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Deck height variation vs. crankshaft thrust clearance

 
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robertob
Turbo Slant 6


Joined: 24 Jul 2008
Posts: 533
Location: Long Beach, CA

Post subject: Deck height variation vs. crankshaft thrust clearance (Mon Sep 16, 2013 1:39 pm) Reply with quote

I finished gapping the rings and assembled the rods & pistons into my motor yesterday and my deck height is all over the place.

My target was .010 out of the hole (based on measurement of .003 average with my tester piston & rod, I had the machinist take off .007)

Here's what I have (center of the piston average with piston rock):

1 011
2 015
3 015
4 015
5 016
6 015

This block has been decked and align honed to the deck, so there is not be an issue with deck-to-crank alignment. New K1 rods & Wiseco pistons, though I did not measure, should not vary by .005.

I swapped #1 and #5 and got the same number (.011 on #1 and .016 on #5). So I assume it's a problem with the reground crank - has anyone seen a cylinder-to-cylinder variation in stroke of .010?

I went with the crank that's in there now because it had less thrust clearance (.004 versus .012 I think, can't remember) than the other one I have and it's an early crank so I don't need an adapter bushing or later flex plate.

I guess I could swap out cranks, but no guarantee it will be better.

Any advice? Just ignore it and run?


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Dart270
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Joined: 21 Oct 2002
Posts: 10437
Location: Blacksburg, VA

Post subject: (Mon Sep 16, 2013 9:38 pm) Reply with quote

I think you are within tolerances there. I have seen variations such as that. Mark Etheridge saw bigger variations on one of his motors/cranks. Most likely it is a crank journal offset issue?

Lou



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Joshua Skinner
Supercharged


Joined: 27 Jan 2005
Posts: 6943
Location: Portland, Oregon

Post subject: (Tue Sep 17, 2013 7:45 am) Reply with quote

According to my machinist, it's usually the crank. He built a fixture to quickly check for variations in the crank throws. It's really tough to get a crank where all the journals are spot on.



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Joshua Skinner
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robertob
Turbo Slant 6


Joined: 24 Jul 2008
Posts: 533
Location: Long Beach, CA

Post subject: (Tue Sep 17, 2013 9:25 am) Reply with quote

Thanks, that's what I figured. The other crank may or may not be better, and it has excessive thrust clearance.

I know I could find another crank and have it reground to centers by a specialist shop but that's adding a lot of dollars for likely minimal gain.

Alternatively I could have the machinist cut the pistons down to the same height. Again more money...

.005" variation is something like .7 cc, or about .1 point of static compression.

CR is going to be higher than I wanted - 10.50 to 10.67 depending on how the combustion chamber variation lines up with the deck height variation.

But I think it will be OK if I back off the cam timing from 100 to 102 degrees. ICL.


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CNC-Dude
Turbo Slant 6


Joined: 20 Feb 2010
Posts: 877
Location: N. Ga.

Post subject: (Tue Sep 17, 2013 9:55 am) Reply with quote

Since your dealing with an open chamber style head, trying to get your deck heights all the same or even close is really a moot point since the depth of your combustion chambers are going to vary more than that. If your were dealing with a quench style head, then that would be a good practice to follow. You'll be fine with the readings you have. As you already pointed out, it is more money to cut pistons or swap crank and for no more gain to be seen from the extra effort.



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robertob
Turbo Slant 6


Joined: 24 Jul 2008
Posts: 533
Location: Long Beach, CA

Post subject: (Tue Sep 17, 2013 11:57 am) Reply with quote

Very true. While head CC is within 1 CC, or as close as I can measure, the depth is all over the place.

Sometimes I wonder if it's not better to be ignorant of stuff like this... If I had just not bothered with the dial indicator I wouldn't know any better.


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CNC-Dude
Turbo Slant 6


Joined: 20 Feb 2010
Posts: 877
Location: N. Ga.

Post subject: (Tue Sep 17, 2013 3:24 pm) Reply with quote

Its always good to follow sound techniques and methods when engine building, you just have to remember that things like this don't always have the same effect on all engines, based on said parameters mentioned. Its still good to know how to do those things though.



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