Jump to content
fjord-tii

tii pistons

9 posts / 550 viewsLast Reply

Recommended Posts

I inherited four USA tii pistons and seek expert advise on whether they are worth using in a rebuild. I could not find the specs on the pistons, so list here what I could measure (I’m not a machinist!).

 

There is very slight scuffing on the sides, three of the four have corrosion that I assume could be media-blasted  (plastic beads? walnut shells?) to clean. The inside casting says Mahle 89VII, the diameters (marked 89.97 at the top) with “roundness” +/- 0.1mm using a low-quality calipers. Weights are all about 515g., ring gaps 1.81, 1.98, and 3.99mm. Worth keeping?

8DB413E1-0735-4033-93F5-37D7D3BB752D.jpeg

Edited by fjord-tii
Photo added

Share this post


Link to post
Share on other sites

I'd get a gallon or two of "carb dip" and let each one soak for a few nights. I wouldn't use anything abrasive on them, except maybe a red scuff pad - used VERY gently every night before re-dipping them, just to get off the major corrosion. The crucial measurements will be the ring lands / ring grooves and wrist pin bores. From the photos, they look pretty rough. I wouldn't personally use them, but it depends on how much you're willing to gamble (based on your own time or a machinist's time for the rest of the engine build process.) 

 

IMHO: The skirt scuffs are less worrisome than the crucial measurements of the ring grooves and wristpin bores. 

 

EDIT: Those look taller than USA spec compression pistons... I bet they're slightly higher. Measuring the height of the crown and raised portion would be the tell. Rarely do aftermarket pistons come in USA compression. If they're aftermarket Mahles (and they must be, if they're 89.97mm, since BMW didn't sell that size) then I bet they're Euro spec compression. That's a good thing....if you can save them.

Since these are 89.97mm... that means a 90mm bore - and once you go that big, you're pretty committed. If they fail, you're into it for a new block or custom sleeves, for the next rebuild. 

 

I have found a source for NEW OEM Goetze rings in the correct 90mm size with correct profiles and thicknesses, to re-ring these. BMW never offered this overbore, so sourcing them in the OEM aftermarket is your only option, unless you just want to slap some Deves or other rings on them. I did a lot of research on the TotalSeal rings, and decided I wasn't comfortable with them, based on some reviews and information regarding their dynamic sealing qualities. (great for static leakdown numbers, but flexing during use was the theory that worried me.) 

 

Hope this helps at least a little bit!

Edited by wegweiser
  • Like 1

Share this post


Link to post
Share on other sites

Factory specifications for all 2.0 liter OEM (Mahle / KS) pistons is as follows:

 

top compression ring thickness = 1.75mm

top compression ring clearance as installed = .060-.082mm

total permissible groove width = add the above numbers

 

second compression ring thickness = 2.0mm

second compression ring clearance as installed = .030-.062mm

total permissible groove width = add the above numbers

 

oil control ring/scraper thickness = 4.0mm

oil control ring clearance as installed = .020-.052mm

total permissible groove width = add the above numbers

 

Wrist pin bore in piston = 22mm with permissible .002-.006mm of play. 

 

Get out those feeler / bore gauges!

Share this post


Link to post
Share on other sites
(edited)

Paul (Wegweiser), above, said it already: assuming the pistons clean up beautifully — which might not be the case here — do you really want to overbore your block to fourth (third?) oversize if it only needs to be over-bored to first or second oversize? Don’t do that!

 

These are grand piano pistons, suggesting they are for an E12 head. Do you have an E12 head?

 

Regards,

 

Steve

 

Edited by Conserv

Share this post


Link to post
Share on other sites
3 hours ago, HBChris said:

Doesn’t scuffing indicate overheating in the past?  I would buy new from Walloth.

I agree that new pistons would be the way I would go. Does W&N have the Mahle oversized E12 pistons in stock? Good to know!

 

As for scuffing, you are correct. Not sure how this might affect future performance of the piston, unless the ring lands were damaged (as was the case in one of my cars 30+ years ago, after overheating). A painful and expensive lesson to learn. I had to drive an AMC Eagle station wagon while the repair work was done. To this day, I taste bile every time I see one.

 

I will never try to limp home an overheating 02 ever again.  :)

Share this post


Link to post
Share on other sites

Thanks everyone for the feedback. Sounds like they might make good paperweights, along with the valves from my last rebuild. Will clean them and see what they look like dimensionally.

Share this post


Link to post
Share on other sites

Polish them up and use them as trophies at your next club competition..

  • Like 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.




×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.