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Exhaust valves: in-spec but need replaced? Pitting and how long will they last?

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2005 CRF450X

Took head off to replace piston. OEM piston and head with 100+ hours.

Checked all valves 44 hours (when I took ownership. No idea how long ago the valves were checked/adjusted) ago and they were all in spec. Checked again this past week for disassembly and the left intake was down to .002 ; time to get the intake valves replaced. The exhaust valves perfectly in spec.

Figure I'll just have the intakes replaced and recut. Local shop doing work calls and tells me that my exhaust valves are "pitted" and need replaced as well. Tell them they were in spec the last two times I checked them, including right before I took head off and dropped it off to them.

1.How often have you all seen or experienced the exhaust valves going out?

2. How are they in-spec for the last 44+ hours, and in-spec for this last check ...yet need to be replaced because of "pitting"?

Recommendations?

Edited by mikesbaron

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Exhaust valves are really cheap.

I would feel badly if I fixed everything but the exhausts, and the engine didn't last because I didn't replace such an inexpensive part while I had it apart.

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Just change the valves - not much point having it all apart and worrying about saving a few dollars on new exhaust valves...

Unless there is a need to remove a significant amount of material (which there shouldn't be) then a quick clean up cut of the 45 deg seat is sufficient and will have a negligible effect on the size and location of the other angles. Once the seat is cut a quick lap with fine paste will confirm whether everything is ok.

Shouldn't have to worry about touching the other angles in this case - particularly on the exhaust. Where it is worth looking at is when you fit larger 36mm stainless inlets as this requires significant cutting of the existing seats and cuts into the transition between the seat and chamber roof and increases the size of the 45 degree seat with redundant area on the 45 between the seating area and the 60 deg.

Edited by mtrehy

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I agree, change them along with the seals and check the guides. You already have it all apart, do it all and it will be fresh.

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What do you think about putting new valve springs in for the TRX intake valves? I always thought you should put TRX springs with the TRX valves since they are slightly heavier (vs the stock Ti valves). The shop said it's not a big deal to use the old intake springs with the TRX valves.

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Never reuse old springs, they are tired and no longer have the tension required to provide proper seat pressure.

If the shop recommends that you use old, tired springs with new heavier valves, find a new shop.

Make sure the shop is qualified to cut the seats properly to accept the larger valve. You'd be amazed at how many shops screw that up.

The TRX intakes use the same springs as the CRF intakes.

Edited by CRF DOC
  • Like 1

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Use the correct springs for your application.

Too heavy equals unnecessary parasitic hp losses and wear. Too light equals valve bounce at lower rpm.

The lesser of 2 evils is the former.

If you increase the mass then the spring stiffness may need to be increased accordingly.

Edited by mtrehy

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What do you think about putting new valve springs in for the TRX intake valves? I always thought you should put TRX springs with the TRX valves since they are slightly heavier (vs the stock Ti valves). The shop said it's not a big deal to use the old intake springs with the TRX valves.

&%$#@!??? That shop is clueless. Your valves will float at 5000rpm .

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&%$#@!??? That shop is clueless. Your valves will float at 5000rpm .

Valve float occurs when the force created by the accelerated mass of the bucket, shim, valve, cotters, retainer, spring etc is greater than the force of the valve spring. Acceleration is a result of engine rpm and cam profile. We know the standard setup is good to over 10K so to have problems at 5K would need accelerated mass more than double standard.

Density of stainless steel 7480 - 8000kg/m3

Density of Titanium 4500kg/m3

Obviously there will be a safety margin and you wouldn't expect the valves to float immediately on reaching the rev limiter.

You can calculate safe RPM based on valve train component geometry and mass. If you have all the cam data, measurements, geometry and spring rates there are lot's of software packages that will simulate it accurately enough.

I don't know much about atv's but my guess would be that the retard chariots aren't revved as high as the bikes so they get away with using the same valve spring with the heavier valve (just an assumption though - not gospel).

Safest to buy springs that are rated and proven to work with the stainless inlet valves and there are no shortage of options.

Edited by mtrehy

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I appreciate all the advice I get from this forum but I do like to take everything with a grain of salt, checking and double checking recommendations at any level.

With that, I called the shop (who shall remain nameless but let's just say they are well renowned in the ATV racing industry, racing in the WORCS series to name but one series they race and build engines for all manner of bikes and atvs) and told them to go ahead and do my exhaust valves as well. Then I told them i would really feel better if I had new intake valve springs to go with the steel TRX valves, that I was worried about them being slightly heavier than the TI ones and it causing issues. They assured me that there would be no problem and my springs were measured within spec. Plus...the TRX springs were the same as the OEM CRX ones!! What? Blasphemy!!! Everyone here swears you must have TRX springs to deal with the extra weight of the TRX valves vs the lighter Ti valves.

So I go online to microfiche and check part numbers. THEY ARE INDEED THE EXACT SAME PART NUMBER. Can anyone explain this, the rumor that the CRX intake springs will only work well with the lighter Ti oem valve and not the stainless/heavier valve and if I don't get the magic heavy duty TRX springs I will experience all manner of valve float and bad performance?

For reference, here is the OEM CRX intake spring part number:

14751-MEB-670 CRX

And here is the TRX 36mm intake spring part number:

14751-MEB-670 CRX

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So I go online to microfiche and check part numbers. THEY ARE INDEED THE EXACT SAME PART NUMBER. Can anyone explain this

it's already explained above mate. required spring rate is based on the force produced by the accelerating valve and components. force=mass x acceleration. mass is the "weight' of related components, acceleration is produced by RPM and cam profile.

There are several possible scenarios as to why the same spring is used in 2 different applications and how the forces produced remain the same and i've listed a few of them in the order of likeliness.

1. the applications rev range is different thus reducing acceleration

2. cam profiles are different thus reducing acceleration

3. installed height of the springs are different in each application

4. mass of the related components are different in each application

5. honda built the crf with springs that are unnecessarily stiff because they had a warehouse full of them and wanted to save money

6. honda built the trx with springs that are unnecessarily soft because they had a warehouse full of them and wanted to save money

To say that the same valve spring is optimal for the exact same application for 2 vastly different valve masses shows a lack of understanding of basic engine design and operation. That's not to say that the application won't function as you need it to but there is no way it can be optimal for both. i'm surprised that your 'renowned' race shop is looking for functional and not optimal.

Not sure why i bothered to explain that though as you will take it with a grain of salt anyway. :thumbsup:

Edited by mtrehy

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Actually reading people's posts may help you in your search for knowledge.

You'll notice in post # 10 above, I stated that the TRX springs were the same as the CRF springs. This isn't news to us.

When Krannie said you'll be floating the valves at 5000 rpms, he was being somewhat sarcastic. He didn't really mean that at 5000 rpms the valves will float, but he did mean that with old worn out springs, the valves will float at a much lower rpm than designed.

Your shop, so called ATV race experts, is dead wrong if they are suggesting that you reuse old springs. That would alert me to a problem immediately. When they measured the springs, did they merely measure the height of the spring or did they actually measure the seat pressure applied?

I cannot believe all this fuss over saving $ 12.00 on new springs. Penny wise and pound foolish.

Edited by CRF DOC
  • Like 1

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I cannot believe all this fuss over saving $ 12.00 on new springs. Penny wise and pound foolish.

No doubt.

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