A forum for owners of Yamaha TTR250 trail and enduro bikes!

miltos · Member

Conrod big end seizure

TTRfan · Super Guru

Sorry to hear that cry

Is the cylinder head off and, if so, are all the valves in place and in good order? My guess is that the piston has hit the valves which caused the caps to crack. This will be obvious from marks on the piston and bent valves.

What caused it? A worn camchain breaking or jumping the sprockets, a nut or bolt coming loose and jamming between the balancer and bigend, and several other things.

But I am sure all will become clear as you continue to strip the engine down. Photos are always helpful.

Brian

miltos · Member

Hi Brian,

The cylinder head and the cylinder itshelf are already off.

There is no sign of collision between piston and valves.
The camchain is not broken. It was not possible to turn the crankshaft so we cant be sure if the cam timing had changed.

For a more complete understanding of the situation, the cylinder and the piston have no seizure type worn. They are not in perfect condition, but I could say that they have mostly signs of normal worn.

-- Edited by miltos on Wednesday 25th of August 2021 03:33:02 PM

mossproof · Super Guru

I have heard of a couple of conrod failures, both on highways, on hot days, after an extended high speed run, followed by a sudden stop (for a toll booth/peage)

I can't explain why, but guess very hot thin oil, with sudden engine revs fall to tickover caused low oil pressure at the conrod which caused the aluminium thrust washer to "pick up" or bind on the bearing cage.

I am very careful to slow down gradually towards toll booths now, on the rare occasions when I'm thrashing down European autoroutes. Doesn't help you now though, sorry!

Conrods can be rebuilt and main bearings replaced. Make sure the engine is thoroughly cleaned out and oil pump checked for metal swarf. I would also strip and clean the starter sprag clutch - metal debris in there will cause premature failure rapidly and it's the most expensive bit to replace. Ensure the conrod is rebuilt to the tolerances in the manual.

TTRfan · Super Guru

I had a big end seizure once caused by a couple of nuts loose in a newly rebuilt engine. I am pretty sure they were "posted" into an open crankcase by a helpful grandson who used to love "helping" me in the workshop evileye

The balancer gear was banana shaped!

Blue_engine (2).jpg

These were the culprits:

Crank 008.jpg

mossproof · Super Guru

Ouch!
What a little darling, so helpful!

miltos · Member

mossproof wrote:
I have heard of a couple of conrod failures, both on highways, on hot days, after an extended high speed run, followed by a sudden stop (for a toll booth/peage)

That's exactly my case!!! And yes, the aluminioum thrust washer seems deformed (looking from the bore where cylinder fits).

I noticed that before the failure the engine seemed to have noticeably better performance until 7000+rpm

I did not know that conrods can be rebuilt. I 'll check the manual.

I am in great concern about the condition of the trasmition gears, due to low viscocity hot oil the minutes-seconds before the seizure. The same for every bearing ect. I have already found that at least one crankshaft bearing needs replacement.

-- Edited by miltos on Thursday 26th of August 2021 11:38:13 AM

-- Edited by miltos on Thursday 26th of August 2021 11:39:27 AM

TTRfan · Super Guru

These are the parts you need to rebuild the crankshaft : https://totallyttrs.com/epages/699105d9-e4cc-4b32-b236-84e72cd67f84.sf/en_GB/?ObjectID=1718311

miltos · Member

hmm... I misunderstood. I thought mossproof meaned that I could use the same conrod...

Thanks anyway!

mossproof · Super Guru

Sorry, I should have written "cranks can be rebuilt".

Parts of the broken big end will have probably gone through the main bearing balls. Just inspect everything closely for damage. Sometimes aluminium bits stick to the transmission gears, but can be scraped off.