04 800 RMK top end/rebuild question

[bootsbti]

Howdy. I'm new to sledding, I bought my first sled last winter in January. A 2004 RMK 800 Edge 159" with 1100 miles on it. The purchase may have been a little hasty considering I know little about it's use and maintenance. I put about 200 miles on it before it quit and left me stranded on the mountain. I finally got the time to pull the head off and find out what happened. On the clutch side the VES valve broke and slid into the jug and the piston smashed into it several times before the engine quit running altogether. I'm wondering if I can just rebuild the top end on that jug or if I should rebuild the entire top end or do I need to rebuild the whole engine? Any advice, tips, links or the like will be a great help. I'm no stranger to engines having been a diesel mechanic for the last decade but am new to these little gas 2-strokers…

Thanks

 

[oscar680]

the main thing you will have to look at is the connecting rod, depending how bad the piston got jammed up it could have bent, if it is you will need a full rebuild.

if it was just a small piece of the valve that broke of and it didn't jam things up too bad you should be ok just rebuilding that side of the engine,
get an exchange cyl from someone like millennium technologies along with a new piston and gasket kit, once you get your parts check the piston to cyl clearance and ring end gap (your local dealer should be able to hook you up with the proper specs) after that its pretty straight fwd, you might have to replace the dome if its chewed up really bad, if its just pitted clean it up with some emery and it should be ok to use again.


one other thing I would recommend is pulling the motor out of your sled when you do the rebuild, with all the shrapnel from the broken valve and damaged piston it would be good to flip the motor upside down and give the crankcase a good wash out with solvent and compressed air.

also make sure you check your other power valve guillotine if its aluminum, replace it with an upgraded stainless guillotine's from Polaris.

good luck, post back if you have any more questions

 

[Jay]

The 800 twins are good engines but they do have a few weak points. The cranks can, and often do, fail on the pto lower rod pin. Mostly due to improper squish on the crank bearings. Pull the primary clutch and check the runout on the end of the crank with a dial indicator. Polaris says up to 0.004 is acceptable, IMHO anything over 0.002 and your living on borrowed time. If it's not to bad run it but make sure your primary clutch is in good shape, a worn or out of balance cluch just hastens the demise of your crank. Check the line up of your clutches and the front left engine mount, they are another common weak point. If you want, an engine plate instead of the two straps under the engine is a great investment. All designed to make the crank happy. Stock exhaust valves are aluminum and what happened to you is not uncommon. The updated version is stainless steel and is much stronger. If the rest of the engine is in reasonable condition you can just replace the cylinder, piston and exhaust valve, the head is probably toast as well from your description. The swap meet on here is a great source for parts, try an ad in the wtb section, there is a bunch of parts for the old BB's out there and LOTS of guys on here have tips and help for you.

 

[bootsbti]

Thanks for the replies. As best that I can tell without removing the con rod and measuring for a bend it does not appear to be bent. There wasn't actually much shrapnel involved most of the pieces that appear to be missing were caught in the pipe. Of course, I will still be giving the case a very thorough cleaning. I'll try and post some pics soon. The dome is also pretty clean, the valve broke at the skinny neck where the guillotine and diaphragm connect, and appears to still be in one piece aside from the few small shavings that I got out of the pipe. Thanks for the tip on the stainless guillotines, I'll definitely be upgrading. Jay thanks for the tip on the crankshaft run out, I'll be doing that soon, I'm sure the clutch can stand to be cleaned and balanced. I'm also going to look into the torque arm and engine mounting plates. Does anyone know what causes the aluminum valves to fail? Are they too weak to handle the stress of their duty or was it a lack of proper maintenance/bad fuel or oil?

 

[bootsbti]

Is that hole in the guillotine that's plugged with carbon a possible culprit?

 

[Goinboardin]

The aluminum valves break because aluminum is the wrong material for that application. They made them out of aluminum, then titanium, and now stainless. Not a lack of maintenance type problem. I ran them for over 3000 miles but replaced when I saw they were deforming (lucky to catch it). You can order two new kit's from Babbits and they include new: ss valves, VES springs, diaphrams, and valve nuts (part 17: http://www.polarispartshouse.com/oe...-s04nk8cs-a-b-c-s04nl8cs-a-b-c-s04nm8cs-a-b-c )

Definitely check the run out of the crank. If its getting up there I'd pull it before it breaks, send the bottom end and clutch to a good shop and at a minimum have them line hone the case, true the crank (skip welding the rod pin), rebuild the clutch (just replace what's worn) and give it a good balance job. I've had good experience with Indy Specialties out of Welch, MN and Dan has a reputation second to none. He has a fancy bottom end for that motor, but it's $$$ for an old sled. If you build the motor right the first time, run good fuel with plenty of oil, and stay on top of primary clutch maintenance, it should last you some time if you're a lucky one

Cheers

 

[bootsbti]

So I finally got around to measuring the crankshaft run out and it's a scary .0075. I'm not to stoked to be taking that kind of reliability into the Cascadian backcountry. Instead of shelling out all the dough to make this 800 a runner I've considered swapping it out for a 700. Has anyone done this? I hear they're a lot more reliable, and the parts are certainly cheaper. Any input would be killer...

 

[Jay]

A 700 ves would drop right in, all you would need is a different cdi for the 700 and change your jetting and clutching. If you go that route make sure you get a good engine to replace the 800 or it's just more money wasted. You could sell the 800 to offset some of the cost, I'll take the cdi if you want to sell it. It all depends if you want to keep the sled or sell it and move on, personally I'd look into rebuilding it but most guys would say dump it. Good luck.