Piston and Rings

[Marine240z]

Hi everyone, I am in the final stages of a complete rebuild of my 72 Z.  133 thousand miles on the odo.  Transmission is complete, head done with a midly agressive cam, block refinished and bored to .030, crank polished, ground and balanced. I had ordered a "complete" engine kit including fuel, oil water pump, all gaskets pistons, rings. This kit was ordered from a well known z car parts supplier.

I received the kit and a thourough inspection by me and my trusted mechanic exposed the following issues:

 

1.  Head gasket was too thick and did not fit correctly.

2.  Pistons were ITM make and skirts ( bottom) was 0.5-1mm wider than the piston head.  They were also 26-30 grams heavier than the stock ones I removed. ( I know, the were 83.75 vs 83.00). Two of pistons weighed 1.5 grams more than the others, and one weighed 2.1 gram more. 487 grams was the base weight. 

3. oil pump did not rotate completly

4. Timing chain guides had 1 small air bubble underneath the rubber coating

5. Timing chain was thin and not as robust as the stock one removed. 

 

After an exhaustive internet search, the following corrective action(s) were taken:

 

1. Felpro oem gasket ordered ( $56.00) vs $22.00 kit  Fits perfectly.

2. New  " Silv-O-Lite pistons ordered from United Engine & Machine Company, Carson, Nevada. They have been in business since 1922.  I also ordered new " perfect circle"  rings from Mahle sizes  2mm 2mm 4mm.  The web site for the pistons is:   uempistons.com

2-5. New oil pump and timing chain kit from Beck Arnley

 

The pistons and rings should arrive Friday so I will update at that time.  Attached are a few picks I will also downloads more soon, and document rebuild as much as possible.

Also am replacing the stock exhaust with a new 35 year old original "new old stock" ABARTH. See attached ad from the  August 1973 Road & Track magazine.

 

Lon

Edited August 12, 2015 by Marine240z

[cbuczesk]

From what I understand the pistons are tapered. Narrower at the top so they can expand properly when they get up to temperature. The skirts don't get nearly as hot. I don't know how much taper there should be.

 

Chuck

[madkaw]

The pistons were made that way. The machinist should size bore based on skirt size. The biggest issue is rings. Rings usually are sized for standard overbore. Since the skirt is over the overbore size, and you allow clearance too, then you are too big for rings.

[Zed Head]

 This kit was ordered from a well known z car parts supplier.

There's nothing wrong with putting the name out here, especially if you've done the homework and taken the proper measurements.  We need to know who the good and bad suppliers are. 

 

The "How To Rebuild..." book )pg. 76) says the skirt should be at least 0.0005" (.013 mm) bigger at the skirt than at the wrist pin area.  No upper limit mentioned though. .75 mm (the middle) seems off, about 60 times bigger than minimum.  Even your smallest measurement seems big.  The question though is what are the ITM specs.?  It will be interesting to see what the Silv-O-Lite pistons measure at.

 

What does "did not fit correctly" mean on the HG?

 

Looks like fun.

[Marine240z]

I really feel that the pistons were just Chinese or Taiwan poorly made copies.  The kit itself was junk as well and should never be sold by a reputable z supplier.  These items should be sourced from American, Japanese, or European suppliers who work with exact detailed measurements from Nissan. 

Madkaw, I read your rebuild thread, and am surprised that after all the issues you had that your suggestion the skirts are usually wider than the bore was made seriously.  The piston should be perfectly sized from top to bottom.  The expansion will be determined by the alloy make up and silicon content.  According to Silv-o-lite the skirt should only exceed the diameter of the head if the skirt is coated with a dry film lubricant and then it will only be .001. 

Good cast pistons ( forged is a whole other topic and expense) should be uniform in measurement and weight and the copies from Asian producers are crap and should not be sold by any reputable supplier especially when you are dealing with investments such as ours.  There are really only two good suppliers for cast pistons in the US.  UEM and Birmingham Machine. Ring sets should come from Hastings, or Mahle or possibly other "American" manufacturers.

Our original pistons were hypereutectic and contained an average 12-14 percent silicon. I was amazed at the overall good condition of mine which is a testament to the build quality of these great cars.     

Edited August 12, 2015 by Marine240z

[Marine240z]

Zed, the piston bore hole in the gasket extended slightly into the hole, the gasket itself was 3 mm thicker than the stock. I have not seen the "How to Rebuild" manual and was unaware the tolerances for skirts were addressed. This is my first rebuild so maybe I am being a little anal.  If I was not so concerned about keeping all the components matching stock original, I would have just purchased a pre-made crate 24L and been done with it.   Much simpler but not as satisfying.

 

Keep your inputs coming. I post some more pics later this evening.

 

Also check out the Silv-o-lite section in the catalogs on the uem home page. It has a ton of great info and calculators.

 

Lon 

Edited August 12, 2015 by Marine240z

[Zed Head]

The kit itself was junk as well and should never be sold by a reputable z supplier.  

 

Madkaw, I read your rebuild thread, and am surprised that after all the issues you had that your suggestion the skirts are usually wider than the bore was made seriously.  The piston should be perfectly sized from top to bottom.  

You're still not helping us if you don't tell where you got the kit.

 

(Edited some stuff out)...The reason the head is lower diameter than the skirt is for two reasons - there's more material there to expand, and it gets hotter than the skirt area.

 

 

Here's some terms - http://www.hastingsmfg.com/ServiceTips/piston.htm

 

Here's a really good link from a well known university, used by another well-known university - http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20Design.htm

Edited August 13, 2015 by Zed Head