Timing chain

Why is people replace the timming chain for the cams? Will it breake?

When I had my engine open and overhauled the top and upgraded to bigbore I replaced all guiding for the chain, but not the chain it self.

If its brake valve will got damage, but that is all?

They tend to stretch and it messes up the cam timing.

baldy110 wrote: They tend to stretch and it messes up the cam timing.

So you will not normally have a instant break down, but bad performance?

I guess you will provoke it easier with harder valve spring and modified cams then stock top?

Like all moving parts timing chains wear
Also they are cheap so replacing it when the motor is torn down is smart money. Some may say overkill but cheaper than a failed chain .
When a chain breaks it can take out the whole valve train up to and including the head .
Also pistons can be damaged and once that series of events start it will ruin block , rods , and more . let alone the metal which can go thru the whole motor
So , Back to where I started . Its smart to replace a chain when you have access to it which means splitting the cases as a proper chain is endless. Not peened together
Hope this helps
Dave

But its not common they break?

No it's not common.

Silent type timing chain in inline fours engines has quite a long lifetime, if not overtightened. My Kawasaki Zephyr 750 has mostly 100000 km on clocks and timing chain is still original and within specifications. One guy from Australia told that his Zephyr with 170000 km still has original timing chain too.

However, there are engines with quite short lifetime of time chain. Few year ago I rebuilt Yamaha SRX600 engine (similar to XT and TT600) with around 50000 km on clocks. There were no specs to check if timing chain is OK, but when I put it back I found that I couldn't align timing marks satisfactorily. I checked tensioner and found out that piston was out for 12 notches from 15. Sure I replaced chain with new OEM. The difference between length of new and old chain was quite visible.

I have never actually saw or heard about engine with timing chain broken or slipped due to over-stretching. Thus the main thing about timing chain stretching is , as it was said, mess with timing.

I have seen them fail
No . Not common , but they do fail. I error on the side of caution for a small add to my engine tear downs
Ask Les Holt about the chain that broke on him during our KZ Flag Relay

if in doubt change it out is my motto.

If I were doing a big bore kit I would weld the crank, if I had the crank out I would change the timing chain too.
If it was a matter of just freshening up the bores with new rings and staying close to stock I would get a new cam chain tensioner and idler sprockets and leave the crank and cam chain alone,
My 2 cents

I'd measure and replace the chain if it's out of spec or close.

About a year ago during the rebuild of a Z1B I had the engine torn down to the cases. I checked the timing chain and it was within the spec mentioned above. So I left it in and rebuilt the engine. I noticed when I installed the top guide that there was more slack than usual but it seemed to tighten down OK so I went with it. During startup the chain jumped, messed up the timing and bent three valves. I tore the engine back down and replaced the chain as I should have done in the first place.