Will this cause any problems with clearence at the oil pan rail or anywhere else or will i have to have a costum set of pistons made etc..

Whats your comments with using 6" rods with a 3.875 crank

Also will i have to run a Canton Oil pan with a 3.875 stroke crank? Or can i use the stock oil pan?

I think this is good question for Advanced Tech or i dont mind if its moved to LT1 Tech section just need the a few ? answered
Thanks Lance

 

The clearance issues relate to the stroke, not the rod length. For a given stroke and rod length (as well as block deck height) you will need a specific compression height for the piston to get the desired piston deck (distance from the flat part of the piston crown to the block deck).

block deck height - 1/2(stroke)+rod length+compression height = piston deck

For example, my 3.75" stroke combo has a 9.020" block deck height and uses 5.7" rods. My desired piston deck is -0.005" (.005" down in the hole). So the compression height has to be 1.440":

(3.75/2)+5.7+1.440 = 9.015"

9.020" - 9.015" = .005"

Most people go with a 3.75" for their stroker due to difficulties clearancing the block, oil pan, etc. But a 3.875" is do-able. As you have implied, there is a better selection of off the shelf pistons for a 3.75" stroke. A 6" rod and a 3.875" stroke leaves a short compression height. This makes for a light piston/rod combo but may compromise ring support, especially important for a power adder setup. In any case, pistons with the needed short compression height should use "buttons" or some other method to support the rings as they intersect the pin bore.

Rich Krause

 

Rich -

On that kind of motor, how thick would the top land be? At least .150 maybe?

 

The taller compression height gives the piston designer more options wrt the rings lands, for sure. I don't have a piston in front of me to measure, but I think they are at least that thick.

FWIW, the standard compression height for a 350 piston (3.48" stroker, 5.7" rod, 9.025" block deck height) is 1.550". Do the math to see what piston deck this gives. Most engine builders like ~0.040" of piston to head clearance for steel rods. This gives optimal quench while avoiding possible interference. If you go throught this exercise, you will see why many engine builders like to "zero deck" the block. Don't forget to add in the compressed height of the gasket when calculating piston to head clearance. The other ways to get the desired piston to head clearance are to adjust the gasket thickness or the piston compression height. For my new combo, I needed a custom piston anyway (10 over rather than the usual 30 over) so I ordered a custom compression height.

Rich Krause

 

Yes, I know that an good "ideal" quench height is .040, with .020 of that being the gasket.

What I'm wondering is how thick (or thin) the top land is when you use a 6.125" rod in these engines.

 

Well, if you look at good piston for a long rod what they have is some kind of a ring support where the ring interects the pin bore. Usually a "button". That's how they get acceptable thickness for the ring lands with such a short compression height.

Rich Krause

 

what is the quench in a stock lt1?

 

It's too big. The piston is 0.025" in the hole to start with and the gasket is what, 0.040" (I'm am blanking on it for some reason). I am not sure whay it's set up this way. Production tolerances maybe?

Rich Krause

 

Rich -

I think you're misunderstanding my question.

In any piston you have a given pin height from the centerline of the pin to the top of the quench deck.

A wiser man than me once did some extensive testing, and this was with older materials technology, and he discovered that the best general ring pack setup for endurance engines is:

top land = 150 - 180
compresion ring - 1/16
2nd land = 150 - 180
scrape ring = 1/16
third land = 090 - 110
oil ring = 3/16
bottom of oil ring groove to top of pin bore ? 010

.150 + .150 + .090 + 010 = .400 = 4/10

1/16 + 1/16 + 3/16 = 5/16

4/10 = 64/160
5/16 = 50/160

64/160 + 50/160 = 114/160 = 57/80"

57/80" = .7125"


So....
If I wanted to run a 6" rod with a 3.875" stroke in my LT-1, and I wanted to make sure I met the minimum tolerances that Smokey said were needed for an endurance engine to live, I would want to know if I would have a pin height of at least .7125"
(Assuming I have at least a 9.010 height after zero-decking.)

 

Math baby.....


Deck height - 1/2 stroke - rod length = compression height.

9.00 - 1.9375 - 6.000 = 1.0625

Usuallay 9.00 is what you are going to get on a zero decking.

Smokey did that research YEARS ago. Top ring land depth is all dependant on how much power and heat you have to deal with along with the type of rings you use.

Bret

 

It's ok for the rings to intersect the pin bore as long as there is a button for support of the oil ring. Good pistons will have this when needed.

Rich Krause

 

LOL, that book has a permanent home on top of my toilet tank.
Every time I read it I get something new.

 

Kick started the CAD program cuz I wanted to see what the side clearance was (I've been considering a similiar combo but thought 5.7 rods were the largest that would fit without notching the bottom of the bores).

Anyhow, with the 3.875/6" combo, I'm showing a minimum of 0.94" clearance between the rod journal and the bottom of the cylinder. This occurs at 22.5° btdc abd again at 22.5° atdc. Judging from the reduced rod angularity of the longer rods, this should be enough to clear the caps.
Edit: After checking several rod manufacturers websites, I found that big-end thickness is generally around 0.94", meaning with the above stroke/rod combo notching the block would be necessary.


Anyone know the distance from the crank centerline to the camshaft centerline and specs for the pan rails (vertical distance from the crank center and horizontal distance from the block centerline)?

 

Smokey's book is a great read. The old man is getting the autobiography for Christmas (when it gets here in the mail)

The cam to crank centerline is 4.521

Get the Chevrolet Power book from HP books for stupid info like this. (HPBooks-1087)

Not exactly sure what you are asking for with the pan rails.

Bret