fixing loose valve guide

I'd question whether that bronze guide was original. It would have been fairly unusual in a 1930s cast iron head. (But Italian bikes do specialise in the unusual.) Worth looking into whether the original was cast iron.

Having had awful trouble trying to drill ally bronze how do you do it? It seems to shrink onto the drill and seize. Gave up eventually

Flood coolant. Low speed, maybe half of what you would run for steel, or less. No pilot hole, or at least very small pilot just big enough for the web of the larger drill bit. Definitely do not go up in small steps or the tips of the cutting edges will catch.

Run one lip of the drill over the grinding wheel so it is just a tad longer than the other one, causing drill to run off centre and cut an oversized hole for clearance. Maybe even stone the cutting edge as when drilling brass so there is zero rake.

And bore the hole to final size. If reaming, leave only a couple of thou for final finish with the reamer and again run slow and plenty of lube or Trefolex.

Or just buy a readymade valve guide of the correct bore and machine the OD to suit your engine!

I use a lever feed tailstock and it helps with anticipating if the drill or reamer is inclined to rub with aluminium bronze. I always either sharpen any drills, or use a new drill. I drill a pilot hole about 5/32" diameter then follow this with an intermediate sized drill, then something closer to the finished diameter. The drill always has to be cutting. Then bore the rest out to reaming size. Trefolex is my preferred lubricant. The reamer has to be new, or almost new and razor sharp. If it rubs it will need re-sharpening. For valve guides I've had a lot of success with good-quality expanding reamers.

The problem with ready-made guides is they're usually standard nominal bore sizes, which results in too much stem-to-guide clearance. You could go to the next size down and re-bore the guide, but if you're doing this you may as well do the whole thing from scratch. The exception is that there are some guide blanks that are made either in an alloy that's otherwise difficult to source, or cheaper than buying bar stock. The other issue I've found is that the OD of the blanks is often nor great enough for many motorcycle valves where there's a large diameter change between the main body of the valve and any flange (eg Honda CB450 DOHC guides) When re-using an old valve with a new guide its best to lap the stem using a split aluminium or copper lap to get it parallel and round, so its even more important to be able to size the guide to the valve.

Thanks for all your knowledgeable replies, gents. You have convinced me to forget about a quick fix and go for the over-size guide. Unfortunately, all the spares have to come from Italy which complicates and slows down things down somewhat. I have a good engineering company, Cerney Engineering near Swindon, I would not attempt this myself.

A lot of yrs ago I had a 500cc gold star & the exhaust valve guide was loose in the head [bronze guide,alloy head], I got some copper sulphate & a piece of copper ,cleaned the guide & corked the ends & copper plated it, never had anymore bother.

Graham.

You could take the valve guide to an electroplating firm and they could plate up the relative part while masking off everything else. Then the od could be carefully turned down to the correct interference fit. Turning using a very sharp tool and taking minimum depth cuts. The head would best be heated when fitting the guide.

I had to order a new valve for Italy so at the same time ordered a replacement o/s valve guide. The supplier is a specialist only in these old Guzzis so I will be interested to see if the new guide is bronze or iron.

The main difference is that bronze must be lubricated and iron not, that is why most iron guides on more modern vehicles have seals to keep oil from getting into the cylinders and coking up.

The last bronze guides I had were on Royal Enfield twins.

Edited By old mart on 30/05/2022 21:23:34

Try using a split-point drill, like the one on the left.

Martin.