Is .030 the lowest piston to head clearance in a 420cc? How do you measure a rod length?

[BLACK DEATH]

I pulled the head off my new duromax 16hp 420cc engine, to see the piston deck height.
My dished piston is about .003 out the hole with a stock rod.
It has a .055 head gasket measured at the edge, if that’s where ur supposed to measure it or that’s accurate enough.
I have a flat top piston for a non hemi engine I wanted to install to get a little more compression And maybe a thinner head gasket.
Two things concern me now, the rod length, as it seems may be different in 420s so the flat top may sit lower or higher.
How do you measure your rod length? From what point to what point on the rod?
But is .030 the lowest number you can use for piston to head clearance or you can go lower?
Or does rpms also play a roll on piston to head clearance?

 

[flattop1]

Center to center .
rpm plays a roll .
.030 is safe you can go lower at your own risk .

 

[OVALTECH1]

Check it on the edges too... you will lose some quench distance to piston rock

 

[BLACK DEATH]

Center to center .
rpm plays a roll .
.030 is safe you can go lower at your own risk .

center to center on the rod length? Correct?
Any idea on rpms and clearances?
‘let’s say 5-6k rpms?

 

[flattop1]

Yes . I would not go any less then .030

 

[BLACK DEATH]

Ok thank you sir.

 

[DynoDon]

I'm going to throw a curveball at you guys. True rod length is actually measured dead center wrist pin dead center rod pin. The general consensus of the length of a rod in our sport is measuring it from the top of the crank pin area to the bottom of the wrist pin area. However one should actually torque the rod cap to the rod and measure its circumference and also measure the circumference of the wrist pin on the rod divide those two areas by two and then add it to the total length of the rod from the top of the crank pin to the bottom of the wrist pin area. what I just gave you doesn't really play a role in our sport but that is the actual way to measure the true length of the connecting rod.I hope my college Prof didn't teach me wrong and I get a bunch of alligators after me lol

 

[Kart43]

I will give you a couple of clean up spots Don before that professor gets to you. "torque the rod cap to the rod and measure its circumference " I am certain you mean "measure its diameter" .

 

[Kart43]

If you are comparing one rod to another, top of crank pin bore to bottom of wrist pin bore works fine as long as we assume they both have comparable bore dimensions.

 

[DynoDon]

I will give you a couple of clean up spots Don before that professor gets to you. "torque the rod cap to the rod and measure its circumference " I am certain you mean "measure its diameter" .

Good point thanks

 

[flattop1]

LOL . Center or Dead Center

 

[BLACK DEATH]

Ok guys, I called NR racing about their heads they sell.
So while talking to their engine guy, he said .020 is the minimal you can run, not .030
I made sure and asked, “ so your saying .020 is the minimal or did you mean .030 was the minimal amount?”
He said, no .020 is the minimal you should run, piston to head.

 

[BLACK DEATH]

Check it on the edges too... you will lose some quench distance to piston rock

Can you explain that to me, I’m lost on what u mean.

 

[OVALTECH1]

When you are checking your quench measure your farthest points out on the piston as well at TDC. The piston rocks a bit in the bore and will tighten that clearance up a bit.

 

[OVALTECH1]

with the newest cranks out that are hardened a bit and a billet rod I would not be afraid to run .020 but I lean towards safer not sorry with a stock rod.

 

[BLACK DEATH]

Good point on the billet rod.

 

[alvin l nunley]

Maybe this will help.

 

[BLACK DEATH]

Maybe this will help.

Thanks Alvin