2020 YZ250f top end rebuild question?

1/31/2021 3:53pm Edited Date/Time 2/1/2021 4:32pm
2020 yz250f
Had 2 exhaust valves drop , had everything in the top end in a jam. Timing chain , cams the whole 9 yards. I took everything apart free the cams and timing chain. Broke down rest of the top end also checked the cylinder was good crank and rod was good. I am replacing the piston / rings the head , everything in the top end is getting replaced with new stuff OEM. Question is not knowing what stoke the motor is in at the time it stopped. With no way of checking the valves to see if there loss and the cams facing the right direction. Do I put the top dead center marks on flywheel make them line up then proceeded with building the motor or what is the best way to go about this ?
|
DynoDan22
Posts
773
Joined
9/7/2011
Location
Victorville, CA US
1/31/2021 6:26pm
Two exhaust valves crashed into the piston? That's a pretty catastrophic failure. You said the rod and crank looked fine, but I'd recommend splitting the motor and inspecting the crank out of the motor. That rod took a pretty good hit. The YZ250f's have had a history of cam chains snapping, valve guides breaking and a few other valve train issues. How many hours were on the bike?
1
1/31/2021 6:49pm
The rod went up and down with the piston in the cylinder find and free of being in a bind in anyway. Rod had very little side to side play I know that's normal. Had no up and down play at all rod was straight as an arrow.
kb228
Posts
6161
Joined
1/31/2018
Location
Mansfield, OH US
1/31/2021 8:24pm
Assuming theres no other damage, it does not matter what stroke the engine was in before a long as its in time when you assemble it.

Im with dynodan tho. Make sure your crank doesnt have runout.
1

The Shop

1/31/2021 8:44pm
Yeah I have looked at that crank over and over. Checked the tolerances with filler gauges , checked it over and over for it being bent. Crank bearing is still smooth like butter. No I mean zero medal in the oil when it was drained. I didn't think it would matter as long as the marks where lined up but didn't know. Thank you for your reply to my question my friend.
Leeham
Posts
1085
Joined
10/29/2018
Location
Rochester, WA US
Fantasy
3059th
1/31/2021 9:18pm
I dont believe you can check runout with the crank still in the cases. Never done it. Also those crank bearings likely took a hit. Dropping 2 ex valves? Damn
1/31/2021 9:28pm
I am sure they did just was thinking if they were damaged in any bad way I would feel it in the crank as I rotate it over and over. But it's just as smooth as can be. Should have noted before now also that when the bike locked up it was in a corner and not on any wide open stretch with the motor wide open. But yeah buddy two exhaust valves , two valve buckets shattered , one valve guide tip broke , also chipped small pieces of the top part of the head where the buckets slid in it broke in two spots there as well.
Factor E
Posts
727
Joined
9/22/2011
Location
So Cal, CA US
1/31/2021 10:56pm
Lets see a photo of top of piston and how hard it hit.
Rod might not be straight,
1/31/2021 11:06pm
Hope it's ok like I said before I pulled the cylinder it pushed the piston up and down just fine with no resistance or abnormal binding that I could feel.
soggy
Posts
4810
Joined
12/3/2018
Location
UT US
Fantasy
3783rd
2/1/2021 10:17am
Hope it's ok like I said before I pulled the cylinder it pushed the piston up and down just fine with no resistance or abnormal binding...
Hope it's ok like I said before I pulled the cylinder it pushed the piston up and down just fine with no resistance or abnormal binding that I could feel.
What you can’t check is the runout on the crank halves and what these guys are saying is you might have knocked them out when the motor locked. You wouldn’t be able to tell just by cycling the crank. Your investing a ton to get it fixed I would for sure pull your crank to have it checked and throw a new big end bearing in it while your at it. Would be terrible if your crank went out after this rebuild and your back where you started
2
garagedog
Posts
837
Joined
4/23/2014
Location
Tulare, CA US
Fantasy
802nd
2/1/2021 12:26pm
Guys... you are just not telling dirtflinger what he wants to hear.
3
2/1/2021 2:04pm
All good I don't mind paying 360.00 more dollars for a new crank. To gain the comfort of knowing it is new and good to go. So guess crank , bearing , seals will be in the way as well.
soggy
Posts
4810
Joined
12/3/2018
Location
UT US
Fantasy
3783rd
2/1/2021 4:16pm
All good I don't mind paying 360.00 more dollars for a new crank. To gain the comfort of knowing it is new and good to go...
All good I don't mind paying 360.00 more dollars for a new crank. To gain the comfort of knowing it is new and good to go. So guess crank , bearing , seals will be in the way as well.
Ain’t gotta spend that much, half that should get it rebuilt for ya.
kb22
Posts
144
Joined
8/5/2020
Location
Knoxville, TN US
2/1/2021 4:32pm
Can have it balanced,trued, and welded by crank works for that price. Thing would be absolutely solid afterwards.
1
adam8781
Posts
670
Joined
12/22/2015
Location
CA
2/3/2021 2:21pm
rebuild oem crank webs with pro x rod kit, youre going this far just do it all so you wont have problems in the future.

and i think the gytr ported heads with gytr cams comes ready to bolt on for your bike and costs the same as replacing all oem parts 1 at a time. but you have an awesome performance upgrade for the same cost
dingo94
Posts
27
Joined
6/6/2016
Location
NZ
2/5/2021 7:07pm
valve springs lose tension at high revs after about 50 hours jm2cw

Post a reply to: 2020 YZ250f top end rebuild question?

The Latest