Replacing Valves

[Liamthedevastator]

I got a set of new valves with my bike when I purchased it, and I'm at the point now where I am putting the head back together. The Haynes manual says to re-grind the valves, but everything else I read on the internet is saying no. Can't trust everything you read so I'm hoping anyone who has done a valve change can chime in on this one.

As far as I'm aware, lapping the valves at home isn't good because it messes with concentricity of the seats. Also, when the engine gets warm the valves and seats won't match anyway (again, just what I've read). I have installed them and tried to blow air through the intake, and there was no leaking, so I'm honestly not sure what would have to be done.

So what exactly does one need to do to have a safe and successful valve replacement?

 

[Wolfe_11B]

You should take it to a machine shop. Could you get away with just tossing them in? Probably. But the shop will do an angle job on the seats to ensure the new valves mate properly. When I took my head in for decking, they did the valve seats for next to nothing, was like an extra $20. Make sure to replace the seals too.

 

[xschris]

Also agree on the shop. If you have new valves for it would be a good idea to put in new guides. Yamaha still has them.

 

[Liamthedevastator]

So even if the engine only has 20xxxkm on it it's a good idea to change the guides? There isn't any play, but I suppose it makes sense to renovate the whole shebang if it's going to a shop anyway.

To clarify, the machining matches the angle of the valve seats to the new valves right? How many times can you do this before too much material has been taken off the valve seat and the whole head needs to be replaced?

Thanks for the help, sometimes one needs motivation to get a bunch of work done that may seem like you could get away without doing.

 

[Wolfe_11B]

The shop will let you know if things need replaced. As for the seats, if they go beyond a certain point, the shop can put new ones in.

 

[16VGTIDave]

Well, I'm of the opinion that if the current valves are not bent or burnt, just hand lap them and carry on. 20k km is nothing and the guides should be virtually new. If you need to replace the valves because they burnt, do so, and lap them. They should be ground correctly from the factory and you will know quickly if the seats are damaged. If the old valves were bent, replace the guides and have the seats ground. Properly lapped valves will seal perfectly, and I wouldn't assemble a head without taking the time to lap the valves. Despite what many a machinist will tell you. In theory, if the machinists work is perfect, lapping isn't required. In reality, lapping the valves will prove that the valves seal perfectly, have the correct margin, and will last.

For the record, I trust machine shops no more than I trust used car salesmen. The last shop I dealt with (of many over 30 years of working on vehicles) tried to tell me that being able to see daylight past a closed valve was ok. They even tried to pressure test the combustion chamber to prove their point. Then they tried to get me to pay for replacing the valve seats. In the end, they bought me a new cylinder head. But not until after I got a lawyer involved...

 

[Liamthedevastator]

Yikes, that's quite the machinist horror story Nice that they ponied up for a new head but scary that it even got that far. I'm sure it isn't that common, but I understand your stance on trusting machinists now

I appreciate the opinions and education on the valve seat cutting process and how to go about replacing valves. I got a quote today from my local motorcycle machinist and they say it's pretty much flat-rate $120 for the job (and a light touch on the valves to reeally make sure it's a good seal).

As much as I hate spending money I'm in the wrong hobby to let that keep me back, so I think I'll probably take the darn thing in I could reuse the old ones but what the hell, might as well get the engine buttoned down for the long haul