Top cut and throat cut of valve seats (question for the porting and tuning gurus)

arcticXS

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I have posted a little bit about my XT250 project, including ports/ valve seats and cam, but this question may relate just as much to the XS and other bikes.
On a bike with a single angle valve seat (45 degrees), like my XT250, I understand that a 3 angle (or more) seat will improve flow, and by that increase power.
So my question is if using hand held/ hand driven valve seat cutters for throat cut(s) and a top cut will give acceptable results. I am basically talking about 60/75 and 30 degree cutters.
For a back cut angle on the valve itself, i believe I can do this with a die grinder mounted in the lathe toolpost. Any thoughts on that as well?
 
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Yes you can do the back cut on the lathe, try first though with high velocity and a carbide tool as you might not need the die grinder ;)
For the seat cuts I'd recommend a 60 and below a 75 but more importantly open the inner diameter of the throat to about 0,87-0,89 depending on what cam you plan to use. With certain high lift cams one could go up to 0,92 but then the SSR gets really critical that it's done right

Kind regards and all the best
Christian
 
For a back cut angle on the valve itself, i believe I can do this with a die grinder mounted in the lathe toolpost. Any thoughts on that as well?
Not a very good pic, but that's what I did to cut my valves.... the standard 45°cut. It makes pretty quick work of 'em.



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Not a very good pic, but that's what I did to cut my valves.... the standard 45°cut. It makes pretty quick work of 'em.
That's exactly what I was planning to do, except not cutting the 45 degree, but a 30 degree where the 45 ends, stem side. To hopefully improve low lift flow somewhat. I have 2 new Yamaha valves, so really no need for cutting the actual 45 degree sealing surface
 
Artic I have no experience with back cutting valves but I have read that the 30 degree back cut improves low lift flow as you wrote. This is fine for the exhaust but if done on the inlet valve it can increase reversion. This may be only for XS 650 heads I cant remember.
Hopefully Hudriwudri or others with real life experience will read this and comment.
 
I guess one of my questions was if this type manual cutter is acceptable for the 30/60/75 degree cuts that are only for improving flow. (My personal guess is that they should be ok for that purpose)
 

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The neway Cuttery are very doable and if used with tenderness and a good eye even quite usable for the seat itself. Just don't use too much force.
For the 30 degree back cut it's imho not that easy to do judgment as it depends also a lot on the rest of the port, but usually back cuts on the valve better a hint the low to mid lift flow but it also obviously depends on how long the back cuts are.
That's obviously just my five cent, but there are also other very knowledgeable people on this forum.

Kind greetings Christian
 
A couple of items that help get proper (light, even) pressure and cutting with the Neway cutters:

1) a "bounce spring" sized to slide over the pilot that the cutter head rides on and keeps it just off the seat;
20221123_153121.jpg





2) their rotary driver tool that allows one to hold even pressure while getting multiple turns (rather than partial turns)
valvetools (3).jpg
 
Yes you can do the back cut on the lathe, try first though with high velocity and a carbide tool as you might not need the die grinder ;)
For the seat cuts I'd recommend a 60 and below a 75 but more importantly open the inner diameter of the throat to about 0,87-0,89 depending on what cam you plan to use. With certain high lift cams one could go up to 0,92 but then the SSR gets really critical that it's done right

Kind regards and all the best
Christian
I just checked the intake seat throat diameter, it is 32 mm, while the valve is 38 mm. So the ID should ideally be opened up 1-1,80 mm then? The seat appears to be some kind of bronze alloy. Is there a risk of it coming loose after removing quite a substantial amount of material?
The cam I will use is a new, stock 22 hp camshaft (the old one must have been a German market 17 hp cam) The intake port measures 32 mm at the flange, but the stock carb is a 28 mm pumper, with a V shaped venturi at the bottom, so "smaller" than a round 28 mm. The TT 250 sister model had a 30 mm carb and a different piston with a bit higher CR.
 
