Valve seat size for 34mm exhaust valves, Rebuilding 1.7 heads with larger exhaust valves Options

[Green626]

I am currently in the process of ordering parts to get my 1.7 heads rebuilt-
the 33mm exhaust valves need to be replace so I am going to replace with 34mm.

To go with the larger valves i need to get new valve seats and want to make sure i order the right size. Doing a search I can find 32mm, 35.5mm and 37.5mm and larger replacement seats - but it not clear if this is ID or OD of the -

Has anyone has done this on your heads? if so which is the correct size to order?

Thanks in advance for your input

[brant]

You need a machinist with lots of air cooled experience

Dropped valve seats are a motor killer
And air cooled motors are good at dropping them
The expertise at this is significant

It’s also expensive
Sometimes cheaper to buy a better head
1.8/2.0 heads are out there

We have this done on race motors
The head work including flow porting is the most expensive part of a build

[MikeK]

Other than exhaust valve size, are there any other differences between 1.7 and 1.8 heads? Port shapes or sizes? I'll be going down this path soon.

[brant]

1.8 heads have a little more meat and much better for a 96 mm cylinders than 1.7 heads which are a bit thin

Plus the bigger valve sizes

[Green626]

1.8 heads have a little more meat and much better for a 96 mm cylinders than 1.7 heads which are a bit thin

Plus the bigger valve sizes

I am trying to make a slight improvement to my 1.7 J heads by going with the 1.8 34mm exhaust valves - as I have to replace them any way - which is why i was asking about the replacement seat size that i need

[brant]

1.8 heads have a little more meat and much better for a 96 mm cylinders than 1.7 heads which are a bit thin

Plus the bigger valve sizes

I am trying to make a slight improvement to my 1.7 J heads by going with the 1.8 34mm exhaust valves - as I have to replace them any way - which is why i was asking about the replacement seat size that i need

Makes sense
But likely cheaper to buy used 1.8 heads than it is to put big valves into a 1.7 head
My machinist makes his own seats to size out of quality stock
And I would expect an air cooled machinist to handle sourcing the material. And likely be particular about these materials.

1.8 heads are out there and still cheaper than the machine work/labor

[914Sixer]

You can buy new heads from AA Performance for what it will cost to rework your old heads.

[Green626]

Gents,
I greatly appreciate the input ! Looks like I will consider all options