Damaged valve seat

[suzyj]

I've done a very bad thing.

I've been cleaning my heads (2007 695).  I stripped them down, made covers for all the bits I didn't want sandblasted, then sandblasted all the paint off.

So then the ports were there.  I bought a die grinder and went at it on the intake port of my vertical head.  Anyway, I damaged the valve seat (the port looks lovely).  I had a go at cleaning it up, using the valve and some valve grinding paste.  It's probably okay, but I know it's not 100%.

As I see it, my options are:

• Ignore it.  Finish the work on the head, put it back together, and worry about it down the track.
• Replace the head.  Buy a head off ebay (~$200, plus ~$100 for shipping).  But then I'm back to square 1.  I have to strip it down, sand blast it, and then have another go at the ports...
• Replace the valve seat.  This is probably the cheapest option, as according to my research a valve seat is ~$35. I can scam up liquid nitrogen, fwiw, and have a perfectly good oven...
• Replace the valve seats and valves with bigger ones.  This will clearly be more expensive.

So what are people's thoughts.  How hard is valve seat replacement, anyway?  Is it really as straightforward as cooling the seat, heating the head, and then sticking the seat in, or do I have to machine things? Does a 695/800 head have scope for bigger valves?

[ducpainter]

A new seat would need grinding.

I'd just take the head to a machine shop and have it, and the corresponding valve, touched up.

[Ddan]

A new seat would need grinding.

I'd just take the head to a machine shop and have it, and the corresponding valve, touched up.

+1  Unless you really made a mess of the seat this should do it.   Because I'm cheap and sometimes not very smart, I'd be tempted to reassemble it and do a leak down test to see how everything is sealing up.

[suzyj]

Given that a picture tells a thousand words, here's a picture of the damage.



I'm thinking I'll just pop the seat and replace it.  The cost of a new seat is not very much.

[Howie]

I would pop the valves back in and see if the valves seal.  If so, I think you are good to go. The lighter colored area is where the valve actually makes contact with the seat.

[J5]

looks like you have lost about 50% of the seat

cheap way would be lap the valve and see how it goes

but reality its not likely to be a long term fix

if you have the ability and its relatively cheap to do the seat then change it out


but remember you are also going to be up for some shims as well once you get the new seat in
as the clearances will change

me personally would try a couple of machine shops and see what they think and cost to do

i would prob try seeing that ducati guy at meadowbank as to who he could suggest as i know
he does rebuilds of old bikes , he could suggest the right place to go to or also offer advice as
to which way you should go with it

[ducpainter]

I would pop the valves back in and see if the valves seal.  If so, I think you are good to go. The lighter colored area is where the valve actually makes contact with the seat.

I'm with the 'old guy'...

the damage is not on the part of the seat that the valve contacts...

put it back together and run it.

[Ddan]

I'm thinking I'll just pop the seat and replace it.  The cost of a new seat is not very much.

The seat is cheap enough but you'll still need to get it and the valve ground to match.    Then, of course, you'll figure you might just as well have a 5 angle job done on it, and shit, why not oversize the valves while you're in there.  

I'd spend a little more time lapping it in and run it.

[brad black]

i would do everything possible to not replace the seat.  generally they're a slight back cut to lock in place, and if it goes wrong and they come loose it's not good.  i have a strong paranoia regarding them.  if you do decide to do it then you need a machine shop to do it (or finish it at least), as it needs to be machined (it will come as a round, parallel sided ring i expect) and the valve faced.

you need to get some blue and see where the actual seat is.  i don't think you've touched the seat, i think it's further out as per my arrows.  http://bradthebikeboy.blogspot.com.au/2012/05/photo-for-suzy.html

actually, you should be able to tell when you have the lapping paste on where the seat is.

i haven't had a 695 or 800 head off that i can recall the valve interaction, especially with the shorter duration cams the 800 has (10 deg less overlap), but on a 900 the issue is valve to valve clearance at overlap.  that's why you can only put a 44 inlet valve in, unless you sink the shit out of them or reangle them.  the 695/800 have the 900 valves, but i'm not sure if the chambers are shallower meaning the valves are further apart when seated.  looking at your photo, maybe not.  that's why they have domed pistons too.

[J5]

I'm with the 'old guy'...

the damage is not on the part of the seat that the valve contacts...

put it back together and run it.

i just had a relook at the pic and yes you are right

the damage is not where the valve seats the head

its in the lower part of the valve seat where the valve has yet to reach

bang it back together and will be good to go

[suzyj]

Thanks heaps for the awesome help.  I put some prussian blue on it and spun the valve, and the actual seat is the bit between the arrows:



So the seat width is reduced by about 50%.  It's not going to work very well, methinks.

I've been reading all morning about porting, valve seats and valve guides.  I figure now I'm in, I might as well go to town.  I'll make up a tool to push the guides out tomorrow, and then finish the porting properly with no guides in there.

The goal of the porting is something like this:  http://www.ducati.ms/forums/57-supersport/106431-750-ss-motor-build-started-4.html

That clearly wasn't done with the guides in place.

I'll chuck the valves and cam back in the other head (which is as yet untouched) and see how close the valves come at overlap, before making cam changes.

I don't think it should be too hard to get the seats out.  I have ready access to liquid nitrogen, so I can cook the head in an oven, and then put a rubber bung (pre-cooked with liquid nitrogen to -200) into the valve seat.  This should cause it to contract quite a bit and fall out.  Installation would be similar - make a tool, heat the head, then cool the new seat in n2 before pressing it in...

[ducpainter]

The area in between the arrows is not the contact point of the valve.

The contact point is the shiny part...

it will be fine methinks...

or as the engineer you are you can completely over think it.

[J5]

The area in between the arrows is not the contact point of the valve.

The contact point is the shiny part...

it will be fine methinks...

or as the engineer you are you can completely over think it.

shiny part exactly

you can see the machine marks in the section you have damaged

the valve would never seal there in a million years

[suzyj]

Not shiny part.  I've already used some pretty coarse grinding compound in an attempt to get to the bottom of the trench - hence the 'machining marks'.  I'll be replacing this seat.

I note an 800 here:  http://kaemna.de/cms_en/tu_example.htm?&ct=68 that has 44mm intake (1mm oversize), stock 38mm exhaust, and "fitted camshaft", plus big bore kit etc.  Would they have used 900 cams?

Brad, thanks for the overlap warning.  The stock valves on the stock cam on my 695 get pretty close at overlap - I measure the minimum linear distance between the valves as 1.6mm. The included angle between the valve shafts is 60 degrees, making an equilateral triangle.  By my calculations, with the same valves, I can cope with another 1.6mm of lift at overlap before the valves hit one another.  Do you have numbers for how much lift the different cams have at maximum overlap?

[WTSDS]

My suggestion is first get some fine grinding paste and spend forever lapping the valve in, you might get a good enough seal, there is still a fair bit of contact area next to the damage.

Next is do like ducpainter said, take it to a machine shop who may be able to cut a shallower angle in the seat, and same to the valve.
This will expand the contact area a tad.

As for doing it yourself, good luck. As mentioned, the seats are meant to be a tight fit in the head at motor operating temperatures. Getting replacement seats to fit just right in the kitchen is chancy.