valves and valve seats

#1
Merry Christmas to all,
What would be involved to correctly replace a 27mm intake valve with a 28mm?
What about vice versa? (Both heads NR/Russell)
 
#3
I'm gonna leave em' alone OND!
(I have a brother in Russellville, I'm in B Green, would like to
come by your place nezt year to talk mini bikes with you 10-4?.
 
#6
If the seat is the stock one, it hasn't the material for a bigger valve to seal or to open the throat to the 85-87% commonly used.
If it's a big seat, and the throat is excessive, a bigger valve helps a lot.
 
#7
The point of a big valve is not that you can run a bigger valve, it's so that you can run a bigger seat. If you put a bigger valve in a stock seat, think about what you have done. The air still only has the same area to flow between the seat and the valve stem only now it has a bigger valve "head" to avoid as it's flowing.

If you can get a bigger seat, do it. Otherwise, run the SMALLEST valve that will work with that seat.
 
#8
I'm confused on the term stock seat. That's what, say Russell, starts with when
he builds a race head... What does he do to the seat when somebody orders the
bigger valves? Does he machine the stock ones out and machine the head to accept bigger seats?
 
#10
The stock seat is over a tenth of an inch thick you can easily take out .020 which would open it .040. I do this on a stock 27mm hemi seat to install a 28.5 valve. In addition I use a 75degree throat cutter after the 60 degree bottom cut to give more of a venture effect to the seat (as well as a 30 degree backcut on the valve. This improves low lift efficiency.
 

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#12
The point of a big valve is not that you can run a bigger valve, it's so that you can run a bigger seat. If you put a bigger valve in a stock seat, think about what you have done. The air still only has the same area to flow between the seat and the valve stem only now it has a bigger valve "head" to avoid as it's flowing.

If you can get a bigger seat, do it. Otherwise, run the SMALLEST valve that will work with that seat.
So when I said "bigger seat" I should have specified that that doesn't always mean taking out the stock seat and pressing in a new one. In some cases you just bore out the center of the stock one and recut the faces into it to accept the bigger valve.
 

delray

Well-Known Member
#14

ole4, curious how well the stock one piece Tecumseh seat machine out for you. looks like the insert on that boring bar cut the seat nice and clean.
did you just run the speed real slow and whittle away at it. also did you just open them to the vintage size valves or go little bigger. for example briggs size. when going start posting some of your Tecumseh mod's(machine work)


I believe 27mm to 28mm is .0394 or .03937:) .038 still works......
 
#15
That is a old m head Bridgeport and its slowest speed was pretty fast around 300 rpm, but it cut just fine. That is an old pic and was my first hs50 hop up. I think it was 125crazy who had built some of these and helped with a lot of it. He told me the latest motors had a small exhaust valve and gave me the part number for the larger valve. That is why I opened the throat Up.
 
#16
Ole4 and all,
Should I assume the auto speed/machine shop of mine would know after measuring that
I could go with a 28.5 intake in a GX200 head already machined for a 27mm? And also do
the throat cut and bottom cut?
 
#17
Ole4 and all,
Should I assume the auto speed/machine shop of mine would know after measuring that
I could go with a 28.5 intake in a GX200 head already machined for a 27mm? And also do
the throat cut and bottom cut?
I'll have a talk with the guy about what he can do, will report back.
 
#18
I don't know since I bought all the cutters and replace the intake seats the throat is already the correct size and when replacing a exhaust valve from 24mm to 25 I don't bore the throat but just put a 75/60 to get the seat where I want it. Exhaust flows well on these engines as is. Intake is where a lot of the work is. While I do flow test exhaust it is just to find any issues and to listen to the port when valve is at .0400 to hear any abnormal noises. otherwise a gentle radius of the shortside and the usual work around the guide on the roof of the port.
 

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