Valve seats

[ChrisTheChippy]

Hi, just a general question which will help with the refurbishment of my csr.
I have always been under the impression that when replacing valves and guides it is good practice to recut the valve seats .
I had intended to do this when overhauling the heads on my twin.
I had a conversation on this subject with someone today who said that they just lap in the valves after replacing the guides. ( ajs twin )
I'm not 100% sure this is the correct way so am now a bit confused.
Any thoughts/ advice?

[SPRIDDLER]

As long as the valves seal O.K. after lapping I can't see any reason to re-cut the seats.
However, after fitting new guides it may require more lapping than the extent of one's patience to get a good seal so re-cutting before lapping is far quicker.

[ChrisTheChippy]

Thanks spriddler .
I guess it's just a case of seeing how it goes after fitting new guides then . I probably won't want to spend hours at the job so will probably recut the seats. Had a good look at the heads today and I think I might strike lucky as only inlet on one head seems to require a bit of attention although I will strip the heads for cleaning and polishing so will know more then .

[Joker_Bones]

I have done this three or four times, both new valves to new guides and existing valves to new guides without re-cutting the seats. The seats were good condition and the lapping was not too onerous.
The last head I did, the seats were quite recessed and not having any cutters I got a local garage to re-cut the seats for me.

Recently I picked up a vintage set of valve cutters at the VMCC jumble, so I am having a go at re-cuting a valve seat on the engine I'm currently working on.

This exhaust valve seat was somewhat pitted and slightly recessed.
New valve guides are fitted and for the exhaust I have a new valve, the existing was shouldered, slightly pitted and had a very narrow seating face.

I inked the valve seat face and took a light cut to see what was happening. The cutter started removing material from one side of seat only... So it was not perfectly perpendicular to the seat. When the seat was cut to a full circle, I inked it again and dropped in the new valve and turned it to see if the valve was seating nicely. It wasn't, it only removed ink from one side of the seat.

There are two cutters in the set I have of a size that suits these seats, so I repeated the process with the other cutter. It started removing material from one side of the seat face only, so there was a small difference between the two cutters. This time the new valve sat concentrically in the seat but was only touching on the outside circumference. So there must be a small difference in the angle of the cutter and the angle of the face on the valve.

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That is as far as I have got... I trust (hope) the new valve will lap in nicely from there.

[Expat]

Hi, I bought a new ex valve for my G3 as the original wasn’t closing completely and the stem was worn undersize. I decided to stick with the same guide though as it seems okay. Your post and how the job is progressing is interesting as at first, it seemed a good idea to lightly recut the seat on mine as it looked a bit pitted but my searches here regarding seat cutters put me off that idea mainly due to the cost of new ones and after a lot of negative reviews of all but the most expensive kits, not viable to buy just to cut one seat.
Hope your project works out well, I’ll keep watching progress. Reckon it will help me when I eventually get around to tackling the job, so good luck with it.
Btw, what area of the valve produced that witness ring on the seat?
Steve

Edit: Reread your post and you did say the outer circumference produced it.

[Joker_Bones]

I know what you mean about the cutters... When I wanted to cut the seats on my last project I thought I would buy an inexpensive cutter and have a go. I couldn't find such a thing! So it went to the local garage to be done.
I was really pleased to come across the ones I got at the jumble.

[ChrisTheChippy]

Thanks for relating your experiences with valve seat cutting. It sounds like it's not worth buying the cheap valve cutting sets .
As its looking like I may have only one to do I will probably borrow from my mate who only has good quality tools. Otherwise I've got an awful lot of lapping in to do . Although all the valves will be lapped in just to refresh them . Hope I'm not counting my chickens too much here .

[Joker_Bones]

Took some more cuts to try and get rid of the remaining pitting. I inked frequently to keep track of what was happening.
I found that with the head laid flat you unintentionally put more pressure on the lower half of the seat and removed more mateial from that side. To get an even cut all round it was better to tilt the the head up so that the cut was made vertically.

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These cutters aren't exactly Neway Carbides like Dick Casey's and when the cut was even all round it was difficult to get the cutter to bite. I found by laying the head flat again and deliberately applying pressure downwards the cutter would bite again albeit giving an off centre cut. Then you could stand the head back up and the cutter would bite into the uneven cut and bring it back to an even cut.

[Joker_Bones]

This is four or five quick laps with a change of paste in between as soon soon as it stopped feeling gritty.

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I've never done the thing to check the sealing by filling the well with petrol, I don't know what more I could do if it leaked.

[Ralph]

I would be well happy with that.