[Roadsters] R16-U20 measurements and comparison

[Gordon Glasgow]

Since I have both R16 and U20 parts in my garage right now, I figured this
would be a good opportunity to document things. Eventually I may make a Web
page out of it but for now, here's what I found.

				R16		U20
			   -------    -------
Crank weight        38 lbs     46 lbs
Piston weight      454 g      471 g
Pin weight         125 g      125 g
Pin height         1.61"      1.61"
Rod weight	        792 g      954 g
Rod length         6.010"     5.670"
Crank to deck      8.937"     8.937"
Stroke             2.630"     3.267"

If you look at the following two pictures:
http://www.gordon-glasgow.org/R16_U20_Comparison/R16_U20_Cranks_1.jpg
http://www.gordon-glasgow.org/R16_U20_Comparison/R16_U20_Cranks_2.jpg
You can see why the U20 crank is heavier. Not only is the stroke larger by
about 5/8", but it is fully counterweighted. The R16 crank is not.

Since both engines use the same piston pin diameter (22mm), it is easy to
compare the pistons by putting one pin in both pistons:
http://www.gordon-glasgow.org/R16_U20_Comparison/R16_U20_Pistons_1.jpg
You can see first of all that both pistons have the same pin height. You can
also see that the skirt of the U20 piston has been cut back to clear the
counterweights and to allow for the longer stroke (remember, the piston will
come down farther in the cylinder bore at BDC).

So if the U20 piston is actually shorter in the skirts than the R16 and
doesn't have a dome, how come it's so much heavier?
http://www.gordon-glasgow.org/R16_U20_Comparison/R16_U20_Pistons_inside_2.jp
g
The walls of the U20 piston are twice as thick as the R16 - 0.125" vs
0.0625". And I measured the material thickness of the crown in the center of
the piston at 0.371" for the U20 and 0.295" for the R16.

I used the same "common pin" technique to compare the rods:
http://www.gordon-glasgow.org/R16_U20_Comparison/R16_U20_Rods_piston_pin.jpg
It is clear that the R16 rod is longer. Also, the pin boss is actually
beefier.

So why is the U20 rod heavier than the longer R16 rod? Here's the answer:
http://www.gordon-glasgow.org/R16_U20_Comparison/R16_U20_Rods_crank_journal.
jpg
I pulled the caps off the rods and set them up on a couple of blocks of wood
so that the big ends would be at the same height. This not only shows
clearly how much longer the R16 rod is, but it also shows how much stronger
the U20 rod is. Look at the thickness of the beam and at the extra meat
around the big end. 

It looks from the measurements like the R16 actually has a higher deck
height than the U20:
Deck height = Crank to deck - (Stroke/2 + Rod length + Pin height)
R16 - 0.0020"
U20 - 0.0235"

I don't have any Z pistons around to compare to, but if one of the theories
is to use the R16 rod and a Z piston in a stroker motor, the pin height will
need to be around 1.27".

Gordon Glasgow
Renton, WA
www.gordon-glasgow.org