Engine Noise

[LanaTheGreat]

I have overhauled my Yamaha XT350's engine. I replaced the following parts:

- Main bearings
- Big-end bearing & thrust washers
- Con rod
- Gudgeon pin
- Crank pin

The piston and rings are still fairly new with about 500Km work.

After the overhaul the engine ran fine for about a week to two weeks, and then the knocking sound (like when the big-end bearing was worn) started again. When the engine is cold the noise is almost non-existent, and gets worse when hot. The sound seems to come from the middle of the motor, not the top or bottom end. Has something broken again, or is it something minor? Where should I start looking for the problem?

[darkangel]

Check the piston to bore clearance it might be out of spec.
Did you get it done at a garrage?

[LanaTheGreat]

I will strip the engine tomorrow and check the clearance. I doubt whether it will be out of specifications, as it is still new. I did not have it done at a garage, I did the engine myself with some guys from the base, following a workshop manual for the bike.

[Ariel Badger]

Did you make sure that the oilways in the crankshaft etc were clear when you stripped her down? Sounds like the big end has gone again .
____________________
Bikers make great organ donors, get 115 on your licence today.

[Odie]

[LanaTheGreat]

I stripped the engine. The big end is still fine, and the clearance according to specs. The piston to wall clearance is also within spec, although there are pit marks on the cylinder wall at the front end of the engine. It did this before with the previous piston and cylinder, and I have replaced it with a brand new piston and cylinder etc. Why does it do this at the same place all the time?

I did check the oilways. They are clean as I double-checked it all and cleaned everything thoroughly when the engine was apart.

The other thing which is bothersome is when I removed the magneto side cover, quite a lot of oil came out. As far as I know that side is supposed to be dry. I did replace the crankshaft oil seal on that side too and was surprised to see so much oil come out.

[Finglonga]

[LanaTheGreat]

There is no free play, as I replaced the main bearings.

[Ariel Badger]

You do not have the ignition timing over-advanced do you? That can give a nasty knocking.
____________________
Bikers make great organ donors, get 115 on your licence today.

[Ichy]

I'm with Ariel on this.

Timing can cause it but its generally there all the time if this is the case. The wrong fuel mixture or spark plug can also cause the same problem due to pre ignition because of overheating, so it tends to appear when the engine has warmed up. Knocking is also likely to cause damage to the barrel wall because your forcing the piston down before its ready.

Bearing and the like tend to be noisier when cold and might quieten down a bit as things warm up. They are unlikely to be quiet at startup and then become noisy as the engine warms.


Just thought, not using the correct gaskets can have an effect as well if they are too thin due to increase in pressure.

[Vincent]

Did you check the crank for run-out when it was out of the bike?

[LanaTheGreat]

I doubt whether it could be the timing. The workshop manual says that the timing cannot be adjusted on this bike and is controlled electronically. I am also using the original Yamaha gaskets for the bike.

I am not sure which run-out you are talking about on the crankshaft can you be more specific please?

[Vincent]

[LanaTheGreat]

There is almost no side play of the con rod. I cannot check the run out of the crankshaft without taking the bottom end of the engine apart (please save me that trouble). I accepted that it should be okay since the bike shop owner assembled it using a 30 ton press, and he told me that it was done using micrometers and the like.

[Paddy Blake]

How does the oil pump look.Check it out.

Paddy.

[LanaTheGreat]

Hiya Paddy, been a long time...

I will check the oil pump tomorrow. It is kinda late now (01:15 am) here, and let you know what I find.

[LanaTheGreat]

I stripped the engine down to the bone. The oil pump is still fine, no problem there. Run-out on the crank seems fine, nothing notably wrong there.

I am including pics of the only things I found that could possibly cause the engine noise:

Main Bearing.jpg --> Brand new bearing replaced about two months ago. The bearing is spinning inside the crankcase where it is fitted. There is no noticeable play between the bearing outer trace and the crankcase, yet if you turn the crankcase over the bearing simply falls out. Now please, tell me anything that gives me hope, I am NOT going to buy a new crankcase... Besides the main bearing, there are four other bearings also a bit loose in the casing, and they have slight play. That includes the bearing for the balancer shaft. I will replace those as well.

