Valve guides.

[Beach Life]

Hi, I have recently embarked on the restoration of what was my fathers 450 Desmo. I am putting in new valves and valce guides, my question is, I see various oversize guides for sale. I havn't removed mine yet, did Ducati fit various sized (o/d) guides? Or are the heads reamed to suit an oversize when replacing them.
I remember hearing many years ago that they assembled their bikes from A parts, B parts, C parts etc. If you got a bike that was all A's you got a minter, if they were getting near the bottom of the parts boxes, they tended to mix and match what was available with inevitable results.
Cheers all

[Duccout]

There are some experts who will give you more information, but be very wary when replacing the valve guides. The guides have very little area holding them in place, particularly on the 'short' side of the shoulder, and it is very easy to get the guides in crooked. Every time a guide is removed and replaced, the bore in the head enlarges, so often an oversized guide is required to maintain the correct interference fit. AFAIK, Ducati selected the appropriate sized guide to fit the bore in the head, so measuring the guide after you have removed it will be a good starting point

Some people prefer to use a puller to ensure that the guides are straight, but the trouble with this method is that it is much slower, so the guide heats up and tightens as it is being fitted. A well-respected Ducati tuner told me years ago of his procedure when fitting guides: drift out old guide cold, ream bore in head in line with valve seat, turn up new guide to fit the bore, heat head and drift in guide. Not easy!

[Jordan]

Ducati is the only engine maker I've come across that suggests removing valve guides from a light alloy head in a cold state.
The usual recommendation is to heat the head first, so it expands a little and releases the grip on the guides, at least partially.
As far as I know, guides from other makers are not normally made in over sizes.

What I do know is that if guides are removed from a cold alloy head, they will cause the guide bore to become larger than they were.
I discovered that when, not knowing better, I once knocked some out cold from a Matchless aluminium alloy head.
The guides for it had plain cylindrical outer diameter, and were retained only by an interference fit. Shortly after installing new guides and running the engine, one of them moved downwards and touched the valve, causing loss of compression and need to repair again. The interference fit was spoiled.
This problem doesn't occur with cast iron heads.

Ducati's system was to allow cold removal of guides, and to provide oversizes to compensate for enlargement of the bores.
That's workable, but any convenience advantage is negated by the need to ascertain and procure the appropriate oversized guides.
I don't know if there's a practical reason for the Ducati method (seats might fall out?), but I heated my Ducatis' heads before guide removal.
At least there's a flange on the guides, so they can't drop out of place in use.

[marsheng]

I always measure what comes out and the the hole size as a guide. With these old bikes anything could have happened to them in the past so I never trust them. I have just removed guides from 2 heads cold. They hardly took any force to get them out. I'm going have to scratch my head a bit to remember the interference fit that I have used in the past. I normally make new to size from PB1 or PB2 bronze. I also heat to 180 deg C and put the guides in the freezer on reassembly.

From my other post, a 100 deg rise in temp with the crank and casing caused a 0.04 mm change over the width of the crank due to the different rates of expansion. So I don't think there is going to be much of a change in diameter on a valve guide when heated together.

Quite different if there is a difference in temperature between the 2.

Probably the most important item in reassembly is the drift. Needs to guide the guide in very well.

Cheers Wallace.

[Duccout]

Hot or cold removal is debatable. Ducati vee twin workshop manual has instruction to heat the head before removal and Steve Wynne told me to heat the head before removal (remember that the drift, especially if put in the freezer first) will cause the guide to contract; however, AFAIK, phosphorous bronze has a higher expansion rate than aluminium, so heating the head first seems wrong. Anthony Ainslie ( who was the UK's top Ducati Single tuner) told me to remove the guides cold.

So, unless Eldert or Nigel chime in with a definitive, it's a case of you pay's your money.....It is a lot more convenient to knock them out cold. I believe that the reason why Ducati offered oversize guides was because there is so little support around the guide, so a more accurate interference was necessary; imagine if the guide bore was 50mm deep, then the fit would be less important.

[blethermaskite]

I "sort have" the same job to do on my 350 mk3, I have had in stock a completely destroyed desmo head ( it dropped a valve at very high rpm) had it for nearly 30 years, however while the casting was scrap most of the bits inside were in v.good order, so recently I bought from America a really nice condition used bare desmo head....and I mean bare the only removable things left in it were the valve seats! I have been slowly gathering up the parts needed to build this up and I bought of Ebay a couple of new old stock desmo single valve guides, but I have not as yet measured them or the guide holes in the head so have no idea what sort of fit or not I will run into, I have removed and fitted hundreds of valve guides but this is the first time I have needed to do them in a desmo head, so I am very interested in any advice in this thread.
Cheers,
George

[DBDBrian]

Not pertinent to Ducati I accept, BSA recommend Goldstar guide removal with the head cold.
I have fitted many guides over the years, with the head hot and the guide cold, I find speed of operation is a must, as the guide will pick up heat and expand if not installed in one quick move. It helps to have a good solid drift, a good fit in the guide, but not to tight so it becomes trapped when the guide is installed, and reasonably long as it gives a better indication of alignment when first entering the guide into its bore.

Oversize guides and interference fits, are very dependent on the condition and construction of the cylinder head.

[blethermaskite]

Other than the interference fit tolerance, the issue I see with desmo single valve guides is getting the milled flats that accommodate the closing rocker nicely aligned parallel with the rocker....I assume a fine felt tip pen line on the head and on the guide would suffice as a guide?
Cheers,
George

[ranton_rambler]

For my Guzzi, I was recommended to K-Line the original guides as there was no discernible wear on the stems. So far so good although it’s only done 500 miles so far. Not really a DIY option as far as I can tell.

[Duccout]

I second that, although they cannot be used in desmo guides. I know that Eldert is not a fan of K-line liners, but to me they are a safe way of renewing guides. I had my 750 Sport done and the bike ran better than it had ever done.