How bad is this cylinder pitting?

[Luis A.]

The M30 engine on my 3.0 Si has about 84,000 miles and was showing about 155 PSI compression for all cylinders with #3 being a little lower at 150. Upon dis-assembly of the block I see some pitting on cylinder #3.

What do you guys think the options are to address this? This is the original, number matching, engine and I'd like to preserve it.

I'm concerned about putting new rings in and having them be ruined (or worse) with the pitting.

Thanks for your advice.

 

[x_atlas0]

Do you know how deep the pitting is? Sound like your best bet may be to re-bore and get the +1 pistons.

 

[61porsche]

If the pitting is in one cylinder only, sleeve the one cylinder is one solution. with so many new ways of applying metals these days some shops can add material in very specialized ways. for instance there are shops in CA that can rebuild Porsche jugs by adding chrome, metalizing, etc. Probably cheaper than finding pistons. By you, US Chrome, in Milwaukee does snowmobiles, etc.

It looks deeper than a hone job to me. But you need to measure everything else as far as wear too to make an informed decision.

I think this is just one case to err on the side of caution.

 

[Luis A.]

Not sure how I can measure the pitting...? No way to get in there with my calipers. The overbore pistons are crazy expensive at $135 each...

 

[Luis A.]

Jerry, I'll look into US Chrome, thanks. I'll be hauling this and the head to a machine shop sometime soon, but wanted to get thoughts before then. I imagine he'll be able to completely scope out the cylinders for every measure.

 

[Dirk]

Luis, why did you open up the engine ?
You know you won't find money inside.

 

[Luis A.]

Luis, why did you open up the engine ?
You know you won't find money inside.

I was hoping to find a double rainbow --all the way--...

You know it's like one thing leads to another...it's 35 years old, it's out of the car and I'm not pulling that thing out ever again. My goal is to finally have a vintage (almost) daily driver in the summer, with AC and an automatic, so I want to be persuaded that everything on it will be as reliable as I want it to be.

 

[deQuincey]

Do you know how deep the pitting is? Sound like your best bet may be to re-bore and get the +1 pistons.

+1

luis, i did it three months ago, and i am the happiest guy !
its not only the pitting but also how oval had they become, so, head it to a shop, check, machine it and put new pistons, in my case those were +0,25, but they are available up to +0,5 in bmw

in my humble opinion they were not so expensive, 112eur per piston, and the include the rings, good brand MAHLE, should you need +0,5, they are more expensive around 180eur, but at least they are available

check the rest of the moving parts, you may discover some surprises

good luck

 

[Ed G]

I would inspect piston thoroughly as well, with attention paid to ring land wear. Metallizing and re-machining the bore sounds like a good alternative.

Good luck.

 

[x_atlas0]

Not sure how I can measure the pitting...? No way to get in there with my calipers. The overbore pistons are crazy expensive at $135 each...

That's crazy expensive? I found that price to be pretty good. Door seals are crazy expensive at 500$ each side.

 

[Luis A.]

Thanks for all the suggestions guys. Yeah, I know $135 doesnt sound like much compared to $500 door seals. It's when you multiply by six and get to $810 that it starts sounding like real money.

So as long as I'm looking at new pistons, I'm inclined to go with the Euro pistons and get the doming taking me from 8.1 : 1 to 9.0 : 1 compression ratio? Would the Jetronic be able to keep with it or would there be anything else keeping me from realizing all the Euro horsepower?

3.3 Li pistons can be had for a lot less money. They have the same bore sizes as the 3.0 but the wrist pin location is offset from center by a bit more. 2.0mm vs. 1.5mm for the 3.0 pistons, IIRC. Also the skirt appears to have a bit longer side to it on the side that the skirt extends further down; sorry don't know how to better describe this. Has anyone used them? Look at a picture here: http://www.ebay.com/itm/26065973133...=ADME:X:RTQ:MOTORS:1123&vxp=mtr#ht_1330wt_954 That small "tang/beaver tooth"" that extends down is not so pronounced on my pistons.

 

[threewebers]

Pitting

Grab a long block from a wrecking yard....

 

[Dirk]

I was hoping to find a double rainbow --all the way--...

You know it's like one thing leads to another...it's 35 years old, it's out of the car and I'm not pulling that thing out ever again. My goal is to finally have a vintage (almost) daily driver in the summer, with AC and an automatic, so I want to be persuaded that everything on it will be as reliable as I want it to be.

The reason for me asking that question was that with an acceptable compression ratio and so little variation between the cylinders, I personally would not have opened that engine, unless you were suspecting something else.

Mine had 80 in one of the cylinders (major corrosion from sitting years without use and a leaking head gasket) and I had no choice, a complete engine rebuilt, and indeed 6 x $135 plus, plus, plus .....

... but all that doesn't help you now, Im afraid.

 

[deQuincey]

Grab a long block from a wrecking yard....

....and ?

who tells you it will be in a better condition ?

i am not sure that this is a good idea, ...:confused: moreover, probably it is not so simple to find one

 

[Luis A.]

The reson for me asking that question was that with an acceptable compression ratio and so little variation between the cylinders, I personally would not have opened that engine, unless you were suspecting something else.

Mine had 80 in one of the cylinders (major corrosion from sitting years without use and a leaking head gasket) and I had no choice, a complete engine rebuilt, and indeed 6 x $135 plus, plus, plus .....

... but all that doesn't help you now, Im afraid.

Fair enough Dirk. Here's the scoop; the engine was coming out for an engine compartment and body repaint anyway ...And the head was coming off since it is really cruddy from all the EGR crap it's been breathing for years ... and the exhaust valves were giving up about 20% on the leakdown test... As a side note, I emailed the Spanish company that made those replacement heads (AMC) and they indicated my head was manufactured around 1984-5, and the car had around 30k miles at that point so the head is pretty fresh with ~50k miles on it and the valves and most gear should be able to be kept.

So, yes it was running well, but not perfectly. On the bright side, the crankshaft and main bearings are good with radial clearances well within spec at around .050 mm or less with the spec being .030 to .070mm.

I want the engine to look as close as i can to the day it left the factory, block repaint (so boil it off first), replate hardware, etc. All in all, not one of these was *requiring* that I tear it open, but I wanted to in order to ensure it will be as realiable a driver as I can get and that it will look as good as when it was new. That what I really enjoy doing...:grin:

As for the wrecking yard suggestion, agree with DQ... Most of these cars, and engines, have been in a state of perennial decay for so long that it's hit and miss with a "new" block. Maybe different if I lived in California. Plus, call it silly, but I really enjoy knowing the number Fritz stamped on that engine is the same as that found on the car...

 

[siscobmw]

rebore cilinders, new pistons...imho.
if you want to put it to euro spec, 9:1 pistons, get rid of thermal reactors, use euro exhaust manifolds, use high pressure injection system(maybe giannicsl has something here in italy) and throw in a bin egr hardware....
you will get almost those 195 bhp that euro 3.0Si with l-jetronic had when new.
Francesco

PS:expensive but really desiderable opinion, mine.....