The Honda PCX / Forza / ADV150 / SH Forums

I have a small question regarding engine building. Feel free to chime in.
I don't have special tools to measure the piston to wall clearance. I recently brought my puny engine from 125 to 150 and got a nice 2.5 HP bump out of it from 7.5 to 10HP.
Problem is the stock KZY piston is 58mm with 14mm pin. Stock 125 crankshaft is 13mm pin. I bought an aftermarket 58mm piston that resembles the stock 58mm piston to attain the stock 150 compression ratio. This is my problem.
The stock Honda KZY piston slides and drops right down the cylinder bore, dry, no oil. It drops right down like a rock.
As my circumstances allow, I used the stock Honda piston with stock 150 cylinder. I changed the 150 connecting rod into the 125 crankshaft. However, that throws the crankshaft out of balance and make the entire thing vibrates like a 150cc vibrating dildo so I have to stop with that setup.
I switched back to stock 125 crankshaft and used aftermarket 58mm, 13mm pin. This piston seems even looser in the bore than the stock one (I reuse the 150 cylinder as I have ran it for only 2 days, less than 100km). I tested the piston rings, they are up to spec. But the aftermarket piston seems looser, the piston drops right down the bore maybe even faster than the stock Honda piston.
Question is, can too much piston to wall clearance do any damage and if there is what are the signs? I have heard the term piston slap but not sure I'm getting that in my engine. I haven't heard a piston slap noise before. In a perfect world, the piston should not even touch the cylinder wall, it should rides on a thin film of oil. But I need input.
Thanks for any help. Greatly appreciated.

First time I ever hear about someone who swaps rods and cranks in a pinch, yet can't quite
figure out a piston's clearance.

Ever heard of feeler gauges? (cheap and good enough):.0015"=perfect, .004"=piston slap, even if not evident to the ear -- brand new can be be very thight, but quickly settles to around .0015" and will stay there for years given proper lubrication. Measures should be taken at the bottom of skirt,
front or rear.

Then why mod a PCX ?

Because I don't have the proper tools for it.
Switching rods and cranks is child's play. Measuring stuffs is harder. I'm not an educated machinist so this is an honest question. When you say 0.015" doe that mean the result from the feeler gauge on 1 side of the piston or is that figure a total number and it's actually 0.0075" ?
About switching rods and cranks, the pcx crank is not so hard to take apart. I'm a part time enthusiast so spending lots of money on a proper gauge is difficult.

aguim wrote:First time I ever hear about someone who swaps rods and cranks in a pinch, yet can't quite
figure out a piston's clearance.

Ever heard of feeler gauges? (cheap and good enough):.0015"=perfect, .004"=piston slap, even if not evident to the ear -- brand new can be be very thight, but quickly settles to around .0015" and will stay there for years given proper lubrication. Measures should be taken at the bottom of skirt,
front or rear.

Then why mod a PCX ?

I did check that using a feeler gauge and I got 0.001 inch on each side which rounds up to be 0.002 inch total PTW. That's acceptable right? Because I'm working mostly on small engines and I don't know if the rules applied on bigger car pistons can also be applied on these puny 58mm pistons.
Thanks for your input.

.001" front, then .001" rear, if taken independently, give .001" , not .002" clearance.
Pretty much as new.

(measure should be taken ONLY at ONE place, either the front of the piston, or the rear (as
pistons are slightly oval-shaped -- for reasons not to be discussed here).

So it looks like this .001" piston/cylinder is perfect.

As an example I can relate to : using only feeler gauges, my 70,000 km Ninja 250's pistons
both showed .0015" (near perfect). I then used the micrometer/dial gauge gimmickry, only to
obtain the same results +/- .0001".

Dial gauges are best suited to check tapered and/or ovalized, worn cylinders (my Kawa's were
perfect, proof of a very good engine design and materials).
I wouldn't worry about the PCX plated cylinder. If it looks good to the eye, it is (that plating
is so hard as to be nearly wear-proof).

A .001"-loose piston will slide through its cylinder totally free (of any friction), indeed. But not
with the rings on, evidently !).

aguim wrote:.001" front, then .001" rear, if taken independently, give .001" , not .002" clearance.
Pretty much as new.

(measure should be taken ONLY at ONE place, either the front of the piston, or the rear (as
pistons are slightly oval-shaped -- for reasons not to be discussed here).

So it looks like this .001" piston/cylinder is perfect.

As an example I can relate to : using only feeler gauges, my 70,000 km Ninja 250's pistons
both showed .0015" (near perfect). I then used the micrometer/dial gauge gimmickry, only to
obtain the same results +/- .0001".

Dial gauges are best suited to check tapered and/or ovalized, worn cylinders (my Kawa's were
perfect, proof of a very good engine design and materials).
I wouldn't worry about the PCX plated cylinder. If it looks good to the eye, it is (that plating
is so hard as to be nearly wear-proof).

A .001"-loose piston will slide through its cylinder totally free (of any friction), indeed. But not
with the rings on, evidently !).

You're a diamond mate.
Yes with the rings on inside the cylinder turning it was heavy, like usual. I'm not very confident with using feeler gauge initially, mostly due to my inexperience, so I was a bit worried. Thanks again.

so 0.0015 is good but are we talking about mm or inches?
(about to repair mine after 170.000km or 106.250miles)