There is a useful ‘decision tree’ in this thread on another forum: **LINK**

https://www.mig-welding.co.uk/forum/threads/best-loctite-for-a-valve-guide.94507/

MichaelG.

inlet might respond to loctite but exhaust will be too hot . Putting a knurl on the outside might work. Noel.

That would be one way to do it ! Noel.

The method I used to retain PB guides in an ally head on a model engine was to drill and tap halve in the guide and halve in the head and insert a grub screw.
The guides were fitted into a heated head so what I did was belt and braces.

Emgee

PS In fact, looking at pictures of 1935 Guzzis they appear to have the typical of the era exposed valve gear with no lubrication on the guide. You might want to seriously consider cast iron guides for that, in line with almost every other bike of the era. There might be a reason your bronze guide came loose in the head, like overheating and picking up on the valve stem and getting ripped out of the head. In my experience, 1920s inlet-over-exhaust Harleys and OHV Big Port AJS's ran cast iron guides in similar unlubricated set-ups with never a failure. Never seen bronze used in a dry set up like that though.

But I am sure there must be vintage Guzzi cognoscenti somewhere who have done all this before if you can track them down. My experience is mostly Brit and US bikes and a few BMW's. Never dabbled in the Italian stallions.

Edited By Hopper on 27/05/2022 09:58:26

ISTR my old BMW "Airhead" has bronze guides and those things run forever without problems.

I must have been unlucky then, as both the "post 81" R100s I have owned wore the valve guides out by 50,000 miles.

I moved the first one on without fixing it, but fitted supposedly "superior" guides from a BMW specialist supplier to the second – which wore out in under 10K ! Apparently most users who fitted these guides didn't actually use the bikes much afterwards – so that was OK then ! Second fix was with OE BMW guides, which were fine for the next 5K or so that I did before moving that one on. The OE replacements were shorter than the originals fitted by BMW when the bike was built – apparently changed by the factory as a fix for the valve head seperation these models have a bit of a reputation for (stems friction welded to the heads). Heads heated to 240C to fit the replacement guides, using a s/h oven bought off Ebay for a fiver which was recycled afterwards.

I would look to fix the Guzzi properly with new oversized guide, fitted by someone with a proven track record. Choose your specialist carefully, though – one I made the mistake of using to change the guides & valve seats on T'Wife's R65LS was not as good as he thought he was. He fitted one of the valve seats with too tight an interference & it broke up, with the broken bits dropping into the cylinder. Even though the break-up occurred during a cold start (and the engine didn't fire, as it locked up) that scrapped the cylinder, piston & head . I was (eventually) able to source used parts, but I doubt it would be easy to do so for a pre-war Guzzi. I would not take any risks with such a bike.

Nigel B.

The late, great Phil Irving did not say that, just the opposite in fact, Al bronze is eminently suitable for valve guides, good wearing properties and a good conductor of heat.

What he did say was not to use phosphor bronze, as others have said, unsuitable for the application.

If the OP can get hold of some Colsibro (free cutting Al bronze), make an oversize guide. Don't forget to check the hole before making the quide. It may well be necessary to re-machine the hole if its damaged or worn, the set up for this needs to be done very carefully to keep seat and guide in alignment.

You could try using JB Weld, which has good high temperature properties. I used the regular stuff on a cast iron multi fuel stove after extensive bronze welding in order to level the surface after dressing back the welds. It's done three years at over 300 degrees centigrade towards the top, with no issues at all.

I consider anything other than fitting an oversize guide a short-term replacement. You may be fortunate in that an epoxy or retaining compound repair lasts, but it depends a lot on frequency of use and many other imponderable factors.

Many of the bronzes used for guides nowadays are difficult to obtain in the UK in small quantities. I've successfully used CA104 aluminium bronze to make guides for my Guzzi Spada and Lario (alloy heads) and these have stood up to high mileage. I would now perhaps favour Colisbro if I could get it at a decent price. Another alloy that I've found to be good for guides is PB104.

One consideration with an iron head is the difference in expansion of a bronze guide compared to an alloy head. With an alloy head the guide expands at a similar rate to the alloy, so the guide-to-stem clearance can be quite close. With an iron head the guide expands at a greater rate than the head, so if fitted with too tight a clearance can cause the guide to grip the valve stem when hot. The bore shrinks down and the guide extrudes due to the constraint of the 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

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.