Fixing single valve guides

andyblue · Post by **andyblue** » Tue Mar 29, 2005 10:53 am

I thought I'd get the '55 18S out for the first run of the year this weekend, but it wasn't to be because on attempting to kick it into life I found that there was no compression!

It had been working OK when I put it to bed last year and the last thing I'd done was let it tick over to empty the carb before putting it away. A little investigation revealed some suspicious clearance on the exhaust tappet. Some taking appart later I found that the exhaust valve guide had moved down far enough to hold the valve off its seat. The seat and valve face look alright.

I'd had new spares club guides fitted and seats recut etc. last year and so I suppose the new guides aren't as tight a fit as the old ones. I've read somewhere that to counteract this, guides were introduced with a groove for a circlip which stopped them creeping down. I think I remember that the new guides had this groove (can't see it now 'cos it's slipped out of sight...and I think the inlet guide has a bit too) but I didn't know what it was for at the time and my old guides never had it.

So is there any reason that I shouldn't heat the head up, drift the guide(s) back to reveal the circlip grooves and just bung some clips on? It seems OK to me but there may be hidden snags and I'm sure that some of you out there will know all about this and have some words of wisdom. Also does anyone know the part number for the circlips so I can get some in?

It still beats me how the old crate managed to tickover when I last used it, unless the guide shifted all at once right at the end and that's what stopped it rather than running out of petrol?

Cheers

Andrew

Chazzyb · Post by **Chazzyb** » Tue Mar 29, 2005 6:06 pm

I'd drift it out down into the port and refit it with a circlip. Pop the guide in your freezer for a few hours and head in oven (cylinder head, silly).

If the guide has been moving for a while, you might find the hole in the head has worn. In which case you may need an oversize guide made or if you're unlucky, the hole in the head may need machining back concentric too. Not disastrous if it came to it.

Biscuit · Post by **Biscuit** » Tue Mar 29, 2005 8:38 pm

I have removed/replaced valve guides using local heat, but the oven is the best bet, try 30 mins. at no. 9 for starters, the guide needs to come out relatively easy to prevent more wear in the hole. You need to remove the guide completely to check the position of the oil hole, fit the circlip before re-inserting the guide. The manual suggests copper plating if the guide is too loose. The circlip No. is 011111A ( page 7 in your spares catalogue)

Merlin · Post by **Merlin** » Tue Mar 29, 2005 8:39 pm

Who needs a hole in the head when you run a classic bike[:)]

Biscuit · Post by **Biscuit** » Wed Mar 30, 2005 12:43 am

I thought Chemists were born with them!!

Seriously Andrew, it is essential that you follow Charles' advice and drift the guide down through the port, as any carbon specks left on the guide would damage the hole. If you feel the need to clean the guide with emery or similar abrasive before replacing, work the emery up and down the length of the guide, not round it, as you leave minute rings around the periphery wich act as a very fine but effective broach.

Merlin · Post by **Merlin** » Wed Mar 30, 2005 10:58 am

Only useful ones Alan[:I]

autosprink · Post by **autosprink** » Wed Mar 30, 2005 12:36 pm

Tightened up a pair of 18s valve guides in the head with a knurling tool in the lathe some years ago - not ideal but still going after 2000 miles

John Powell