Piston to Bore Clearance

[Rosco400]

Seems differing opinions to piston to bore clearances from few different guys I ask, any set rule of thumb, just run the feeler gauge down the bore of the maico with piston flush with top of barrel with .004" (.1mm) clearance.  no ring on piston of course

[Noel]

HI rossco ,
Was there a hole in the piston or just no rings left,
most would say that 4 thou, is a bit sloppy, and using a feeler gage is not the way to  measure properly,
but I would think 4 thou is not beyond the realms of serviceability ,(you may have heard my CR rattle),
Cheers
Noel

[Doc]

mmm..does sound a little loose at 4 thou. Ring gap??

[Rosco400]

HI rossco ,
Was there a hole in the piston or just no rings left,
most would say that 4 thou, is a bit sloppy, and using a feeler gage is not the way to  measure properly,
but I would think 4 thou is not beyond the realms of serviceability ,(you may have heard my CR rattle),
Cheers
Noel

Everything intact Noel, no holes in piston but exhaust port side of ring worn down considerably, seems like soft spot as plenty of compression at start of day, ring 3mm inlet and sides of piston but worn down 2.5mm exhaust side, probably lucky didnt implode, ring gap now Doc would be 3mm just looking at it without measuring, is it possible for rings to have soft spots and wear that quick, was ringing its neck pretty hard been just over 12 months since new ring put in though

[Freakshow]

YOU want to be as tight as you can get it.  I would have said correct  me if im wrong lozza, 40 thou is way  too loose, you might even find its rocking on the thrust, u want to be as close as you can get in there to seal it up = more power and less chance of it heating up.

The wider the clearance the worse it will disapat heat and if it get gases passing your on a down hill slide to shits ville.

you want that bore as tight as you can get it at running temp. and 40 would be miles out IMHO

[Noel]

MMMM,
I think I would be putting in a new set of rings gapped to about .5mm and checking the chamfer around the exhaust port,
sounds like the rings are having a hard time there.

I did notice that there was quite a lot of oil staining /wetness around exhaust flange on Sun.


Noel.

[Lozza]

That's not the place to measure, every piston will have alot of clearence on the timing edge as that's the bit that expands the most.Where  it needs to be mic'ed is 10-15mm up from the skirt at 90deg to the gudgeon and the bore measure with a proper bore gauge just above(and below if you can) the exhaust port and subtract one from the other. Plus without the ring you will move the piston when you poke the feeler in.Are you sure there is a 3mm ring end gap sure it's not missing a bit??? Never known rings to have soft spots, the rule there is if they loose the chamfer it all over.I have an 88mm piston here I'll check what the box says as far as clearence for big bores go.
Dressing of port windows correctly is essential to extended ring life and using lots of oil is the other.Ring end gap rule is 0.1mm/4 thou for every inch/25mm of bore width. To much is better than not enough up to a point.
Is there a lot of black/brown blow by under the ring??

[gorby]

Is there a lot of black/brown blow by under the ring??

sometimes after eating a lot of licorice

sorry lozza, just could not resist repling to that one.

[Tim754]

Licorice ice cream turns it green............                 How do i know these things?

[vmx42]

Why do you look??

[Lozza]

Perhaps you should change your avatar then gorby

[Colin Jay]

Have a look at this site, it gives some good general info on piston clearance etc.  As the guy says, there are many factors such as type of piston, i.e. cast or forged, bore diameter etc that determine how much claerance a piston should have. 

Finally, there is also no substitue for a good service manual, and following what the manufacture said regarding clearance.

CJ

[Graeme M]

CJ, what's the site?

[Nathan S]

Don't listen to 'em Rossco.

Those engines only come good with around 20 thou of clearance, and they're at their best with 30~35 thou.
If you've got a lousy 4 thou of clearance, then you really need to get the hone out (if you don't own a decent hone, then a collection of half-round files will do the job but will take more time).

Trust me on this one.

 

[Rosco400]

Don't listen to 'em Rossco.

Trust me on this one.

 

 

Lozza no blow back under rings i.e. no brown/black bypass stains