Question about top end dissassembly

Hi guys,

I've got a 2008 WR250F... I've got 120 hours on it, valves haven't moved since last year's adjustment. Quick question for those who have disassembled the top end - I am wanting to do a piston/rings/valve inspection, not necessarily any parts replacements unless required. I've got a top end kit (gaskets, rings, piston, timing chain). If I follow the book and disassemble the top end, do I need any special tools to get it back together? Aside from the valve spring tool I mean. I've got all the gaskets.

For example I read a lot about cutting the seats, etc, I presume this is only necessary if replacing the valves?

I've disassembled many two strokes, and I can follow the book, but I just want to make sure that there are no really special tools required. Thanks guys.

Nothing special is required. If you are planning on removing the valves for inspection, it's a good idea to replace the guide seals. Don't forget to inspect the guides and make sure that all the valves and buckets go back in the same spot they were removed from.

If there is a lot of carbon buildup on the seats, you can very gently lap the valves with some fine (320 grit) lapping compound. Don't get carried away with any lapping. The Titanium valves are hard coated and if the coating is lost, the valves won't last long.

You are correct about the seat cutting. That's only required if you are fitting new valves or guides.

Edited by 2grimjim

to inspect the piston and rings you have to remove the cylinder. at that point just put in a new cam chain and new piston and rings.

why put in old when it is the same work for new?

to inspect the piston and rings you have to remove the cylinder. at that point just put in a new cam chain and new piston and rings.

why put in old when it is the same work for new?

Right, and I don't mind doing that, I've got all the stuff here already.

I'm looking at the microfiche now - the "guide seals", are these the seals that are around the valves, numbers 3 x 4TV-12119 and 2 x 4SV-12119?

This guide seems pretty thorough: http://www.dirtrider.com/features/protips/141_0412_yamaha_yz250f_dirtbike_rebuild/

I've ordered a leak down tester, I'd like to take that measurement before proceeding past a valve check. Thanks guys.

I don't really consider them special tools as anyone doing much mechanic type work on cars or bikes should have them, but torque wrenches for getting everything bolted back together to proper specs. One for in/lbs and one for ft/lbs.

I'm looking at the microfiche now - the "guide seals", are these the seals that are around the valves, numbers 3 x 4TV-12119 and 2 x 4SV-12119?

This guide seems pretty thorough: http://www.dirtrider...rtbike_rebuild/

I've ordered a leak down tester, I'd like to take that measurement before proceeding past a valve check. Thanks guys.

Yes, those are the guide seals. They can be emoved with a pair of needle nose pliers. You can use a small 1/4 drive deep socket with an extension to reinstall the seals.

Guides are a complicated process. You would need the removal and installation tools, and reamers or hones to finish size the guides. Not to mention a set of small hole gauges and a micrometer for figuring out your guide clearance.

Thanks guys - Yeah, I've got good torque wrenches. I think what I'll do is just pop the head off and leave the valves off like they do in the dirt rider guide. I'll just do the chain, piston and rings. The clearances are still dead on from last year on the valves.

I think I'll wait for the leak down tester which will be here soon and do a before and after... Thanks,

Luc

Hi everyone,

so, little update, and a question to follow.

Prior to disassembling the top end as described above, I did a leak down test using the Motion Pro unit. The result was 7.5%. I replaced the piston, rings, gaskets, timing chain and have reassembled everything less the valve cover. I also haven't filled up coolant and oil. Throughout all this work, I removed the head but never removed any of the valves. I cleaned around the valves with scotch brite but never touched the valves as to not damage them. There was a slight bit of carbon build up but minor.

I checked the valve clearances after putting the camshafts back in, and they're still well within spec.

Before putting on the valve cover, I thought I'd do a leak down test again. Somehow, there's some 60% leakage, I can hear it coming out of both exhaust and intake. Not sure what happened? I don't really want to continue reassembly in case there's something wrong, but it's just hard to see how anything went wrong considering I never took any of the valves out. Part of me just wants to reassemble it, start it up, and then cool down and try the test again.

Either the valves are open, which, I can confirm the cams aren't forcing them open as the feeler gauges still get in there without issue. What about seals? I put some oil on the cam lobes and I can see that it's leaked through the valve seals, as I can see through into the exhaust port. Maybe the air is leaking out through there? Is it typical to have this issue after a rebuild? Maybe a heat up to temperature and shutdown will resolve it?

Anyway, any feedback would be greatly appreciated. Thanks!

Luc

Well, you are going to get significant leakage around the rings of a not run engine, they need a good five minutes runtime to seat. No reason for your valves to be leaking at all unless during your cleaning process you were a little aggressive and forced some carbon between the valves and the seats.

Leaking seals only prevent oil from the cam area to get into the ports. They do not prevent any leakage into the barrel area.

Thanks for the response.

I'll reassemble, run the engine for a bit, and recheck afterwards.

I don't know why I was thinking seals, obviously wasn't thinking.

The air is coming out of the inlet and exhaust, so it's coming through the valve ports.

I used carb cleaner in the vicinity of the valves, to clean the gasket mating surface, could be one or two drops that went in, could that be an issue?

There's no carbon that was forced in there, I was extremely careful.

Luc

So I just checked the valve clearances again... Some have opened up by 0.02 mm, so maybe some carbon did get in there somehow. They're still within spec, so I'll reassemble and check again first 20 minutes of engine time....

Edit: Just ran it up, started fifth kick. Starts first kick every time... Rode it around a tiny bit, seems OK. I'll idle it intermittently and ride around the yard slowly here, and then do a leak down test and check those valves again. If it is in fact carbon that's accumulated there, hopefully it just goes away...

Thanks again!

Edited by lucgallant

Hi everyone, just some closure on this item.

I ran the engine for about 20 minutes, rode around slowly, and redid the leak down test.

It has returned to the same numbers as before, about 8%. Valve clearances returned mostly as well. Somehow, some carbon must have gotten in there, I just don't see how given how much spring tension keeps those valves shut.

Anyway, with 130 hours on the bike, and still the original valves after 4 years, I'm aiming for a valve job in the upcoming winter, which will probably make 160 hours on the valves.

Thanks for all the feedback!

Luc

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