hey all, im workin on my neighbor's old d2 again...took the head off, everything looks to be in great shape as soon as i took two dead spiders out, i had a question regarding piston/skirt clearence its seems pretty large! does anyone have the spec for the clearence?

abram - Cat do not give piston or piston skirt clearance in any specifications in any of the Cat literature I have seen on the older D2's. These old Cat engines are a "high tolerance" engine .. i.e., they can have large clearances and still perform satisfactorily.

As a guide, the service manual for the 3¾" bore engines, states that liners can have up to .015" wear .. and as long as they are not grooved or scratched .. they can be re-used if the wear is under .015" ..

The critical specs relate to ring-to-piston-ring-groove clearance, and ring end gap.
The side clearances are as follows (3¾" bore engine - 5J & 3J S/No tractors) ..

Top ring .. .003" - .004"
2nd & 3rd rings .. .0015" - .0025" ..
Oil control ring .. .0015" - .0035" ..

Maximum permissible side clearance to piston grooves for new rings is .010" ..

The ring gap is as follows ..

Top ring .. .010" - .015" ..
2nd & 3rd rings .. .010" - .020" ..
Oil control rings .. .013" - .021" ..

ok thanks for the info

If you were wanting the piston skirt to bore clearance it should be no more than .004 , much more than that will cause piston slap. The specs that were posted above are for ring groove clearance for each ring on the piston.
Mike

At the dealer we checked for scored piston walls and the ring lands as that was the important thing with them.I looked in my Cat spec. sheet and nothing on piston diameter.I don,t know about now bt Cat at one time came out with cam ground pistons that clearance was only on two sides opposite the pin hole (thrust sides) and they claimed it went round when it warmed up.They also had keystone rings that were tapered.

Not quite there yet:)
Mike, you are correct the .015 allowable liner wear is at the top of the liner only where ring wear is maximum, not at the skirt end. On the other hand .004 seems a little tight but you may be right for that bore diameter. Cat doesn't give much information.
SJ, yes the late D315 & D318 engines used the cam ground pistons but I don't know that the D311 or earlier D2's ever did.
edb!!!! need your assistance again:)

To find out if yours has a cam ground piston, measure at the bottom of the piston (about 1 inch from the bottom) 90 degrees to the pin and that will give you the area that you want the clearance at. Then measure parallel to the pin and you will see a noticable difference between the 2 readings. In the early days most pistons were cast iron, most of them were round ground pistons with little or no taper at all. As aluminum pistons came to be they had to cam grind the pistons and add a little taper to them, usually between .0015 and .0025 depending on the application. A cam ground piston does get rounder as the engine warms up. In a tractor engine with a 3.750 bore you may want about .004 to .0055 in clearance, in your cylinder you will have taper from the lowest point that the rings travel and increasing to the top of the stroke (most of that being in the top inch of travel), at the top .015 is the most that you want and if you have that much wear I would redo the sleeve assemblies. I also have a piston grinder in my shop for grinding pistons and even on some of the cast iron pistons I put a bit of cam to them usually about .006. A good way to check how much clearance you have is to use ribbon guages. (long feeler) lay the end of the feeler gauge in the jug and insert the piston into the jug with the gauge between the piston and wall. Here is the calculation for a feeler and the total clearance. guage thickness + .0015= piston to wall clearance. Say you can put a .003 gauge in and with a slight drag pull the guage out then you have .0045 clearance. If you clearance is over .008 I do not advise on knurling as this usually lasts a very short time. Hope this helps.
Mike

I should mention that when you check that clearance you will want to put the feeler gauge and piston in from the bottom, since that is where you need to check the clearance.

lets put it this way...lol i appriate all the info and will make the calculations but tell me if this is bad... the gap is so wide i can see the rings shine in the sunlight! lol...dunno if that is bad or not. i will see if i can get some pictures.

What gap where??
If your referring to the skirt clearance at bottom of the cylinder that"s bad....five thousands is only about the thickness of a sheet of writing paper.
Even if it is fifteen thousands at the top of the cylinder it doesn't sound good from what your describing....sounds wore out:)