engine and drivetrain Damn! Scuffed Piston Skirts!

[jimk]

Throw some rings on it and let it go!  Minor scuffing though.

Edited February 26, 2014 by jimk

[Rocan]

I'm going to caution against re-using heavily scuffed pistons. Check the diameter against specifications; if they are below spec I wouldn't risk re-using them. It will work, but ring and bore wear will be significantly accelerated. Sloppy pistons have a tendency to ovalize the bore as they wear as well, requiring a larger oversize bore then what would usually be necessary. How much more quickly the bores wear is dependent on a number of things; it could hold for a few weeks, or a few years. I wouldn't take the chance. 

 

Before you re-install anything, measure everything with the appropriate devices (bore gauge and micrometer). There is no point doing this twice; if the wear is significant you'll have to buy rings again as well as new pistons and paying for an overbore. If things are out of spec, but by a small amount, you may just be able to get away with a hone and new pistons, which is cheaper in the long run. 

[FB73tii]

Throw some rings on it and let it go!  Minor scuffing though.

 

A little JB weld and you are good to go

[jgerock]

This engine will be re-bored for oversized pistons

 

[jimk]

jgerock, I am curious as to how plugged are the oil return holes in the oil wiper groove in those old pistions.  Usually with dino oil, the holes become filled with heavy, hard varnish and is the main cause of oil usage past the pistion rings.

[TobyB]

...and I think you should send Rob your parts, and machine his- barring something being way under- spec, that's pretty minimal wear.

 

heh

 

t

[jgerock]

My pics showed the Mahle pistons I removed from the "birdseed" 72tii engine (2nd oversize already).  I have another new set for the block which will require boring.

[thehackmechanic]

Old school Boston CCA member and retired machinst Bruce Machon is coming over tomorrow with his dial indicators and bore gauges to tell me what's what.

[jgerock]

Old school Boston CCA member and retired machinst Bruce Machon is coming over tomorrow with his dial indicators and bore gauges to tell me what's what.

What size pistons do you currently have in your engine Rob (guess I need to do a search)?

[thehackmechanic]

Jim, they're the original size, 88.97.

 

I'm a very practical guy. If the bores are so out of round that re-ringing is foolish, then my hand is forced, I'm done. If the bores are okay, then I have some decisions to make in terms of re-using the original pistons or finding others. I often look at things this way -- running down possibilities until I find they're not the possibilities I thought they were.

[TobyB]

If the block's good and the pistons aren't so hot, there is NOTHING wrong with honing the block and tossing new pistons into it.

The 2002 has a very tight spec to start- and a lot of leeway for wear, so you get an 'almost new' engine for the cost of pistons

(which even come with rings)

 

Done it several times, never a problem.

 

hth

 

t

[thehackmechanic]

Old-school Boston BMW CCA chapter member and retired machinist Bruce Machon came over and laid his expert eyeballs, hands, and gauges on the bores, pistons, and ring grooves. He was not at all concerned about the scuffs, as none of the marks were fingernail-deep. The bores were all within spec. One bore/piston pair is near its clearance limit. Two upper ring grooves are at or near their limit. His overall pronouncement is "if it were my car, I'd ball-hone it, re-ring it, put it back together, and drive it." He also recommends that I touch up the valves, even if it's just a light lapping. As much as I don't want to touch the head, I'm going to let myself get dragged down this part of the slippery slope, then dig in the ice axe and the crampons.

 

The new Deves rings just arrived. I'll test-fit the top ones in the grooves and check the side clearance. But I'm likely to follow Bruce's advice.

 

I've never "touched up" an engine like this; it's always been all or nothing. If nothing else, it'll give me the hands-on and gut-level experience to know never to do it again if it goes south on me.

[Colin]

Good stuff.

[TobyB]

There you go!  cool!

 

Lapping the valves is easy, and you can do new stem seals at the same time.

And if they're ugly, well, then you get to visit the machine shop!

 

t

[thehackmechanic]

Having decided to have the head touched up, I now need to strip it down. Of course several of the nuts on the exhaust studs won't come off without a fight. Of course my oxy-acetylene torch is out of acetylene. Of course welding supply houses aren't open on a weekend. Why, god, why?

 

(the MAPP gas may get 'em hot enough. these are the little 12mm copper nuts, not the big headpipe nuts)