How can an engine have more stroke without increasing the distance from the crank pin center to the center of the crankshaft? I have two engines that have the same crankshaft yet one is supposed to have 1.6mm more stroke. The bore is larger.

Brian check the distance from the center of the crankshaft to the center of the crankpin, if they are the same then the stroke cannot be longer. It sounds like somebody mistakenly called a larger bore a larger stroke.

Hi unless you have V blocks and dial indicators and a height gage its hard to check the amount of stroke ,you may want to try this and it will get you very close .With the heads off and the piston in place rotate the engine over to TDC and measure it with some dial calipers then rotate it to the BDC and measure it again.Do this to both engines and then you will have a better idea if there the same or not .Digital calipers aren't very expensive, Under 20 bucks at most auto parts stores .1.6 mm is .063 rounded off so it should be noticeable even using a 6 inch scale.Hope this helps.

Stroke an engine?? Let's see, hold it in your left habd and gently caress and rub it with your right hand.

If the same crank is used in both engines the stroke i the same. No need to measure.

But if you want to measure it, take the head off as the Wolf said, then using a long stroke dial gauge you can get a direct reading with no effort.

Bill.

William robinson wrote:
No, this is incorrect!
You have to take the length of the crankpin into account as well.
If two engines uses the same crank, the one with the shortest crankpin will have the longest stroke.

/Red B.

Addendum: My statement above is simply wrong. My sincere apologies!

``Red:

Quote:

ORIGINAL: Red B.
...If two engines uses the same crank, the one with the shortest crankpin will have the longest stroke.

Huh??? If the cranks are the same how can the crank pin be any different one from the other? Besides, a longer crank pin just gives more bearing area, it will not affect the stroke. Stroke is controlled solely by the center to center distance of the shaft itself and the center of the crank pin.

Bill.

The length of the crankpin has no effect on stroke. That's like saying two teeter totters having the exact same length and having their fulcrums in the exact same place are different if one has wider seats. Nope, the kid on either end will go up just as high on both teeter totters.

Stroke is determined by the distance of the crankpin's center from the crankshaft's center. A fatter crankpin or longer crankpin will make no difference.

Some content removed by moderator.....

Removed because of temporary loss of mental powers!

/Red B.

let's take the OS120 Surpass. One is pumped and one isn't. The pump is driving by a little extention on the crankpin and is about an 1/8th of an inch longer. By your reasoning the pumped 120 must have a longer stroke. Taint so.

This is making my head hurt![sm=spinnyeyes.gif]
Someone is going to have to explain this to me...
I don't get it...
JLK

That is wrong too. It doesn't matter how long or short the rod is. The piston will move exactly the same distance up and down based on the distance from pin center to crank center. That up and down movement is the stroke.

Bill, It's stuff like this taht almost ruined my hobby when I worked at hobby Shack as an engine fixer. I had to argue with these guys all the time. Finally gave up. I ended up agreeing with all the goodball ideas that came along.

Red:

OK, translation error. But con rod length still makes no difference in the stroke.

The length of the con rod is a compromise, the shorter the rod the shorter the engine can be, giving less engine weight. At the same time, the shorter the rod the greater the rod angularity, causing higher side load on the piston and greater friction losses.

The stroke will be the same regardless of the rod length.

Bill.

All a longer or shorter rod does is lengthen or shorten the overall length of the cylinder. A long rod just means a tall cylinder. The piston still moves the same distance up and down and still has the same stroke. That's why the old Holland Hornet 049 was short compared to the TD 049. They had the same stroke but the Hornet's rod was a bit shorter. Problem is that a short rod puts higher side loads on the piston and cylinder

Removed because of temporary loss of sanity!

/Red B.

The stroke will be the same regardless of the rod length.

Bill.
Can't be put any plainer than that. The distance from BDC to TDC is determined by one thing, distance from the crank pin to the center of the crank times two...

Red

Are you maybe thinking about compression ratio instead of stroke. The compression ratio will change with a change in conrod length.


Dennis

See what I mean Bill?? I put up with this stuff for 6 years. I finally started telling these guys they were all probably right and that even though I was the engine repair tech I knew absolutely nothing about model engines. The other thing I loved was when they asked you a question concerning an engine because your were "the expert" and then they would argue with you. I finally answered with the same line. "If you knew all that, why did you ask me in the first place?" I quit repairing engines for pay and I quit answering questions about them unless I knew the guy in the first place. I only repair engines for club members now and only a few of them. I never charge. That way they can't scream if their engine doesn't meet their expectations. It makes life a lot easier and I have more fun at the field. I'm too old to take a load of crap for helping someone.

Bruce

AMA 13797 modeling for over 50 years and I still don't know what makes them work. I can fix them but don't know how I did it. Maybe I'm an engine Idiot Savant.

I'm very sorry! I really messed up this time!
My apologies to William, who is absolutely correct. I did the math myself and it hit me like a ton of bricks what a stupid statement I did.

Again, please accept my apologies William!

/Red B.

Changing the rod length won't change the compression ratio either. Only if the deck height doesn't change. You could even have a lower compression ratio if while lengthening the rod you raised the deck height. Shave some off the top of the liner and you raise the Comp Ratio. To many variables to make that statement. The only exact thing is the stroke is based soley on the distance of the crankpin and crankshaft centers and it is a constant given a given crankshaft.

It's called geometry and I taught it in high school.

Red:

No problem sir, it's good that you now see the relationship.
------------------------
Dennis:

Lengthening or shortening the rod, with no other changes, does affect the compression ratio. But I will point out that's far from the easay way to do it.

Bill.

Hypothetically, it would be cool to make a longer stroke crank to bump up the displacement wouldn't it?!

But there just ain't no room in the case to allow it anyway, so...
(they make the clearances rather tight between the big end of the rod and the inside of the case )

About the only way you could increase the length of the stroke is to make the crank-pin smaller in diameter and move the center point of it further away from the cranks axis of rotation. (off-set grind it )

But you wouldn't get much, without significant loss of strength of the crank-pin. And you'd need to make a new rod, etc. etc.
Maybe make a new crank-pin out of "Nobendium" so it could be significantly smaller? Naw...then you loose bearing area!

Ah but like you said with no other changes. If you make the rod longer, whamo! it hits the head. Shorter it lowers the ratio and if you raise the head the same amount that you lengthened the rod no change. So it doesn't really mean much as far as compression ratio goes.

Are you right or left handed?

That'll work in a 4 stroke so long as the valves still clear the piston but if you do that in a 2 stroke then you'll also change the port timings. The easy fix there is to shim up the liner by the same amount you've lengthened the rod. Of course, then the compression is back to where you started so lengthen the rod a bit more. Then to fix the timings shim up the......uh oh...I think I see a problem looming here...