[lowbudget]

Last summer my 300 was smoking, and low on power and compression. Time for a top end rebuild, pretty standard practice on a 20 year old machine. I was looking for a little more power, so I decided to go with the 317 rebuild with the Rancher piston. I did that combo on my Brothers 300 with good results.

New LA Sleeve, new Honda Piston, new Honda Rings. Sleeve pressed in and honed by local machine shop with a very good reputation that did my Brothers. I put it all back together and the rings kept sticking in the grooves on the piston causing low compression, took me a little while to figure out what was going on. With the compressed rings the piston skirt came into contact with the cylinder wall, removing the fresh cross-hatch. Not enough to feel with your finger, or a pick, but enough to visually notice the vertical lines in the correct lighting.

While the head was off I also lapped the valves, and took a flapper wheel on the Dremmel and cleaned the exhaust and intake port. Removed 20 years worth carbon build up on the valves, ports, and combustion chamber, also knocked down some casting burrs and seams. Nothing major, took my time and just in essence cleaned things up. New valve seals while I was in it.

Put it all back together and it smokes like a freight train, and is blowing oil out of the exhaust port, lots of oil. If you put your hand in front of the exhaust port it comes back splattered in oil, enough oil to drip down the exhaust studs. I figured the rings had to seat or maybe its oil from the rebuild. So I put the exhaust back on and let it idle in the yard for an hour, the smoke never decreased, the oil level dropped 1/2 a quart, and when I took the exhaust off I was able to pour oil out of the muffler.

Since then I have changed the valve seals with a brand new set, and changed the rings with a brand new set. Tested compression, about 165-170 PSI. Took the head off, filled the intake and exhaust port with oil, let each sit for 10-15 min with no draw down, and no oil coming through the valves or valve seals. Filled cylinder with oil, let sit for 30 min with no noticeable draw down.

I'm out of ideas, seems like everything I have done has ruled out the cylinder, rings, or head. But i'm obviously missing something. What could be causing this?

 

[lowbudget]

He had the piston, and I was told he tailored the bore to the piston. Also if the bore was out of round, wouldn't compression be down? I'm pretty confident in this machine shop, they have a very very good reputation with local mechanics and race teams alike, and have done good work for me in the past. That being said I realize they are not infallible, and everyone makes mistakes or misses something from time to time.

Also, how would the crank bearing get oil into the exhaust stream? Could it cause the piston to be out of alignment on the way up, leaving oil on the cylinder wall? What is the best way to check the crank bearing? By hand on the Con Rod?

 

[lowbudget]

Ring gaps are staggered, ring gaps are within tolerances.

As far as rancher parts, its a common 317cc big bore, really no different than ordering a big bore kit from an aftermarket supplier, except i'm using readily available Honda parts, so I can easily get my hands on replacement parts, and i'm using parts of a known quality.

I think the general consensus is that the head is good, but its got to be a ring/piston/cylinder issue. I'm thinking about buying a .25 over piston and rings, and having the cylinder bored by a second machine shop to remove that from the equation.

 

[lowbudget]

No visible damage, as I said when the rings stuck when I first put it together the piston skirt rubbed the cylinder wall, just enough to remove the cross hatch, and add a vertical line. But you cant feel it with your finger, fingernail, or by running a pick across the cylinder wall. I'm very surprised as well. When I did my brothers 2 years ago it went together fine, fired right up, with no problems which is why I went that route again. I haven't done anything different this go around that I know of. I'm thinking a do-over with a .25 over piston and rings unless anyone has another idea. I just hate wasting $150ish on a new piston, rings, gasket, and labor if that's not the issue.

 

[lowbudget]

I have, got recommendations anywhere from a cracked head, bad valve seals, "time for a rebuild", and it'll go away on its on. So no real help.

 

[lowbudget]

To take all guesses out of the situation pull it apart and measure your piston to bore clearance, measure to see if the bore is out of round.

btw, you wont get a good gauge at whether or not your valves are leaking with oil, your best bet is using carb clean, brake clean, or even gas, dump it in the intake and exhaust ports and see if it seeps through.

My bet is the bore is out of round. even though its new, it should not consume that heavily.

Will try with regular unleaded tonight. But i'm now thinking the cylinder as well. If I had to guess something is up with the cylinder, the compression rings have enough pressure to seal up or meet the imperfection, but the oil ring does not and it is leaving a film of oil on the cylinder wall. That would explain the good compression, and no draw down on the oil I put in the cylinder last night, but still having some blow by during the compression stroke. On that note though, wouldn't the compression rings be tight enough against the cylinder to scrape the oil off?

 

[lowbudget]

I was about to throw more parts at it by ordering a new piston and rings at .25 over and have it re-bored, but i'm going to take it to a second machine shop which specializes in atv and outboard motors and have them check the cylinder out. Will report back.

 

[lowbudget]

Went into the shop last night after work, the exhaust port was still full of oil with no leakage, same for the cylinder. I've pulled the jug and cylinder and my brother is dropping it off at the machine shop this morning for a bore measurement. Will see what comes of it.

 

[lowbudget]

Went into the shop last night after work, the exhaust port was still full of oil with no leakage, same for the cylinder. I've pulled the jug and cylinder and my brother is dropping it off at the machine shop this morning for a bore measurement. Will see what comes of it.

Best of luck. Are you putting it back together or the shop taking a crack at it?

I'll be putting it back together. The machine shop is just checking the cylinder at this point. I dont have micrometers or bore measuring tools, nor do I feel confident using them if i did own them. So i'm handing that over to a professional. If they tell me its out of round, or scarred, then i'll bore it over. If they say its good, then I'll track down a new head and try that.

 

[lowbudget]

Well, machine shop check the cylinder, said one place up top was a smidgen out of round, by just a couple thousandsth. Also said cylinder looked good with no scratches that should affect anything. But he did check my oil ring, it was too small. We pulled the compression rings off and just left the oil rings on the piston and it would pass right through the cylinder barely touching the walls.

I have no idea if this was caused by me grabbing the wrong rings off the work bench, maybe I grabbed my old one. Or by being supplied an out of spec ring. Can not say for sure but knowing me, I probably screwed it up.

Got a new set of rings, put it all together and it fired right up with no smoke. Installed the muffler, and it began smoking as the muffler got hot, just burning off all the old oil inside.

I feel like an idiot for missing something so simple, but its taken care of now, and i'll thoroughly examine my oil rings from this point forward. Lesson Learned.

 

[lowbudget]

Yep, new $20 set of rings, and I took the old ones off, and placed them on the other side of the shop prior to even opening the new ones. I'm just glad it was cheap and easy, I was not looking forward to buying a new overbore.