rebuilt engine now wont start

**snowsledfred** · 09-08-2011, 01:30 AM

PLEASE HELP!!!! ok i rebuilt the engine on my 94 cat zr440 because one of my pistons got all banged up. after i put it back together it wouldnt start. i heard that if you have fluid in your crankcase it wont run so i took it apart got all the stuff out of there, it started and ran, i noticed my clutch was whistling and thought nothing of it and kept driving, 80 miles later i broke the piston skirt off the magneto side of the engine. could the clutch have caused that? so anyway, i rebuilt that side and got all the bits and pieces of the skirt outta the crankcase with pliers, put it back together and now it wont start, it fires sometimes but just wont go! so i took it apart again and i did notice that a piston ring was closed to the piston after i had the cylinder off. it was a little wet in the crankcase, could this be the problem????
any help appreciated!!!! thanks

**DRHSpecialty** · 09-08-2011, 10:37 PM

Did you have the cylinder to piston clearance checked with a dial bore gauge?

Stuck ring right away on your rebuild and a skirt broke off within 80 miles of a rebuild should be checked into. Too much clearance is a good way to break skirts off.

It's obviously flooded but is it because you have a bad needle & seat or piston to broe problems with low compression? (or maybe something in between)

Start with a compression test and check the spark.

**snowsledfred** · 09-08-2011, 11:51 PM

i did check the piston clearance with a digital caliper in my garage, the piston is 67.93 mm and the cylinder is 68.49 mm is this too much?

also i did check the compression right after it wouldnt start just a few minutes after i had it back together the compression reading i got was 105 lbs. but i was informed that a compression test would tell you nothing until the rings seat, is this true?

**DRHSpecialty** · 09-10-2011, 03:31 AM

.25 MM is the same as .010" (inch)

Your more than .020" (inch) clearance and if thats correct you have WAY WAY to much clearance. That much will break off piston skirts and could be a problem with the ring locator pin.

Dial calipers are not very good for measuring bore size mostly because your only measuring what you can reach inside the bore and that would be only the very ends. You need to find a good machine shop (I have one you could try) and have them measure the bore all over to see where your at. The bore will wear the most in the middle, on a two stroke, beacuse all the porting reduces the amount wear surface available in the bore itself.

Even if you get it running and your measurements are right it ain't gonna work very long the way it is.

**snowsledfred** · 09-10-2011, 11:33 AM

so what would i do get oversized pistons or rings or something???

**DRHSpecialty** · 09-11-2011, 12:17 AM

From what I can find, and what I can remember, that cylinder is a plated (Nikasil) cylinder. If you bore oversize it will need to be replated.

I can get oversize pistons for that machine. Maybe it has been bored over and has been replated already? The only way to know is to measure the cylinder with a quality dial bore gauge at a reputable machine shop. Send them to me and I'll measure them at No Charge, just pay the shipping.

After you get the correct measurements the rest of the job will be very simple.

**snowsledfred** · 09-11-2011, 02:02 PM

yeah, how much oversize are the pistons you could get me and what are your prices?

**Maize** · 09-12-2011, 06:53 AM

WOW! I was reading through.

You have .56mm clearance X 25.4 (number of MM per inch) and that comes to .02205" (rounding up) clearance.

That is about 4 times the amount of clearance from a stock piston.

I maybe wrong, and others may want to check this, but if I remember right ( and this is off the top of my head and it has been a LOOOOOOOOONG time since I worked on a stock Cat) I think the piston to bore clearance should be in 0.004-0.005" range with a piston ring gap of around 0.015"

I think DRH has the parts you need to get it fixed properly.

I think standard bore size is 68mm on that, too. Again, not sure as I am work and thinking off the top of my head.

With proper mic'ing, you might be into the 3nd over-sized pistons at +.50mm.

**DRHSpecialty** · 09-12-2011, 08:47 AM

We can get .25 MM (.010") over and .50 MM (.020") over.

$50.36 ea includes; piston, matching rings, wrist pin and wrist pin clips.

If your calculations are "right on" the .50 MM won't have enough clearance and the .25 MM will have too much clearance.

So, with a little room for error on measuring, the bore could be a little bigger than you actually measured? Which would make the .50 MM the one to use or, is it smaller than you measured, say .006" or so, and the .25 MM is the one to use?

Never really know for sure without first using a good dial bore gauge. Like I said before "once you figure out what you need, the rest of the job is very simple".

Hope this helps.
Mike

**snowsledfred** · 09-12-2011, 11:36 AM

yeah, are these pistons .50 of a mm bigger or is it for a bore size of .50 over? because on www.denniskirk.com i found some pistons that are for a bore size of 68.50 mm bore size. the ones that i ordered before were for a 68mm bore because when i look up my sled it said 68mm bore and so thats what i ordered. now these pistons on dennis kirk i looked up my sled and all kinds of 68mm bore pistons showed up and this one that is 68.50mm bore, are these possibly stock they just used a different size, or is this a oversized piston?