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So I have to bore my cylinders...


flagoworld

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Which means I need new pistons... And what else? Looking for insight on what you would do. What setup would you run, on a moderate budget? My head has a reground 292, and I'd like to keep things similar to stock. "Cheap" performance improvements are welcome. Example: Extra $40 for _______ part over the stock because it has ______ benefits.

'74 Verona

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Whatever you do - do *NOT* bore your cylinders until you have the replacement pistons in hand. There are several vendors that sell aftermarket pistons, and some of those may vary from piston to piston, with regard to diameter. 

 

You get what you pay for sometimes.

 

If you opt for less expensive pistons, ask your machinist to bore each cylinder for that specific piston, if they have variations in size. I've seen enough people burned on this one, that it bears repeating. 

 

IMHO - OEM Mahle or KS pistons are the best - both used by BMW when the car was manufactured, but not always easy to find. 

Paul Wegweiser

Wegweiser Classic BMW Services

Nationwide vehicle transport available

NEW WEBSITE! www.zenwrench.com

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So I should ask the shop what the min bore is they can do, buy pistons slightly bigger than that, then give them the pistons to reference each bore? What about connecting rods? Can those remain? How does this affect my compression ratio exactly? Etc.

'74 Verona

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Essentially yes. Your machinist will probably have reference material for the common oversized measurements, as prescribed by BMW. Either +.25mm over or .50mm over....though you can sometimes find OEM pistons in +1.00mm over. (that's the size I now run)

 

I've never had a connecting rod fail. Have all of them inspected, perhaps balanced (BMW rods are usually very close in weight, when compared to other cars from that era) and have new small end bushings installed and fitted to the wrist pins of the pistons. 

 

Unless you're changing the shape of the piston, or it's vertical measurements, this shouldn't affect compression to any measurable degree...but if 9.5:1 pistons are available for the same price as 8 or 8.3 units...it pays to upgrade the compression (within reason.)

 

BMW used several different cylinder heads on these cars - each has unique combustion chamber shape, and requires a specific piston type for optimum operation. You should determine if yours is "E12", "121" or "E21" casting - stamped on drivers side of head. THAT will determine your piston options.

Paul Wegweiser

Wegweiser Classic BMW Services

Nationwide vehicle transport available

NEW WEBSITE! www.zenwrench.com

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E12... piano topped. Anyone who knows of a site not mentioned yet that sells pistons, I welcome the flood. Time to research and, sadly, spend gratuitous amounts of money on pistons. :'(

Edited by flagoworld

'74 Verona

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And here's a tip....that will save you the HOURS of research I spent, last year.

 

If you use the +1.00 (89.97/90mm pistons) and need rings - either because the ones fitted are damaged or rusty, or if they gat damaged during installation - you can use the std size piston rings from a BMW R90 motorcycle. I insisted on using OEM Goetze/Mahle rings on a used set I had acquired, and the ONLY ones I found were these. I checked dimensions, diameter, profile, alloy...and found them to be identical to the original 2002 rings...but in the +1.0mm oversize...which BMW never offered for automobiles. I ended up NOT using the used pistons, and re-sold the new rings later on. 

 

You can get a decent price on these rings from companies like "Motobins" in the UK - still cheaper that any BMW cycle shop in the US - even with postage.

 

And regarding the above ebay pistons... one set is marked "8.3" as in 8.3:1 compression. You might consider searching for 9 or 9.5 compression units....or have the seller verify their claims of "high compression" application, by comparing measurements to true tii pistons. There's no substitute for displacement...except maybe compression. ;)

Paul Wegweiser

Wegweiser Classic BMW Services

Nationwide vehicle transport available

NEW WEBSITE! www.zenwrench.com

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Read piston ads VERY carefully.

 

One of those ebay ads is for original size pistons and another is for THIRD oversize.

 

I doubt if any 40 year old block wouldn't benefit from some oversize pistons. and third oversize is maxed out.  (anyone ever see FORTH oversize pistons?).

.....just sayin'

 

mac

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Thanks, guys.

 

Naz, I'll let you know if I need those.

 

mac, I will be sure to only go single oversize to start with so I have some wiggle room in the future. IIRC this block is as vanilla as it gets. Never been bored before.

'74 Verona

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I personally wouldn't worry too much about the future. It's the last of pistons this car will see in my lifetime. Last flywheel ect. Just build it in the fashion you wish to enjoy it.


Sent from my iPhone using Tapatalk

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Here's an option for your E12 Head.  90mm QSC "Piano" top Pistons.  Apparently a couple of people have purchased them and reported that the cast lines needed additional cleanup, but for $148 for a set of 4, could be worth your time.  These apparently use BMW 2002 rings, so Wegweiser's tip to use BMW R90 Motorcycle rings may be the ticket.

 

http://www.ebay.com/itm/QSC-Pistons-Set-for-BMW-2002tii-E12-M10-72-75-11251261881/261435360374?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D40130%26meid%3Dd8d2d1a11b3a440d935f5f28c4b09ac9%26pid%3D100005%26rk%3D1%26rkt%3D6%26sd%3D142318844866

 

Mark92131

1970 BMW 1600 (Nevada)

 

 

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