Oversize Valve Guide Dilemma

lcrken · Dec 28, 2022

SEd27 said:
I removed my valve guides yesterday to measure for replacement, but ran into a bit of a dilemma…

When I measured the old cast iron guides, I was getting between 0.511”- 0.514”. Granted this is with a digital caliper (I need to pick up a 0-1” micrometer).

However, the project came with x2 new standard bronze guides. I tried for fitment in the bores and it is not even close. If you turn the head upside down the guides will fall out.

Because these dimensions fall outside the available oversize, I am thinking the following are my only options:

1) See if Kibblewhite can produce a custom sizes. I am using their black diamond valves and have already sent them an inquiry.

2) Purchase 850 guides and find a machinist to cut them down to size. I have a lathe but don’t trust my precision.

Has anyone had any experience with this before?

What is the guide size limit 750 head?

I would have not gone down this path but there was noticeable play with the valves inserted. As well, one of the guides had a crack.

The 750 guide standard specification is .5015/.5000". Factory oversize (cast iron) guides are still available from Andover Norton in +.005", +.010", and +.015". Looks like your head already had the +.010" guides installed. If the original guides were a good fit in the head, you could just replace them with a set of new +.010" guides, assuming that worn guides was the only problem. I'm also assuming that you heated the head properly to remove them, and didn't damage the guide bores in the head. If you prefer to use bronze guides, then you will probably have to make, or find someone to make, some larger oversize guides. I've not done it, but it looks like you could just turn down some 850 bronze guides to fit. I've made custom guides for one of my Commando big valve heads, but you do need the right tools. Turning down 850 guides looks a lot easier to me than making them from bronze bar. If the guide bores in the head are now too loose a fit for the +.010" guides, the other option appears to be boring and reaming the holes to take the +.015" cast iron guides or cut down 850 guides.

Another, less desirable, option might be just boring and reaming the head to take bronze 850 guides, but that's a lot more touchy than just turning down the 850 guides. You really need a milling machine (or a lot of ingenuity) to bore and ream the head. Also, if you screw up making the guide, you can just do another one. If you screw up the head boring and reaming it, it's a lot more painful.

In any case, good luck with it. However you do it, you will learn something from the experience.

Ken

SEd27 · Dec 28, 2022

lcrken said:
The 750 guide standard specification is .5015/.5000". Factory oversize (cast iron) guides are still available from Andover Norton in +.005", +.010", and +.015". Looks like your head already had the +.010" guides installed. If the original guides were a good fit in the head, you could just replace them with a set of new +.010" guides, assuming that worn guides was the only problem. I'm also assuming that you heated the head properly to remove them, and didn't damage the guide bores in the head. If you prefer to use bronze guides, then you will probably have to make, or find someone to make, some larger oversize guides. I've not done it, but it looks like you could just turn down some 850 bronze guides to fit. I've made custom guides for one of my Commando big valve heads, but you do need the right tools. Turning down 850 guides looks a lot easier to me than making them from bronze bar. If the guide bores in the head are now too loose a fit for the +.010" guides, the other option appears to be boring and reaming the holes to take the +.015" cast iron guides or cut down 850 guides.

Another, less desirable, option might be just boring and reaming the head to take bronze 850 guides, but that's a lot more touchy than just turning down the 850 guides. You really need a milling machine (or a lot of ingenuity) to bore and ream the head. Also, if you screw up making the guide, you can just do another one. If you screw up the head boring and reaming it, it's a lot more painful.

In any case, good luck with it. However you do it, you will learn something from the experience.

Ken

Thanks for the heads up Ken, I was unaware that Andover had that selection of oversized cast guides.

That definitely gives me some options.