Valve guide Q's

[taloolamx]

Well, I'm throwing in the kibblewhite valves on my YZF250 soon. I have three questions though.

1) Manual says whenever you change valves, that it is necessary to put in new valve guides. Has everyone here done that when they've swapped vavles? If the clearances are still with in spec, I don't see any reason to do that.

2) Valve guides gotta be reamed before installation to get the proper clearance. I work at a machine shop, and it would be much easier to setup some jaws on a manual and ream the guides before installing them than reaming them when they are in the head. Question is, will I get much fluctuation after I press them into the head? Wouldn't think so, but thought I'd ask someone that has experience with it.

3) Stock steal valve guides or aftermarket copper. Any preferences?

I need to pick up a valve guide remover, valve guide installer, and some reamers to do the job as well. Any good sources to pick these tools up at?

Thanks for any info given, you can have a drink on me :cool:

 

[Rich Rohrich]

I never seen a Yamaha 250F wear out a valve guide in normal use. So unless you have been experiencing some really unusual wear my guess would be you'll be OK. Measure them of course to be sure (pin gages work well) but don't order parts just yet. ;)

 

[taloolamx]

Shweet, that is what I'll do. We'll give it a go and let ya know how it turns out. Thanks for the help

 

[DieselTech]

What about the valve seats when switching to KW valves (Stainless Steel Valves)? Do they need to be replaced with something different, or are stock seats ok? I'm considering re-doing my son's KXF250 with KW valves & springs instead of stock valves. I'm also considering buying a new head (stock) and installing the KW valves/springs in it, then re-doing the original head later in the season, to have a spare "ready to go". If I do this, is there anything special I need to be concerned about, or just assemble everything & bolt it on? Should I ap the valves to the seats? Thanks!

 

[Ol'89r]

If I do this, is there anything special I need to be concerned about, or just assemble everything & bolt it on? Should I lap the valves to the seats? Thanks!

Be sure to run the KW valve spring kit with the KW SS valves.

On the Honda and Yamahas, I normally cut the seats and then lap the valves, (Stainless valves). You can lap the valves using prussian blue and check the seats before cutting them to check their concentricity but, on the KXF250 you may find that the seats have moved and will need to be cut. IMO, it would be best to start with a new head.

 

[Rich Rohrich]

IMO, it would be best to start with a new head.

Agreed :cool: but I would still cut the seats given how poor the valve concentricity is on new KXF heads out of the box.:whoa:

 

[DieselTech]

Agreed :cool: but I would still cut the seats given how poor the valve concentricity is on new KXF heads out of the box.:whoa:

Sooooo.......even with a new head, you are recommending that I NOT just lap the SS valves & install them w/spring kit, then bolt it on the cylinder and call it a day? Are the valve seats that bad from the factory? What about the old head? Cut the seats & lap new SS valves, or do I need to put in new seats?

 

[Rich Rohrich]

Sooooo.......even with a new head, you are recommending that I NOT just lap the SS valves & install them w/spring kit, then bolt it on the cylinder and call it a day?

Check them with a concentricity gage, maybe you'll get a good one. Or if don't have access to one small enough to fit, coat the valve and the with machinist's dye and put a small amount of very fine lapping compound (I use 1000 grit but down to 400 will work) on the valve and do a very light lapping. The light cut with the fine grit will give you a pretty good visual indicator of valve seat contact position, width and roundness.

Don't use that super gritty Permatex stuff that was made for 65 Chevys, get some proper machinists lapping compound from McMaster Carr or J&L. It's cheap and it's an invaluable addition to your toolbox. You find it will also come in handy when your KXF inevitably smears a cam bearing.

Don't hold your breath though. I've yet to see one I would use out of the box, but hope springs eternal. ;)

Are the valve seats that bad from the factory?

Yep. I'm convinced it's a big part of why valve life varies so wildly on these bikes.

What about the old head? Cut the seats & lap new SS valves, or do I need to put in new seats?

If you get the seats cut on a Serdi or a similar piece of equipment lapping won't be necessary, plus you'll know for sure if the seats can be made concentric enough to be reliable. If you can clean up the seats toss the head. Heating an used KXF or RMZ 250 head hot enough to change out the seats has a high percentage risk of tweaking the cam bucket bores just enough to prevent them from spinning. When the buckets won't spin they tend to wear a groove in the center and destroy the cam. Even if you carefully measure the bores, you won't usually find this until then engine has run some, and by then parts are trashed.

The heads on these bikes tend to be disposable just like the cases and the barrels.

It an expensive bike to own if you are racing.

 

[taloolamx]

Anyone use one of these for their valve seats? If so how well do they work?

 

[Rich Rohrich]

In the right hands they are excellent tools, that can produce outstanding results. Learning to make your "hands right" is no trivial task, even for a real machinist.

 

[DieselTech]

Thanks for the advice, Rich.

This is my son's bike, and it will be his second year on it (he's almost 16 - raced a YZ80 his 1st year, then moved up to the 250F). He races in the Beginner class, and does well, but he has not reached the limits of the bike yet. It has about 40 hrs on it, and I've adjusted the valves twice now, so I know it's time.

 

[Ol'89r]

In the right hands they are excellent tools, that can produce outstanding results. Learning to make your "hands right" is no trivial task, even for a real machinist.

+1.

Diesel.

Although that is not what I use for grinding seats, I have used those in the past. Like Rich said, they require a very light touch and you may want to practise on an old head first.

The problem with them is when they dig in and start to chatter. Once the cutter 'digs in' it leaves a line across the seat it is almost impossible to get the chatter out without cutting the seat way down.

A little tip to keep them from chattering is to place a peice of paper between the cutter blades and the seat and cut through the paper. Don't know why the paper works, just know it does help. That, along with a very light and consistant touch will produce a nice seat.

IMO, the problem with many of the new heads is the OEM's are trying to make them too light. There is not enough material around the seats to hold them in place with the constant pounding of the valves along with the heat cycles. This has made them disposable. The KXF heads are the worst in this respect.

 

[warrior91]

Paper takes up space behind edge. gives relief to the edge and therefore cutter doesn't cut to full potential right off the bat. Applying enough pressure to cut but not enough to bind/catch/chatter is a fine line.

 

[rpm12505]

Anyone use one of these for their valve seats? If so how well do they work?

Neway cutters! Wow that takes me back to a time I don't care to relive. Cutting brand new hard seats with those puppies :bang:
I guess it's time to kiss the pig dog lizard bosses feet again for buying a SERDI 20 years ago :worship: :boss:

Back to neway: make sure your pilot to guide fit is snug and don't use the pilots that are spring loaded on one end and tapered on the other unless your guide is perfect. Pilots in picture look good. If you lived closer I'd do it for you, provided I had the right pilot size, that's key with the serdi also.Good luck

 

[DieselTech]

How much do these kits usually cost, and where can I get one? Would I be wise to purchase one to do my own valve jobs in the future? I'm not afraid to spend money on tools, nor to do the work on the bikes. But, I also want to make sure that I'm going to use it - and if the results are mediocre or the tools cost too much, then I'd rather just send the head out when needed (which is why I considered buying a bare head).

 

[Rich Rohrich]

This is a general kit for late model MX bikes. You can use this as a guide to purchase just the pieces you'll need for one engine.

http://www.goodson.com/store/templa...IID=5407&SID=64ef1db4e8cd1ac548d35989c8897de6

 

[DieselTech]

Thanks Rich.