Compression & Dome Size on Ported Cylinder

[Matt_E]

You have to watch those local yocal machine shops with their boring equipment.

For the last time, I bought the topend this way. I did not have it bored

:biggrin:

So, why do I seem to have proper piston clearance with the Pro-X 84.25, but the end gap is not working out on the rings?
I measured the pistons in the skirt area. They are exactly 10 thou smaller than the bore (measured at deck level)
The ring gap, however, is 40 thou.

 

[#ZERO]

I read that part, but where do you think the guy you bought the cylinder from had it bored?

Are you sure you're measuring the piston clearance right on the lower skirts?

The ring gap maximum on a stock engine with cast pistons is only .016" and if you're running Wiseco pistons it's no uncommon for gaps between .018" to .028" depend on the engine mods.

 

[Matt_E]

I am measuring with a dial caliper.
I am not 100% sure I am doing this right. I put the skirt area of the piston into the calipers. The piston is circular, so this method should work, right? (Of course, I make sure than the calipers aren't canted or crooked.
I repeated the measurement several times, in different areas.
I will probably take it to a local shop that I actually trust tomorrow to have them measure it.

 

[#ZERO]

Pistons are not round, they are tapered from top to bottom as well as oval from front to back and they are made this way so that they will be round and straight at operating temperature.

Also pistons are not heated uniformly, the crown is hotter than the skirt and the sides are hotter due to the extra mass there from the wrist pin bosses.

Measure the pistons at their largest diameter, 90 degrees to the wrist pin and usually at the bottom to mid way of the skirt.

Measuring the bore diameter is hard to do with a caliper you really need a bore gauge to check the sleeve from top to bottom for taper and roundness.

 

[Matt_E]

Thanks. I measured the piston as you described and come up with 3.311 for both of them.
While I don't have a bore gauge (yet), my measurements at top and bottom shows 3.322 in one hole, 3.321 in the other, throughout, with no out-of-roundness.

 

[Crab]

Not the best way, but you can put the piston in with a feeler guage and get some idea how much bigger the bore is than the piston, as you go up in size it will be obvious when it starts to get snug and you will have a good idea on your bore.

 

[QJS]

Bore is 3.322", pistons are 3.300"
That's a whopping 11 thou clearance.
I need to find a different machine shop.

More pics of the clearance on the rings

Matt, the difference between the piston and the bore measurement is your piston/bore clearance, you are dividing it by two this is why the ring measurement does not add up with your piston/bore measurement.
Chris.

 

[Matt_E]

So wait.....I seem to have a misconception about how to calculate piston/bore clearance.

I thought it was (bore size - piston size)/2
You're saying it's simply (bore size - piston size) ?

 

[Crab]

So wait.....I seem to have a misconception about how to calculate piston/bore clearance.

I thought it was (bore size - piston size)/2
You're saying it's simply (bore size - piston size) ?

I had .0055 with a feeler guage when I assembled my kawi as I recall, with Wiseco's.

 

[Matt_E]

Randy answered my questions.
It's simply bore-piston size.
Leaving me with 11 thou clearance on even the 84.25mm pistons.
Damn
He advised against using the 86mm pistons, so I guess I am looking for 84.5mm.
Anyone got any? :biggrin:

 

[Mouthfulloflake]

do you want to get rid of the 84mm wisecoes?

 

[Matt_E]

Yes.

They've got about six hours on them.

You freakin' vulture.

 

[Mouthfulloflake]

what the hell is 3.2 inches?

did you measure them with a damn tape measure?

 

[Mouthfulloflake]

Im afraid of bore slop in my 760 build with the OEM 760 pistons, im hoping the wisecoes are SLIGHTLY tighter in the holes.

need to do some measureing.

 

[Matt_E]

Yup.

Something wrong with that?















You dork, my dial caliper is not metric.
Since I need the clearances in thousandths of inch anways, I might as well put the piston/bore size in inches.
But just for you:
3.300 = 84mm piston
3.310 = 84.25
3.320 = 84.50

 

[Matt_E]

Im afraid of bore slop in my 760 build with the OEM 760 pistons, im hoping the wisecoes are SLIGHTLY tighter in the holes.

need to do some measureing.

I'll also have a brand-spankin' new set of 84.25 Pro-X's for sale. :naughty:

Btw, you haven't seen slop until you ran 22 thou clearance. :doh:

 

[Kennay]

Boo Yow

 

[Scraggly]

hey matt_ e how did you come up with those numbers???

3.300"=83.82mm
3.310"=84.074mm
3.320"=84.328mm

 

[Matt_E]

Those are the approximate piston sizes.
An 84mm piston isn't actually 84mm. It's a bit smaller.

 

[Mouthfulloflake]

I need to go home and measure, then, if yours ran good though, I know its tighter than that, I might just run it, I had some HELLA small domes, it might work out alright.

shoot me a PM if you want to sell any of those slugs, Ill have to be at home and measure my junk too and see what will fit.