Piston Scuffing on jnr Dragster

[chris b]

Hi, I am a motocross engine builder delving into new territory putting together some jnr dragster engines. All Mcgee PM1 engines at the moment. I Have three in bits, and normally Im used to stripping motocross engines where the pistons come out looking in good shape, sometimes wearing the skirts smooth and the odd small score. But these dragster engines all seem to scuff up the skirts more than I'm used to. Built one together last week, final hone was 280 grit, with 2 thou clearance. Afther a couple of runs had the head off and already can see a few scores up the barrel.
I'm thinking this is down to the methanol washing the oil off the bores? or not enough oil splashing onto the bores? The photo is from an engine that has done most of a season. would you consider this wear to be excessive? or what to be expected with iron liners and methanol. Cylinder still measures round to under hald a thou and piston to bore clearance is still ok.





Chris

[robert1]

.002 is too tight. Open it up to .0035-.004.

[SupStk]

I'm not a jr dragster builder but that piston apears to be growing larger than the clearances allow. Agree with Robert try loosening it up after replacing the piston. Might be the photo but the hone job looks way too rough.

[robert1]

One other thing is you have an aluminum piston rubbing aluminum at the bottom of the bore. Can't do that! In the cool bore (no sleeve) motors they have to run chromed pistons.

[Alan Roehrich]

A good friend of mine is one the top guys with those things, he and I were talking the other day and he said they've tried everything, the jr dragster engines are just hard on stuff, if you're making power. The pistons and cylinders tend to look pretty rough after a season.

[MadBill]

One other thing is you have an aluminum piston rubbing aluminum at the bottom of the bore. Can't do that! In the cool bore (no sleeve) motors they have to run chromed pistons.

Shorter skirts would deal with that, as would skirt coatings although they are perhaps not permitted?

FWIW, that's a kinda low-tech looking piece; is its design highly regulated by the rules?

One thing more: the scuffing looks to extend into the ring lands, a sure sign that the dimensions aren't optimal, as is the fact that the polished area of the bore is as narrow as if it was run with a slipper piston instead of a full skirt one...

Oh, and was all that apparent crud on the skirt present upon disassembly? If so an aircleaner might help..

[greywolf]

I'm not a jr dragster builder but that piston apears to be growing larger than the clearances allow. Agree with Robert try loosening it up after replacing the piston. Might be the photo but the hone job looks way too rough.

X2, maybe the tuneup is off abit and its running too hot???

We run 2 seasons between teardowns on our 7.9 jr with titan motor. approx 250 passes. The cylinder and piston look nothing like the OP pictures. The piston typically comes out looking re useable, but on a 2 year overhaul, the rod and piston get replaced regardless of appearance.

[chris b]

I think I'm going to take the liner out and replace with a longer one. Checked the piston position at bottom dead center and the piston comes down a little past the bottom of the aluminium. There are cutouts for the for the rod, which need to be there, I can narrow them a little. Looked at all 3 engines and they all have the same problem. Thinking of increasing the clearance to 3 thou on this one. The dirt on the piston is not as came out,

[vwchuck]

I would run like .005 in clearance if it is aircooled.
What is the measurement of the bore when you torque the head on?
I would bet that when you torque the head on the engine the cylinder is distorting. Was the cylinders honed with a torque plate?

[chris b]

The one I build last week was, and build with 2 thou clearance. This one I have. Just stripped, I had not built it previously, it measures round without a torque plate. Will bolt the torque plate on tomorrow and check roundness.

[robert1]

Thinking of increasing the clearance to 3 thou on this one.

And you thing .0005" is going to cure that damage? Everyone is telling you to open it up. Use a finer stone to finish.

[user-z68766209]

Couple of thing I see in the pic of the bore..
First like some one else said.. cylinder finish looks like roughing stones...

If you look to the right and were the head bolt is and then look in the bore in line
with it. Looks like bored with no torque plate or that bolt was over torque....

[greywolf]

It does appear to be finished a little too coarse.
Usually finish with a plateau hone as well.

[af2]

Iron liner and no lube.. .004" should be plenty to allow for expansion.. But I say should be... I had to run .009" on a 3.74" bore to make it stop trying to weld itself on a Husky I was running in Early 90's with gas on a Outlaw Cart deal.

Also the fuel used made a big difference on my deal being a 2 stroke....

It does look like you need more clearance. That cylinder is a tell tale....

[cpmotors]

Couple of thing I see in the pic of the bore..
First like some one else said.. cylinder finish looks like roughing stones...

If you look to the right and were the head bolt is and then look in the bore in line
with it. Looks like bored with no torque plate or that bolt was over torque....

I would run like .005 in clearance if it is aircooled.
What is the measurement of the bore when you torque the head on?
I would bet that when you torque the head on the engine the cylinder is distorting. Was the cylinders honed with a torque plate?

x3




Also looks like the bottom edge of the piston could use a light chamfer and how sharp is the bore where the rod notch is?