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fixing loose valve guide

can I rescuer a loose bronze valve guide in a cast iron head?

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c wastell26/05/2022 20:36:02
37 forum posts

I have a cast iron head with a loose (sliding fit) bronze valve guide. Is the a way that it can be held in place? Is there a loctite product for instance? I don't want to go down the oversize guide route as I fear that will be opening up a whole can of worms

Michael Gilligan26/05/2022 20:57:36
20289 forum posts
1064 photos

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


c wastell26/05/2022 21:05:04
37 forum posts

Thanks for that, Michael. A few different opinions there. I appreciate the help👍

noel shelley26/05/2022 21:19:23
1444 forum posts
23 photos

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

c wastell26/05/2022 21:20:55
37 forum posts

Unfortunately it is the exhaust. Do you mean knurl it then shrink fit?

noel shelley26/05/2022 22:40:41
1444 forum posts
23 photos

That would be one way to do it ! Noel.

Hopper26/05/2022 23:45:28
6690 forum posts
347 photos

With engine rebuilding there is only one way to do things: properly. Fit an oversize valve guide.

Edited By Hopper on 26/05/2022 23:46:51

Emgee26/05/2022 23:56:30
2445 forum posts
291 photos

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.


valve gear 2.jpg

Hopper27/05/2022 00:09:40
6690 forum posts
347 photos

Yes that is the question. What sort of engine is the OP referring to? 5cc model or Cat D9 dozer diesel or 14,000 rpm motorbike?

c wastell27/05/2022 05:06:54
37 forum posts

Sorry, I should have been more specific. Its a 500cc single cylinder motorbike. (1935 Moto Guzzi).

John Olsen27/05/2022 05:28:48
1256 forum posts
94 photos
1 articles

Is bronze a suitable material? Some of the bronzes go hot short and will collapse a little and come loose in the head. Phil Irving says in his book that aluminium bronze for instance is not suitable for exhaust valve guides, which is probably why BSA/Triumph used it on my 1971 Blazer SS 250 single. It was failed when I got the bike(which I knew) with about 1200 miles on the clock. It got replaced with a cast iron guide, shrunk in with the head heated as hot as we dared and the guide cooled down with spray on freeze. Never gave me any more trouble.

Funny thing was later, when I was talking to a guy who specialised in fancy cars like Ferraris etc, he said "cast iron is fine for guides but it can hang up at high revs" To which I replied "That's OK, this engine only does 8500rpm." He got a funny look on his face, so then I said "Well, my little Honda has cast iron guides and that is redlined at 11000..."


c wastell27/05/2022 05:36:02
37 forum posts

I'm a pretty amateurish engineer (O level B) but a lot of what Triumph produced was way below standard. On the other hand, I reckon pre-war Guzzi engineering was very high quality and if Carlo thought bronze was best, that's good enough for me🙂

Hopper27/05/2022 09:28:20
6690 forum posts
347 photos

1935 Guzzi - gorgeous! And not something to be bodged up. Unfortunately you don't have much choice other than to open that can o' worms! Loctite can't be trusted to handle the heat. Knurling is a temporary fix usually because it generates raised ridges so you get partial contact which could eventually hammer loose again , and will not transfer heat as well as full metal to metal contact, which is critical on exhaust valves.

The repair depends on how the hole in the head has suffered from the loose guide hammering up and down. If it is tapered and or egg shaped, it will need remachining back to round and straight before a new guide can be fitted. That is best done by an automotive machine shop that does cylinder head work, as they can then get it in line with the existing valve seat. Or if you are a good and experienced machinist, you could do it on a milling machine in the home workshop.

Then, or if the hole is OK but the last guide came loose, you will need an oversized valve guide to fit the hole. With more modern bikes, guides are usually available with OD oversized by one or two thou or so. No idea waht would be available for your Guzzi. Are there aftermarket specialists for the old single bangers? Talk to them if so. Or you could turn one up yourself if you have a lathe. These days you can buy tiny little full carbide micro boring bars perfect for the job. Then ream to final size after installation.

Plenty of bikes new and old have bronze valve guides and most run with no problems. (Valve guide material was the least of the problems on BSA/Triumph 250 singles of the 60s and 70s. But that's another story.) ISTR my old BMW "Airhead" has bronze guides and those things run forever without problems.

These days the preferred bronze is a grade called AMPCO 45. It is a high grade nickel-aluminium bronze made for the job. Phosphor bronze, silicon bronze and lower aluminium bronze grades have trouble with the alloying elements leaching out at high temp in exhaust guides and are generally no longer recommended.

That said, cast iron guides would work just fine in your old Guzzi too. It tends to be more "self lubricating" than bronze on old engines that didn't have the best oil feed to the top end. I use cast iron in road-going Harley engines as first choice. They seem to last longer than the bronze ones. But higher performance guys go for bronze for higher rpm, less friction etc etc that is not a factor on old bangers.

Sorry to be the bearer of bad news re the can o worms, but such a rare and lovely old bike is well worth spending the money to do the job properly. Bodging it up could lead to more engine damage in the long run and cost a lot more.


Edited By Hopper on 27/05/2022 09:57:22

Hopper27/05/2022 09:46:52
6690 forum posts
347 photos

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

bernard towers27/05/2022 09:52:02
691 forum posts
141 photos

oversize guide is the way to go it only needs to be a few thou and Colphos 90 is the stuff to use.

mgnbuk27/05/2022 10:31:00
1207 forum posts
72 photos

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.

Hopper27/05/2022 10:54:59
6690 forum posts
347 photos

Mgnbuk: Yes the post-81 Airheads were known for having soft valve seats that gave trouble and probably led to premature guide wear. Shorter guides maybe let the valve seat recess further into the head before the larger end of the valve stem hit the valve guide and picked up? The older ones seemed to go for hundreds of thousands of kilometres if serviced regularly. It always seemed to be the gearboxes and rear bevels that gave out first. A problem that still plagues the 21st Century BM twins.

Edited By Hopper on 27/05/2022 11:04:20

John MC27/05/2022 12:16:04
385 forum posts
45 photos
Posted by John Olsen on 27/05/2022 05:28:48:

Is bronze a suitable material? Some of the bronzes go hot short and will collapse a little and come loose in the head. Phil Irving says in his book that aluminium bronze for instance is not suitable for exhaust valve guides,

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.

A Smith27/05/2022 12:24:35
85 forum posts
4 photos

What you do depends on the intended usage - to some extent. If it's intended to be trailered to and from rallies and not ridden for any distance, knurling, loctite, wallpaper paste or whatever.

If the intention is to use it on the road at all, it is well worth a permanent fix, especially so, given the highly desirable nature of the bike.

As the valve gear lubrication may be marginal, an oversize cast iron guide and a recut seat, seems to be the obvious choice. I'd give it to a specialist such as T&L in Bedfordshire.


Mick Bailey27/05/2022 12:49:49
34 forum posts

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.

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