Worried about valve wear

[Chris_B4]

Currently doing a top end overhaul on my two diesels primarily to solve a coolant overflow problem I have always had with the starboard engine (1997 Mercruiser 250hp 4.2 litre D-Tronic diesel, done approximately 1200 hours)

As I understand it the rockers are deliberately made to be slightly offset from the centre line of the valve so that it slowly rotates in use giving even wear on the end of the valve and in the guide. For some reason a few of the valves have not been rotating causing them to have a very slight groves worn in them by the rocker action. I have tried to take a pic but not sure if it’s easy to see what I’m talking about.

I guess that once a groove has started to form it will completely prevent the valve from rotating and I can’t make up my mind how worried I should be about this.

My options seem to be:

1. Stick it all back together and hope that they will last ages anyway
2. Buy new valves (probably with a silly price tag, I still haven’t recovered fully from the gasket prices !)
3. Get the existing valves ground flat again (not sure about the tolerances for shortening the valve, have hydraulic lifters)

None of the above will change the fact that the valves didn’t rotate properly in the first place, I have no idea what I could do about that

Have any of you experienced lot come across this before or has anyone got some nugget of advice that will help

Thanks all

Chris

 

[essexboy]

IMHO, any garage / engine recon firm will have a valve refacer, they will have an attachment to reface the stem. Hydraulic lifters will take this up no problem, I would also have the valves refaced at the same time to save effort on lapping, but dont forget once lapped into a seat they must go back in that seat on rebuild.

 

[john_morris_uk]

Its the top of the stem that post is refering to, not the valve face.

My guess is that you could get them machined off.

Shop around for the valves and check the price - you just might be pleasantly surpised. (The valves for my Volvo were only a few pounds each when I expected them to be many pounds each!)

 

[2Tizwoz]

Some valves have a shroud which must be kept in position to create turbulence. This is achieved by having a pin through the stem to locate them. Some engines which I worked on which had that feature I did the valves each 2 thousand hours. My experience was limited to 12 thousand hours with the same valves. The wear which took place in the end of the stem, as shown in your picture I removed by grinding on a bench grinder. That only when it was so marked that valve clearances were difficult to get right. If your valves should rotate and the groove is excessive, and valve clearances are set with the rocker in the groove, you could lose clearance which would result in burned valves. The picture is not large enough to see how much wear you have. As the valve face is ground the distance from face to stem end becomes slightly less so that the minimal wear in the stem end may be removed without problem it would seem to me. If its really no more than polished it should not be a problem.

 

[Chris_B4]

Thanks for the thoughts so far.

Was having a conversation with someone today who looked in an old engine manual they had which suggested that it is the action of the valve spring that makes them rotate, which seems plausible.

I'm still none the wiser though as to why many of mine have not been rotating correctly

 

[Heckler]

not a problem , dont worry about them, the valve grinding machines that we used to use always had a v shaped doofer that you put the valve stem in and pushed it against the grinding wheel while rotating the vlave to get a smoth flat service, the hydraulic liftes will take up any extra slack. you could get a piece of angle iron, clamp it square to a grinding wheel and just dress up the ends.
stu

 

[andy_wilson]

The engine manual will have a wear tolerance for valve stem wear. I found it very difficult to measure accurately when I did my cylinder head.

In the end I measured the wear when ROCKING the stem end from side to side instead of MOVING the whole stem from side to side. Rocking amplified the wear measurement, which was still within tolerance anyway.

If you bite the bullet you will probably end up changing valves and guides. If the valve stem oil seals are doing the job right, I would be tempted to stick it back together again, possibly after gently removing the worst of the wear profile from the stem with emery cloth.

Another option might be to mix and match them to get the best combination of wear measurements.

 

[Col]

See answer to your PM.