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@arcticXS

Will try to answer ya tomorrow.
Sorry but tonight I'm quite tired.
To not leave ya in suspense though I do not think that the seating might come loose, but please lemme know the ring thickness and if somehow approximately measurable also the depth.
Have a wonderful evening christian
 
@arcticXS

Will try to answer ya tomorrow.
Sorry but tonight I'm quite tired.
To not leave ya in suspense though I do not think that the seating might come loose, but please lemme know the ring thickness and if somehow approximately measurable also the depth.
Have a wonderful evening christian
I'll happily wait a week or a month for your input! I am not at my place tonight, so cannot measure seat ring OD and height tonight. In any case, I will have to fabricate a fixture for the head before sending it off to a machine shop, as it is a one piece head.
The plan is to spend time getting it done right, instead of rushing it.

I really have a weak spot for these "mini XT" models, as the general styling is not too far from XT500, but in a much lighter package, with the advantages of monoshock and a balancer shaft.
 
@arcticXS

Will try to answer ya tomorrow.
Sorry but tonight I'm quite tired.
To not leave ya in suspense though I do not think that the seating might come loose, but please lemme know the ring thickness and if somehow approximately measurable also the depth.
Have a wonderful evening christian
OK, today I managed to measure the valve seat dimensions:
Intake seat has 41.0 mm OD, 32 mm ID (throat), and 8.0 mm height. Material is some kind of bronze alloy.

Exhaust seat has 35.0 mm OD, 27.0 mm ID (throat), and 8.0 mm height.
Material appears to be different from intake seat, grey in color.
I somehow forgot to measure the exhaust valve diameter, but according to several eBay ads, it is 32 mm. Intake valve is 38 mm, as previously mentioned.

So opening throats to 33 and 28 mm respectively, will give a 87% ratio. I assume a somewhat conservative approach may be a good idea? Opening the inlet throat will definitely help with regards to removing a step on one side of the intake port.

Also, I believe where the sealing surface is located will be of some importance, i.e the OD and ID of the 45 degree cut. Any input on that?
 
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@arcticXS
Yes yes imho you can easily take out 1mm in throat diameter, in particular if it is a bronze seat as they have to my knowledge similar (or more similar expansion then steel material) heat expansion as the head material.
Opening the throat also might get you more possibilities to work on a smoother SSR.
Personally I always try to move the seat as far out on the valve as i can and maybe leave 0,3(in diameter) the valve seat smaller on the 45°cut.
The seat itself i wouldn't make any bigger than 1,5mm width, in particular considering the small valve diameter.

Hope that helps.

All the best
Christian
 
@arcticXS
Yes yes imho you can easily take out 1mm in throat diameter, in particular if it is a bronze seat as they have to my knowledge similar (or more similar expansion then steel material) heat expansion as the head material.
Opening the throat also might get you more possibilities to work on a smoother SSR.
Personally I always try to move the seat as far out on the valve as i can and maybe leave 0,3(in diameter) the valve seat smaller on the 45°cut.
The seat itself i wouldn't make any bigger than 1,5mm width, in particular considering the small valve diameter.

Hope that helps.

All the best
Christian
Thanks again! Today, I received a factory manual for this bike, and the actual seat width is specified to only 1.1 mm, but centered on the valve sealing surface. But I will try to get it a bit further out, i.e a slightly larger seat diameter.
 
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Today I drove 241 km (round trip), to drop off the XT250 head at the machine shop. I asked them to first open the inlet throat area up from 32.0 to 33.0-33.50 mm, to get rid of a ridge under the seat on one side, then to do a regular valve job on both seats with their Serdi machine. Unfortunately, they could only do 3-angle seats, 30-45-60. So I am looking into buying a Neway CU607, 75 degree cutter for the final touch. I also asked them to grind a 30 degree back cut on the intake valve, hoping that will improve low lift flow a bit.
As the XT250 head is a one piece deal, I had to fabricate a jig to hold it .
 

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