Valve guide replacemnt - Help please

[Scullies]

I have taken my mates TT head to get the valve guides replaced. Now the machine shop says they need to grind the valves and seats as the valve angle will not be the same with the new guides.
I may be wrong but this doesnt make sense to me. Surely if you remove the existing guide and press the new one in, the valves are going to be in the same position.

Are they correct or are they spinning me a story to make an extra buck?

 

[towcestervag]

they are correct
the valves will sit at a slightly different alignment so you will need to reseat them to get a correct seal
if hes doing this then get them to redo the valve seats in the head

 

[16Klappe]

Imagine the valves have been sliding up and down the inside of the guides; they will gradually wear themselves a small groove which they often run through. This slightly changes the angle the valves run through the guides at over time. Because it is gradual everything wears together.

Replace the guides with brand new perfectly straight guides and this wear has been taken away, meaning that the angle the valves travel has changed slightly due to the lack of wear. But the valve seats remain untouched.

Very simple when you think about it.

 

[<tuffty/>]

After a guide replacement they need to be at least to be re-ground... seats may need re-cutting if wear on guides was excessive enough that there is pitting on the seats/valves or they have worn that badly that the valve mad a new profile..

Re-grinding valves is a given if its apart tbh...

<tuffty/>

 

[badger5]

never yet had to recut seats byond just normal lapping in..

jardo.. the wearing bit is the inside of the guide not the outside of the guide to head, so if pressed in they should still align with valves seats, but their tooling for reaming to size will determin the actual angle.... which if they set them up aligned to begin with they should align afterwards.. Its not uncommon for seats to need a slight tickle.. My local m/c shop checks the valves for seating with engineers blue to confirm they will need normal lapping. They do say if they wont lap in bring it back and he will recut the seat slightly to suit the new guide. Not had to do this bit as yet

 

[superkarl]

I plan on only lapping them back in.

 

[Scullies]

After a guide replacement they need to be at least to be re-ground... seats may need re-cutting if wear on guides was excessive enough that there is pitting on the seats/valves or they have worn that badly that the valve mad a new profile..

Re-grinding valves is a given if its apart tbh...

<tuffty/>

When you say re-ground, do you mean lapping them in with grinding paste?

never yet had to recut seats byond just normal lapping in..

t

I plan on only lapping them back in.

That was my plan.
The valves and seats are in good condition. All valves have a nice consistent ring where they have been sitting on the valve seats. No pitting on the valves or seats.

Guess I will try another machine shop and see what they think.

Thanks for all your input guys :thumbsup:

 

[<tuffty/>]

When you say re-ground, do you mean lapping them in with grinding paste?

Yes

<tuffty/>