Pistons

[jasonb]

I've been giving a lot of thought to this. I was originally going to use 4.0 rods and Bulltear pistons but I think I'm changing my mind. I don't really want to spend the money on forged pistons and I'm not real keen on the idea of piston slap, all that beside the fact that I don't need forged pistons. I have a set of 258 rods sitting here and while I understand long rods are better in theory, I can't imagine they'd make any real world difference in my application. So, I've been looking at the other available pistons and am leaning toward the H802CP or H825CP. As far as I can tell the only difference is metric vs standard ring grooves so why is the 825 2x the price of the 802?

Jason

[Russ Pottenger]

The metric rings are thinner and have less radial tension.

If you want to do a low-cost option, I can do a 24cc dish in
a 677 or 2229 piston. That gives you the option to deck your
block for quench and still have a good static compression ratio.

$200.00 shipped to your door.

If you send me your email I'll forward you pictures of
the pistons


Russ

[jasonb]

I see now that I was forgetting to put the 258 stroke in the compression calculator.... Makes a big difference! So yes, looks like I'll need an extra cc or two in them. So, of the cast and hyper pistons, only the 2229 and 677 can be dished? What's the difference between the two? Which do you recommend? How do you feel about coated skirts? So many questions....

I'll pm you my email.

Thanks,
Jason

[Russ Pottenger]

The short answer is the 2229 pistons are made by United Engine & Machine,
The 677 pistons are made by Sealed Power/ Federal Mogul.
Any of the Cast/ Hypereutectic pistons can be dished.
Because you said cost was a consideration, The two examples I mentioned
are the most cost-effective .

Russ

[jasonb]

Pictures look good! So I take it you do that on a cnc mill?

Chances are I'll be getting some from you. If you were closer I'd have you do all the work for me.

Maybe you can explain something, all this I read about the short rods pulling the pistons out of the bottom of the bore more, doesn't make sense. No matter short rod / long rod, 3.895 stroke is still 3.895 stroke?

Jason

[jasonb]

I understand that the wrist pin centerline is .250" lower in the bore, but if we compare an 802's compression height of 1.592" to a 944 at 1.353", that's a difference of .239", and that means a difference of only .011" in piston position at BDC(and TDC for that matter). So is it this .011" that everyone is worried about, or is it wrist pin position?

Anyone, feel free...

Jason

[gradon]

I'm running the 258 rods and 825s, but will use Eagle 6.150" rods and custom full-floating pistons in the next stroker.
Edited my misinfo

[jasonb]

Respectfully, the pins are going to be where the combination of stroke and rod length put them. I agree that with long rods the pins will be higher in the bore, but by .250" because the rod is .250" longer. I'm trying to get to the bottom of the (mis)information I've read about the pistons being lower in the bore using the short rods.

Jason

[jsawduste]

It's not mi information. Run the numbers carefully and you will see the piston is pulled down more with the short rods.

[jasonb]

It's not mi information. Run the numbers carefully and you will see the piston is pulled down more with the short rods.

I did, five posts up. I come up with the pistons being .011" farther down with short rods. If I'm making an error then please correct me.

Jason

[gradon]

I effed up: the pins will be be .25" higher with the 4.0 rods than the 4.2 rods. Pistons have different comp heights and even skirt lengths, so yes the difference at tdc between the two is .11", but that .25" difference in where the pivot point is will put more lateral stress on the pistons and rings at bdc with the short rod combo.

[jsawduste]

I effed up: the pins will be be .25" higher with the 4.0 rods than the 4.2 rods. Pistons have different comp heights and even skirt lengths, so yes the difference at tdc between the two is .11", but that .25" difference in where the pivot point is will put more lateral stress on the pistons and rings at bdc with the short rod combo.

The actual number (amount of skirt contact/engagement to the bore at BDC) depends on the piston design. What you add to the top takes from the bottom and vice versa. In any case you cannot change the pin to crank centerline. That is a given set number. Since the 4.2 rod is shorter the small end is .25 down lower at TDC and at BDC. You cannot get away from that. The skirt can be only so long and the pin only so high.

[jasonb]

I effed up: the pins will be be .25" higher with the 4.0 rods than the 4.2 rods. Pistons have different comp heights and even skirt lengths, so yes the difference at tdc between the two is .11", but that .25" difference in where the pivot point is will put more lateral stress on the pistons and rings at bdc with the short rod combo.

Agreed, as long as you meant to type .011". I am aware of the side load increase with the shorter rod, my whole point here is that the only way the pistons would be pulled out the bottom of the bore more with a short rod is if the skirts are longer and therefore sticking out more or if aftermarket pistons (like the 944) were used on short rods.

Jason

[jsawduste]

You may not wish to recognize my reply as I did not offer up any hard numbers.

If you use your numbers the piston is pulled .11 out of the bore at BDC. That is almost half of the difference in rod length. The piston used as your example places the pin just below the ring lands.

The dynamics of the rod rotating the pin (press fit for your example) within the piston as it cycles between falling and rising places tremendous side load. Cocking the piston to the thrust side as the rod sweeps through it`s arc. As RPM increases the transmitted load will increase by a factor squared.

Bottom line, I am just not satisfied of the potential longevity of a short rod 4.0 based stroker.

[jasonb]

You may not wish to recognize my reply as I did not offer up any hard numbers.

If you use your numbers the piston is pulled .11 out of the bore at BDC. That is almost half of the difference in rod length. The piston used as your example places the pin just below the ring lands.

The dynamics of the rod rotating the pin (press fit for your example) within the piston as it cycles between falling and rising places tremendous side load. Cocking the piston to the thrust side as the rod sweeps through it`s arc. As RPM increases the transmitted load will increase by a factor squared.

Bottom line, I am just not satisfied of the potential longevity of a short rod 4.0 based stroker.

I understand that there is more side load with a shorter rod. That is not the purpose of my question. My question was about comments on this site stating that with a shorter rod, the piston will be pulled out the bottom of the bore more, which makes no sense. I'm just trying to clear it up.

By using my own numbers from before, the piston would be .011" further down not .11", that's 10 times what I stated. .011" is roughly the thickness of 3-4 sheets of paper, or a fingernail.

Here's another way to think about it without getting all caught up with rod length:
Stroker crank = 3.895", therefore the piston will travel up and down 3.895" regardless of rod length.
If the engine is built with 0 deck, then at TDC the piston is at 0 and at BDC the piston is at -3.895 regardless of rod length.
There is no other geometry going on because at both TDC and BDC the piston, pin, and rod are all in alignment with the main journal.

That being said, the piston is further down the bore with a stroker than a stock 4.0 crank. 3.895-3.413=.482, so almost 1/2" lower, but that's crank not rods, so that applies to all strokers.