Cylinder Bore Ok?

gizmoe107 · 11-03-2009, 10:39 PM

These pics are of a 640 that was recently rebored and the pistons replaced. The motor has only run for about 40 minutes while breaking it in. The motor ran well for the first 30 minutes but then it started bogging. The compression is still 140 psi. I pulled the exhaust in order to pressure test the block and noticed this strange wear pattern. The cylinders looked scored in between the ports. Why could that be? Both cylinders look exactly the same. The bores were measured by a machinist friend of mine. They were straight and cylindrical. The hatch pattern is visible in the rest of the cylinder. Could this scoring be related to the bogging?

engr146 · 11-03-2009, 11:26 PM

why was top end done? Low compression or holed piston?

lownrangr · 11-04-2009, 01:18 AM

yikes, those look like seizure scores (which may explain the bogging). Are you running pre-mix or oil injection?

gizmoe107 · 11-04-2009, 08:48 AM

Motor was rebuilt due to oil injection failure and subsequent piston/cylinder damage. It has since been converted to premix. Break-in was done following SBT procedure with 32:1.

Yah, the marks sure look like the cylinder is too small in that area. What piston to cylinder clearance do ya'll run? Mine was supposed to be .004-.008". For a recreational ski can you live with a greater clearance?

dank · 11-04-2009, 09:36 AM

Yes it is seizing and without looking at the sides of the pistons it is hard to tell why. Was the carb rebuild and fule system cleaned out?

gizmoe107 · 11-04-2009, 11:24 AM

I don't have a picture of it but the sides of the pistons that pass by the exhaust port look ok with no scarring. I took advantage of the fact that the motor was out to remove the gas tank and completely clean it out (I got the ski for cheap and it had been sitting for 6 years). The fuel filter was replaced and the carb was rebuilt.

dank · 11-04-2009, 04:54 PM

It could be cold seisure? What pistons are in it? Did you let it warm up at least five min. before going over 1/4 throttle?

gizmoe107 · 11-04-2009, 05:45 PM

They are 1mm over WSM pistons and rings. In hindsight, the one part of the break-in procedure that we forgot to do was let the motor warm up for 10 minutes (which is what SBT says). But we never went over 1/4 throttle in the first 30 minutes. I assume "cold seizure" happens right after startup. Are the cylinders shot?

lownrangr · 11-04-2009, 08:02 PM

yeah it's primo importante to let the engine warm up. The piston and cylinder have different expansion rates so it's key to slowly heat up the engine. You'll have to take a closer look at the cylinder walls to see if there's noticeable grooves.

gizmoe107 · 12-07-2009, 09:20 PM

Update - turns out my cylinders were NOT round!!! Setup on a milling machine and checked TIR (Total Indicator Runout). In some places it was as much as .005". The places with the cylinder scuffing were definitely high spots. A buddy of mine had tried to bore it on his mill but obviously it did not turn out right.

Since these jugs were already at the max overbore, I bought a used set on eBay. I took them to a small engine machine shop and they bored and honed. Rechecked their work and TIR is less than .0003" in most places (only one spot showed .0006").

Moral of the story - spend a little more and find a good machine shop with the proper tools.

I'll have the engine back together soon but will have to wait until Spring for break-in.

Cheers.