Piston to Cylinder clearance

[Redman]

Got head off (other bad story) and noticed that can actaully see a gap between the cylinder and the piston. Had not noticed such on other bikes I have had head off. Measured the gaps with feeler gauges, two have places that are 8 thousands and two have places that are 12 thousand.

SO how serious is this?

Only fix is oversize pistons and boring the cyclinders, right?

Other comments:
- 82 1100GK
- Zuki manual says service limit is 5 thousand and normal is .0020 to .0024
- compression test was mid 120s to mid 130s
- does darken the oil in a couple hundred miles
- does use oil
- does not seem to have all power an 1100 should.
- I quess if find nice looking bike, doesnt mean it is in good condition >>heavy sigh<<

[Nessism]

Did you measure the clearance at the bottom of the piston skirt? If so, you need a rebore. The compression numbers are not all that bad though so you might want to slam it back together and run it a while longer.

[Redman]

Measured with piston all at center of cyclinder, reaching in from top (cyclinders still on crankcase). Didnt think to measure at other places. I assume the extra clearance is from the cylinder wall wearing away, not from piston wearing away.

[Nessism]

You measured at the top of the piston which is the wrong place. The piston has a smaller diameter at the top because it get the hottest and expands the most. You need to measure at the bottom of the piston skirt. Only way to do this is to take the cylinders off.

[49er]

Measured with piston all at center of cyclinder, reaching in from top (cyclinders still on crankcase). Didnt think to measure at other places. I assume the extra clearance is from the cylinder wall wearing away, not from piston wearing away.

Piston to bore clearance. Your pistons should be measured 15mm from the bottom of the skirt with a micrometer. The mating bores should be measured with an internal mirometer at three different depths and also at 90 deg to those positions. Take an average of the two readings at each depth. Take the skirt measurement from the bore average and this will give you the piston to bore clearance. The Zuki manual sounds right.
I gather that you were measuring between the edge of the piston crowns and the bore. This area always has greater clearance to allow for extra expansion as a result of the heat that is generated by combustion in the chambers.

[Redman]

-Doah- okay, yah I was measuring at piston crown (what can see with head off).

Okay, I see what you guys are saying. And I now see that in the Zuki manual. THe clearance spec is based on having the pistons out, measuring with calipers, and measuring bore and finding differernce.

I am not intending to remove cyclinders (although I know some will suggest I should to replace base gasket). So I will say that since the compression test is fairley okay, that I neednt take any further action on this.


OR. . .
Or . . .
Would just replacing the rings be something I should concider?
Might that provide some improvement in compression, or improivement in power, or improvement in oil usage/darkinging?
.
.

[Dave8338]

It all depends, if the bores look great (I've seen cases where at 40k, there were still cross hatch patterns intact), rings won't hurt the compression numbers and may, in some cases help them. This would be a good way to check the piston to wall clearances for your future plans. If the walls look good clearance wise, I'd run a hone through them, throw on some new rings and put it back together.

[49er]

It all depends, if the bores look great (I've seen cases where at 40k, there were still cross hatch patterns intact), rings won't hurt the compression numbers and may, in some cases help them. This would be a good way to check the piston to wall clearances for your future plans. If the walls look good clearance wise, I'd run a hone through them, throw on some new rings and put it back together.

I agree. While you have the head off, give it a decoke and re-lap the valves. This will also help to improve the compression/performance. The other advantage to lifting the cylinders is you can replace the base gasket and cylinder O-rings to ensure you have no more oil leaks.
Incidently, IMO if an engine still has signs of the cross hatch pattern after 40K, the engine has never bedded the rings in properly.