Buffer Boss Crack.jpg --> Yeah, that is what you call that thing, got the name in the parts catalog. Three parts to that thing, the gear teeth outer part, the springs, and the inner part. They can be separated. The arrows point to where the inner part is cracked. The boss goes directly onto the crankshaft and I HAVE to replace that, no doubt. Bloody expensive at about $55.

Sleeve Pitmarks.jpg --> The pit marks I mentioned earlier. Regret no picture of the piston. Excuse the poor quality of the pics, I took them with my cell phone.

Now the question part. Regarding the bearings, I am not buying genuine Yamaha bearings, they are madly priced, I am buying them from the bearing shop in town. Is this a good idea or not? Then, taking into account that the main bearing is still new, do I need to replace that one too, if so, is it possible to get one just a wee bit bigger that will fit snugly into the casing? Will it be possible to get slightly larger bearings for the other four having the same problem?

Then the sleeve pit marks. Will simply honing the cylinder be enough to fix this problem? And what caused this damage to the cylinder wall?

Something worth mentioning too, i found some metal grain at the strainer and the oil filter, not a lot, but it worries me, as I cannot really determine where it came off. Not sure if it is aluminum from the crankcase or other metal from the other parts. I inspected every little spring, screw, gear etc coming out from the engine.

Sorry for this mouthful. Given all that I wrote above, what is your professional advice, as I am getting fed up opening this engine every few months.

[Phil_P]

It's never good for a bearing to spin in the casting. As long as you aren't able to detect any play between the bearing and the casing you are probably ok. To tighten it up, you could use the old trick of carefully using a centre punch on the casting, and punching all the way round the circumference a couple of times. This will raise small rings round each punch mark that will tighten things up. Using an anaerobic bearing fit compound (loctite bearing fit) will hold everything secure.

Bearing shop bearings will be fine, something like SKF or NSK are good.

I'd replace the cracked part, I'd have thought it's just waiting to fail. Don't know if it could be welded.

Can't really tell from your pic if the scuffing is severe enough to require a rebore, or if honing might be enough. It may be that it has picked up some alloy deposits off the piston which makes it seem worse, and honing will fetch that off. You could try running a flap wheel on a drill through the bore and see what it looks like after.
____________________
It is not enough to win, others must fail.
Moto Guzzi 1100 Sport Injection '96

[Ariel Badger]

Is the new main the same outside diam as the old one (use a micrometer to verify) if there is a variance tou will need to get the correct bearing (very expensive Yam one) Loctite bearing fit is very expensive and will fail anyway. It could be that the nice people at Yam use a bearing a couple of thou oversize to stop people using standard parts.
____________________
Bikers make great organ donors, get 115 on your licence today.

[LanaTheGreat]

Thanks for the replies.

The guy at the bearing shop also mentioned punching the casting to make the bearing fit, but never mentioned loctite bearing fit, surely he should have heard of it before. The bearings I got for replacement are NSK's. I was wondering if the slipping bearing could be the cause of oil leaks from the magneto end, as the oil seal is brand new, and oil keeps leaking from it. As mentioned before, the bearing is slipping, but there is no play as it still fits snugly into its housing.

The scuff marks are not too deep, you can see it, and lightly feel it when running your fingers over. They are not alloy deposits, but grooves. I am going to hone it anyway, I am not replacing anything there as yet.

The new bearing is the exactly the same size as the original. Oddly the bearing on the other side fits snugly, and does not rotate in the casing as the one in question. I see all the bearings are made in Japan - NSK, SKF, Yamaha, and the smaller bearings that are in the engine are branded Koyo.

I am trying to get a secondhand part for the buffer boss. Hopefully I can find one in some bike scrapyard.