Kibblewhite Valves

[tightwoods]

I am considering installing Kibblewhite valves/spring kits in an '03 YZ250F and '05 CRF450. Can good performance be achieved by simply lapping the valves into the existing valve seats or do the seats need to be recut (Serdi) for optimum performance? I don't know the condition of the seats yet since the top ends have not been disassembled. How critical is it to maintain Kibblewhite's specs for Installed Height, Seat Pressure, Open Pressure at ?" Lift, Max. Lift? How do you measure Seat Pressure and Open Pressure, with a special force gauge? Thanks.

tightwoods

 

[Faded]

Can good performance be achieved by simply lapping the valves into the existing valve seats or do the seats need to be recut (Serdi) for optimum performance?

You can do anything you want, but the desired outcome may be a far cry from what is considered a good valve job. This runs the increased risk of having negative side effects such as decreased reliability, decreased performance and in general falling short of your expectations. Lapping (if necessary) should be looked at as one of the final 'finish' steps, not a first past 'rough cut'. There will be a good bit of difference between a half-azz job and a quality job done right.

How critical is it to maintain Kibblewhite's specs for Installed Height, Seat Pressure, Open Pressure at ?" Lift, Max. Lift?

These specs are designed to keep you and your motor safe as well as keeping the supplied parts working as designed. Deviating from the specs (assuming you don't know what you're doing) will most likely result in broken parts. Not enough installed height could lead to coil bind of the valve spring. To much seat pressure could prematurely wear out the valves and/or valve seats. Exceeding max lift could also cause coil bind. Use your imagination on what could happen give any of the above.

How do you measure Seat Pressure and Open Pressure, with a special force gauge?

Seat pressure and open pressure can be determined by testing/knowing the spring rate of the valve spring and plugging in the the installed height and the height that they spec for open pressure.

 

[Rich Rohrich]

If the valves are worn have the seats properly machined (preferably with carbide tooling) to the correct angles and widths and install new valves.

If you are installing stainless steel valves you can lap them after the machining but if the seats are machined correctly there is no need to do this, and you limit your risk of leaving and abrasive material behind that is difficult to clean up.

If you replace your worn OEM valves with stainless and just lap the seats without machining them the stainless valves are unlikely to ever seal properly and you'll reduce the life of the new valves. Stock Yamaha valves last long enough that the seats are undoubtedly worn out by the time the valves need replacing, and lapping won't cut it (pun intended ) when it comes time to reface the seats.

Even if the seat wear is minimal, lapping the seats won't give you a proper sealing surface for a new valve, and you won't be able to get the seat narrow enough for good airflow. It's a very narrow seat that you need to have machined to bring it back into spec. Valves and seats that don't seal properly bleed off pressure and really kill performance at high rpm , and seats that are too wide really hurt airflow which can cause all kinds of weird performance issues with intake reversion near TDC.

It's a Formula 1 inspired race engine and it needs to be treated like one. Maintenance practices that folks use on lawn mower engines just don't cut it. :cool:

Setting the spring installed height is very important. If the installed height isn't correct you risk having too little seat and open pressure and won't be able to properly control the valve.

You can measure the installed height a number of different ways. One of the easiest ways to do it at home is to take a piece of .090 brazing rod or something similar and cut it to the exact installed height. The you can use it as a go/no go guage.

The Kibblewhite spring kits have a very good diagram showing where you to measure.

Most good performance shops will use a Rimac valve spring tester to verify each spring before assembling a cylinderhead. I've tested a LOT of valves springs and I've Kibblewhites to be very consistent and true to spec. You should have no problems just setting the installed height and assembling it.

 

[tightwoods]

Thanks a bunch, Faded and Rich. Sounds like good advice.

tightwoods