Valve guides, bronze or iron

OK. The old rule, and I don't think it has changed, is to use bronze guides for racing and iron ones for the road.

The important thing about removing and replacing valve guides is to remove every bit of carbon from the guide before removing it, since carbon will score the hole in the head and allow oil to go down the outside of the guide. Heat the head in the oven to about 400 degrees F before removing the guides. Use the correct tool if you have it, but if you don't, a double diameter drift will do a good job. When you have the guides out, measure them carefully with a micrometer. If the guides pretty much fell out of the head, use oversize ones to replace them, if not, then ones the same size should be good.

The only reason to fit 850 guides to a 750 is if the holes in the head are so enlarged that there is no oversize 750 guide big enough to give a good fit, or if you didn't remove all the carbon from the old guide and scored up the head .

The knock-on effect of replacing guides is considerable, so I wouldn't do it unless they're loose in the head, or you're pretty sure oil is getting past the guide/head joint(s). When I rebuilt my (current) 850 engine I thought the stems were a bit loose in the guides. My engineer agreed, but instead of fitting new guides which would have changed the valve angles necessitating removal of seat material, he bored out the guides and fitted bronze sleeves using a technique that maintains alignment better than when fitting new or oversized guides. I hadn't heard of that technique before but he said it's been around for a good few years now and results are reliable.

It has been sugested to me by our local engine refurbishing shop to do this, but does it not leave the bottom edge of the existing guide dangerously thin once it has been bored out to take the insert?

Previously wrote:

I'm just about to refurbish my cylinder head and have the options of phosphor bronze or cast iron valve guides. I fitted bronze guides when it was the trendy racing mod about 28 years ago and then cast iron subsequently. These are now past their best. Whilst there were some who would advocate fitting bigger 850 guides, I would rather avoid machining if it's not required, I wonder if anybody has any valid opinions on this matter. I seem to recall that the bronze guides didn't seem to last very well.

Is there any opinion with regard to tapping the guides in and out of a hot head with a mandrel compared with drawing them in with the correct tool?

Thanks in anticipation

Dave

Hello FIT Bronze Valve Guides you will have no trouble from them , Cast Iron ones can brake in two , Its happen to me and Cost me a small fortune to fix , Now I have Bronze Valve guide theres on more trouble and will last me out I think, Yours AJD

I have always used iron guides with no real problems. Cast iron is to an extent self-lubricating (due to tiny particles of carbon in the iron) unlike bronze. For road use (and indeed I have used them racing) they seem fine. Gordon.

The relining of valve guides in cylinder heads has been around for years in America. The V8 small block Chevrolet engines have their valve guides reclaimed this way.

Smokey Yunick, renowned as the most innovative and brilliant development engineer for USA automakers and racing engines etc. said that cast iron was the ideal material for valve guides.

Good American builders of Norton engines shy away from English-made phosphor bronze guides as having a short life compared to cast iron and they favor guides of tougher bronze alloy made by some USA companies.

If you have a quality guide that has concentric inside and outside diameters and are a good mechanic with the right tools you can do a great job of putting new guides in.

Because the bronze expansion rate is closer to alloy than cast iron, they can be fit with less interference. Racers like bronze guides because if they float a valve and it kisses a piston, the flexing valve stem will not crack or fracture the end of the valve guide and have it fall into the engine.

That said, with proper maintenance a cast iron guide is going to last a long time for practical street riding and used to have a cost advantage to boot.

I just knocked some nos iron guides into my 1962 650ss head. The valve seats cleaned up with very little grinding. In fact when I took a vernier caliper and measured from the tips of a new set of valves to the spring seats, the measurements were so close to those taken off a spare head which had never had it's seats ground from new, that it might fall into the realm of error. The height for the valve stems were within a few thousandths of each other, which means the installed spring height will be also.

If you get poor quality parts and a bad mechanic to put them in though, things can certainly get far out of alignment and seriously affect the performance and longevity of a cylinder head.

I have seen poorly done heads with the valve seat ground close to a quarter of an inch wide on one side and to the very minimum on the side opposite, indicating something very wrong with the way the valve guide was installed. This means that one seat is now at an angle and sunk, so the flow, geometry, timing, rev limit and sealing for that valve may all be different than it's three siblings.

In summary, if you do not have the tools and confidence to do a very good job yourself, then it will serve you well to be patient and make sure that whoever you are giving your money to has a spotless reputation and many good references as some do much more harm than good to the cylinder heads they put their hands onto.........

Smokey Yunick, renowned as the most innovative and brilliant development engineer for USA automakers and racing engines etc. said that cast iron was the ideal material for valve guides.

Good American builders of Norton engines shy away from English-made phosphor bronze guides as having a short life compared to cast iron and they favor guides of tougher bronze alloy made by some USA companies.

If you have a quality guide that has concentric inside and outside diameters and are a good mechanic with the right tools you can do a great job of putting new guides in.

Because the bronze expansion rate is closer to alloy than cast iron, they can be fit with less interference. Racers like bronze guides because if they float a valve and it kisses a piston, the flexing valve stem will not crack or fracture the end of the valve guide and have it fall into the engine.

That said, with proper maintenance a cast iron guide is going to last a long time for practical street riding and used to have a cost advantage to boot.

I just knocked some nos iron guides into my 1962 650ss head. The valve seats cleaned up with very little grinding. In fact when I took a vernier caliper and measured from the tips of a new set of valves to the spring seats, the measurements were so close to those taken off a spare head which had never had it's seats ground from new, that it might fall into the realm of error. The height for the valve stems were within a few thousandths of each other, which means the installed spring height will be also.

If you get poor quality parts and a bad mechanic to put them in though, things can certainly get far out of alignment and seriously affect the performance and longevity of a cylinder head.

I have seen poorly done heads with the valve seat ground close to a quarter of an inch wide on one side and to the very minimum on the side opposite, indicating something very wrong with the way the valve guide was installed. This means that one seat is now at an angle and sunk, so the flow, geometry, timing, rev limit and sealing for that valve may all be different than it's three siblings.

In summary, if you do not have the tools and confidence to do a very good job yourself, then it will serve you well to be patient and make sure that whoever you are giving your money to has a spotless reputation and many good references as some do much more harm than good to the cylinder heads they put their hands onto.........