cylinder head valve recutting question

[airtight]

Just a simple question. Can anybody inform me what the max depth of cut normally allowed on a valve seat ? I have an engine and one of the seats is badly pitted in one very small area. Depth of pitting is approx 25 - 30 thou. Can i have this recut ? Engine is a diesel 4 litre 4 cylinder ford industrial. I think the seat is approx 8 mm deep.

Thanks in advance. Neville

 

[patprimmer]

No clear cut answer, as it depends on a few things.

It sounds like there will be plenty of metal left for structural integrity, but you will lose some compression. It is a simple sum to calculate how much.

How much you are prepared to lose vs the cost of a new insert is your choice.

Also you might need to shim up the valve spring as when the valve sinks, the compressed length of the spring will be longer.

You could sink all valves the same amount then cut the head to restore compression. You might even do a more precise job of it than the original factory job if you took the extra care.

Cutting heads on modern overhead cam engines can cause problems with cam drives, so the consequences and corrections for that might need a good think through.

Regards

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[Fabrico]

I don't have the spec on that, but that's pretty deep. It's also in addition to any wear or previous cuts. The size of the affected area is not relevant.

Since you already have the head off and the valve out, why not get at least the one seat replaced and not worry about it? Diesels are not forgiving in that area.

 

[airtight]

Thanks for the replies. The engine is not a particular modern overhead cam, its pushrods. I had not thought about the compression change though.

It is turboed to over 2.5 times its normal naturally aspirated version so the compression ratio has been lowered in any case.

Our trouble is getting parts quick enough. It has to be in action sooner than we can get a new seat so hence the reason that i thought about just recutting the valve !

 

[Fabrico]

It may have a lower C/R but the turbo will put the pressures back up and then some over the N/A version.

Looks like you have the choice of checking around to see if anyone has another brand of seat that could work, have someone make a seat, or grind it as-is, slap it back together and do it right later.