons and rings in the box. Is this engine stock bore or has it been overbored?  

BoxheadTim
BoxheadTim MegaDork
11/22/21 4:21 p.m.

With the lack of contact marks on the valves, maybe someone slapped a good used head on that motor?

How are the pistons from your other motor?

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
11/22/21 5:15 p.m.

It could also have been valve float, too. 

 

I would feel a lot more comfortable with the exhaust valves if they were removed, chucked in a drill, and spun in front of a CRT television or monitor.  The strobe effect of the CRT's screen will make it easy to see even the slightest bending.  (Have never tried in front of a flatscreen, it may work too)

AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) SuperDork
11/22/21 6:37 p.m.

Over revs?  Or too much boost holding the intake valves open?  I'd make sure those valve springs are up to the task.  

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 7:07 p.m.
RacetruckRon said:

I have a set of brand new pistons and rings in the box. Is this engine stock bore or has it been overbored?  

I need to confirm, but supposedly it's stock. What do you want for them shipped to 39530?

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 7:09 p.m.

In reply to BoxheadTim :

Well they're .010 oversized(at least supposedly, I haven't measured them), and #4 is pretty pitted from I presume the head gasket leak. 

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 7:10 p.m.

In reply to AnthonyGS (Forum Supporter) :

Hopefully I don't have to use this head, but it'll depend on Challenge budget & what else I discover. 

bentwrench
bentwrench SuperDork
11/22/21 7:37 p.m.

My only concern would be that the dent deformed the ring lands and is compromising the ring seal.

If it pinched the top ring that might also explain all the oil showing in the cylinders...

It looks to me like a junk yard head to replace a head that broke a belt/cam/chain and shat upon the pistons.

Ill bet it was smokey and pushing out oil.

 

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
11/22/21 9:14 p.m.

In reply to bentwrench :

That's a fair point. I trust the guy I bought the engine from - he friended me on FB afterwards, so I don't think he was hiding anything - but he bought it as a running used engine from someone he knew, but didn't end up using it. 

bentwrench
bentwrench SuperDork
11/22/21 10:18 p.m.

I was not implying any skulduggery, just an objective look at the pics.

You have a bottom end with significant miles on it and a recent valve / piston interface failure.

The head looks like it has a similar amount of miles on it (but not crashed or repaired, so a replacement for the blowed up head)

Why are the dents new, yet both the head and motor appear to have similar miles? 

Where did the oil come from?

It was obviously not run much after replacing the head, if it showed signs of some good miles after the crunch I would be less suspect.

Put the head back on it and do a leakdown test if you are going to try and use it as is.

Then you are only out the cost of a head gasket if the rings are pinched. (unless you can reuse it, often you can)

It's gotta be right if you're going to put the boosts to it, if it's not you will need a jumbo catch can on the crank vent!

Boost_Crazy
Boost_Crazy Dork
11/23/21 2:35 a.m.

It was not uncommon with these engines for people to break a timing belt, fix the head, and leave the pistons alone with the dings without any noticeable side effects. Dealerships did that a lot back in the day, as early cars had some timing belt issues when new. Those dings might be 30 years old. That said, I'd take off the sharp edges at least. With stock pistons, you are at only 7.8 to 1 anyway, 20psi isn't a lot if fuel and timing are good. I wouldn't worry about the ring lands- they are way below the top of the piston, it would take a lot more than a nick. 

Mr. Peabody
Mr. Peabody UltimaDork
11/23/21 7:41 a.m.

There's so much material where the pistons have been hit I wouldn't be even a little bit concerned. I've seen and reused a lot worse.

 If there's going to be any problem it's going to be the valves. Definitely check those

gearheadE30
gearheadE30 Dork
11/23/21 10:31 a.m.
Pete Gossett (Forum Supporter) said:

If I run these pistons as-is at 20psi of boost, will the contact marks cause hotspots that could lead to detonation?

Is tapping/filing down the high spots an option? If I do, will the weight difference cause problems at 7000rpm?

Yes, sharp edges will get hot and can cause detonation. I would definitely clean up/round off the edges. It doesn't take a ton of radius to make a big difference.

Others have covered the important stuff. I usually use rubbing alcohol for the valve seat seal test; much less explosive than gas fumes. I did have one engine with very lightly bent valves (mechanical overreved BMW S52) that passed this test because the bent valves had worn the valve seats enough that they actually sealed pretty well, but the guides were very loose and the valve stems were all torn up. I only went in there digging for it because of oil consumption and there was evidence on those valves of oil making it past the seal/guide. In general though, the alcohol test works great and is easy.

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) SuperDork
11/23/21 1:20 p.m.
Pete Gossett (Forum Supporter) said:

In reply to Paul_VR6 (Forum Supporter) :

Also did you just use a wire wheel on the pistons, or something that removed more metal?

Definitely something attached to a die grinder once we got the bigger bits off. Roloc disc is most likely.

obsolete
obsolete Reader
11/23/21 1:54 p.m.

Let's look on the bright side, you can run a bigger exhaust cam now.

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