L28 Crankshaft Modifications

[inline6]

So, I'm spending a bunch on building a 3 liter bottom end. I bought a standard L28 crankshaft and had it sent off to Castillo's Crankshaft Service in CA to be worked on.

 

Here is a pretty cool article in Turbo magazine about them and the work they do: http://www.turbomagazine.com/tech/0610turp_castillo_crankshaft_service/index.html

 

I am having the crank offset ground, lightened, and balanced. I thought the note I got from them earlier this week was really interesting... The stroke on the stock crankshaft is not quite "stock". Read on:

 

 

I read your info sheet to what you need on your nissan crank. Stock stroke is 79mm = 3.110, increase to 81mm = 3.188.

 

I checked your crank with my stroke gage (Starret 0-5 inch travel). What it actually measures is:

•#1 Rod 3.103

•#2 Rod 3.103

•#3 Rod 3.104

•#4 Rod 3.104

•#5 Rod 3.102

•#6 Rod 3.101

 

So, your lowest stroke is .009 short from 3.110=79mm. Your stock rod journal size is 1.967 std. Finish size for 3sgte rod is 1.8892/1.8898 from the Clevite book. So, actual difference is .078. That is exactly what I need to make the 81 mm stroke, but we are short on stroke from factory...

 

Remember, the #6 rod is 3.101. So, if I add .063 to that, rough stroke is going to be 3.164: the .063 is the difference from stock journal size 1.967 to 1.904 which is .015" from finish size 1.889. I rough finish rod journals .015" oversize from finish spec, then I straighten the crank prior to final finish size because the crankshaft will bend upon stress relief. I might be able to get another .010" on stroke when finishing the journals... so it should be 3.174 stroke when finished. This will still be short .014" from 81mm = 3.188.

 

So, I have read several posts here on hybridz.org indicating different amount that pistons stick up above deck (L28). Range has been .012" to .022" I think. Seems like if my crankshaft was off by .006" to .009" then we have found part of the problem. Variability of manufacturing tolerances is something I have known about - that it exists... but this hit home in a different way. I think center to center on the rods is likely to be more accurate to factory spec. than crankshaft grinding. Not sure what I base that on, just seems to mee like crank grinding wouldn't be as precise.

 

Anyway, I thought I would pass this info along. Good thing I haven't ordered the pistons yet because the pin height (compression height) just changed.

[260DET]

That is more variation than I would have thought was within factory tolerances. Perhaps the L cranks were made using equipment that is not as accurate as current equipment.

[Tony D]

That's a production variance, for sure.

This is one of those things you look for when building a performance engine.

 

The closer you look, the more suprised you will be!

[black240]

oh boy...

[Gollum]

And people make how much power on those wide production tolerances?!

 

Obviously when building a motor closer and closer to the limits of intended purpose the more fine the detail comb needs to be, I just find it ironic how you can get super anal about certain things that WILL make a difference, yet it's always amazed me how much you can get away with not doing when you know WHAT you're doing.

Edited May 8, 2011 by Gollum

[cygnusx1]

Maybe they short stroked cylinder 6 because they knew it ran the hottest.