A forum for owners of Yamaha TTR250 trail and enduro bikes!

problemaddict · Member

Valve Guide replacement procedure?

Haggis Hunter · Veteran Member

Get yer spec's on Cubber! Reface the valve seat not replace biggrin

You would have to recut the valve seat after fitting new guides to ensure valve seats correctly. Lapping alone may do it but in my experience it never has.

A quick turn with a valve seat cutter saves a lot of lapping

I suspect the reason for heating the head is to prevent metal in the head being stripped out with the old guide which could make the new guide a sloppy fit. Some hydraulic control valves I have worked on in the past had a valve running in a bushing. If the valve was damaged the valve and bushing would have to be replaced as the tolerances were fine to say the least (The bush and valve came as one item as they were lapped together to a fine tolerance). The bushes were colour coded so when you pressed the old bush out you checked the colour marking and ordered the next size up (which was another colour) The reason for this was of a loss of metal in the housing when the old bush was pressed out. You were talking of 10ths of a thou, not much but enough to cause leaks if the same size bush was re fitted.

Peter

-- Edited by Haggis Hunter on Thursday 2nd of August 2012 11:23:24 PM

TTRfan · Super Guru

Page 4-21 of the workshop manual covers removal and replacement of ther guides. Bascally it involves heating the head in an oven to 100C to make removal and installation easier and "to maintain correct interference fit".

Then use two special tools to extract and install. These look like simple drifts.

Never had to do it myself so sorry - can't help more cry

Brian

Cubber · Super Guru

Here's the page from the manual, for reference.

Guide%2520replacement.jpg

I wonder if the last NOTE: is essential - to replace the valve seat? confuse confuse

I would have thought that lapping the valve in would be sufficient.

Martyn

Cubber · Super Guru

Just checking to see who'se checking! biggrin biggrin

My old Trumpet valve seats have been re-cut that many times they eventually got pocketed. cry

Thanks.

Martyn

TTRfan · Super Guru

Haggis Hunter wrote:
A quick turn with a valve seat cutter saves a lot of lapping.
Peter

I do a lot of valve lapping so just looked at valve seat cutters on ebay. They aren't cheap cry

Haggis Hunter · Veteran Member

They are not cheap. I can remember a set costing about £150 in 1978. It was a big set which covered car sized seats up to large diesels with valve heads around 2", Sorry, 50mm.

You can, or used to be able to, get ones that were a grind stone at the correct angle for the seat with a guide which were much cheaper but didn't do such a good job.

The last set I used on a MAN natural gas engine were made by Sykes Pickavant. They were very good, took no time to lap the valves in which was just as well as it was a V12. The palms of your hands tingle for a bit after that!

We did run some V16 Cats and they had 4 valves per cylinder. Stayed out the way when they needed doing!

Peter