A couple of days ago I decided to change the oil and filter on my newly acquired Serow 250. I unscrewed the three oil filter cover bolts half a turn each, then successfully removed long bolt 1 and short bolt 2. Bolt three would turn but didn’t come out any further. I pulled as I turned and it began to come out along with some black substance that I didn’t immediately identify as silicon rubber!!

Looking closely I could see that the threads were damaged on the bolt!! Being the first time to do this on the Serow I wasn’t sure if I was in a blind hole or what so poked a thin rod in and it seemed it is a hole that goes all the way into the area behind the casing. I had to be sure what was going on so I removed the casing only to find a big blob of silicone rubber inside and learned that the hole does in fact go all the way into the clutch area (see photo). It seems that the last person to change the filter rung the threads in the hole and in an effort to seal the hole and retain the bolt, injected black silicone rubber into the hole!!! In the picture the blob has fallen down after I poked the hole.

Luckily the threads weren’t damaged all the way through and I guess there is about 20mm of good thread at the end of the hole. Using a longer bolt to reach the end of the hole seems to have me sorted.

However, I have a couple of questions I’m hoping the collective knowledge here will help with….

First, are the two o rings at positions 3 and 4 the same? The one in number 3 hole was not an ordinary o ring but has a body to it that extends into the hole.

Second, is the bolt at position number 3 just an ordinary bolt? I seem to remember reading somewhere that it should have a shoulder.

I have put it all together using two similar o rings at positions 3 and 4 and an ordinary bolt at position 3, started the engine and all seems ok with no oil leaking.

I’m reluctant to ride it until I get answers though….

Thanks in advance…

What year is your bike and given that you're in the UK, what origin? Parts fiche should help you determine what's supposed to be where.

From what I see for a North American XT250, all 3 bolts are different, but the o-rings are the same.

https://www.ronayers.com/oemparts/a/yam/500478f3f8700209bc798841/oil-pump

Thanks for your input. Bike is 2008 efi.

Although I bought the bike in the UK, I'm in Ireland. The bike is a grey import into the UK from Japan. They were never sold new here in Ireland or in the UK. I think they were only called a "Serow" in Japan.

Here's a link to OEM YAMAHA PARTS. The diagrams and annotations will help with sorting out identical part numbers. Good luck.

Hi Garfieldus:

The usual configuration for yam oil filter cover is to use the lower bolt as a drain for the filter cavity. The idea is that there is a drain hole from the lower part of the filter cavity to the sump. The hole is plugged by the threads of the lower bolt when it is in position.

The idea is that when the lower bolt is loosened the hole will open and the oil in the filter will drain to the sump and not spill out. That's the reason for the shoulder on the bolt and the O ring on the flange.

It's critical that the bottom bolt be the right length and with a shoulder. If the bolt is too short the drain port remains open when the bolt is tight and there is no oil pressure.


Just a caution...

patuca

Hi Patuca, that is a superb explanation, thank you very, very much.

I have ordered a new bolt and will have it in a few days. I'm just hoping it's long enough to get to the undamaged thread at the inside end of the hole.

I got the new bolt and it reaches all the way to the end of the thread but it turns out that the thread is damaged all the way in anyway.

Now I have to tap it bigger or get a new crankcase cover which turns out not to be too expensive at €59.00. Putting a helicoil in is not an option because of the drain hole.

However a question first....

@Patuca, I've taken your diagram and pointed out that the shoulder is 7mm and the standard thread on the bolt is M6x1.

If I were to tap the damaged thread from M6x1 to M7x1 does anyone see a problem with putting an M7x1 bolt in there?

I'm unsure of how important it is to have a shoulder.

Use a heli-coil and keep the M6 thread.

Putting a helicoil in is not an option because of the drain hole.

Hi Garfieldus:

I think the purpose of the shoulder is just to provide a sealing surface for the O ring when the bolt is undone. I think that tapping the existing 6mm hole to 7mm and then using partially threaded 7mm bolt should work okay.

That way the existing drain system and O ring would still work as normal except the small O ring might get damaged pulling the 7mm thread through. That might not be a serious problem.

A heli-coil would work too if you could access it and make sure the hole is sealed. That would mean that the filter would no longer drain and oil would spill outside the case when the filter was changed but that might not be a big deal if you're not doing it in the living room.

That way the existing bolt could be used but the little bolt O ring would no longer have a function.

I think the important thing is that the oil filter cover can be tightened so the big filter O ring is sealed and it's critical that the oil drain port is sealed when the engine is running.

In both cases debris from drilling and tapping might fall into the drain port and into the sump, but I think it will be trapped by the sump-drain screen or the oil filter before it's a factor.

I would check carefully for oil pressure at the check bolt when the bike is first started. If the drain-port in the filter cavity leaks with the bike running there will be no oil pressure.

Those are my best shots Garfieldus, I've never faced that problem.

patuca