Valve guide oversize limit for 750

Hi All,
I recently acquired a used cylinder head for a 750 Commando. It came without valve guides. I measured the guide bore to be 0.507 +/- a few ten thousandths. This leads me to believe that new guides in an oversize greater than +0.006 will be needed. I note that Kibblewhite and other suppliers do not list anything larger than 0.006 oversize. I also note that the Commando workshop manual references oversize guides up to +0.015.
My question is whether there is reason not to exceed +0.006 oversize?

Thanks for any info.

Ed

APRRSV said:

Hi All,
I recently acquired a used cylinder head for a 750 Commando. It came without valve guides. I measured the guide bore to be 0.507 +/- a few ten thousandths. This leads me to believe that new guides in an oversize greater than +0.006 will be needed. I note that Kibblewhite and other suppliers do not list anything larger than 0.006 oversize. I also note that the Commando workshop manual references oversize guides up to +0.015.
My question is whether there is reason not to exceed +0.006 oversize?

Thanks for any info.

Ed

Click to expand...

No problem over .006.

I have had Kibblewhite make me guides in .015 OS in the past.

I have also made a few of my own from Ampco 45.

750's are easy. Just cut an 850 guide down to the size you need. 850 guides are .626 to start with.

Thank you Comnoz for the quick reply. Your info is greatly appreciated. I wonder, though, is there any advantage or disadvantage in using cut down 850 guides as opposed to 750 guides? I will probably use bronze in either case if it matters.

Thanks again,
Ed

APRRSV said:

Thank you Comnoz for the quick reply. Your info is greatly appreciated. I wonder, though, is there any advantage or disadvantage in using cut down 850 guides as opposed to 750 guides? I will probably use bronze in either case if it matters.

Thanks again,
Ed

Click to expand...

Well, Just that I don't have a source for Ampco 45 guides in .015 OS without special ordering them from Kibblewhite.
But Ampco 45 guides for an 850 are readily available. I have not had as good a result using other grades of bronze. Jim

After more measurements, I find that the four different guide bores vary in size from 0.507 to 0.515. The two exhaust guide bores are the largest.
So, can the 750 head guide bores be safely bored to accept the standard 850 guides?

Ed

APRRSV said:

After more measurements, I find that the four different guide bores vary in size from 0.507 to 0.515. The two exhaust guide bores are the largest.
So, can the 750 head guide bores be safely bored to accept the standard 850 guides?

Ed

Click to expand...

I would not do that.

If you bore the guide holes you will make cracking more likely -like the RH4 head..

Cut the guide down to fit. Jim

Let me add a word of caution on cutting the 850 guides down to 750 dimensions. Be very careful with your setup to get the OD concentric with the ID, otherwise you might have to drastically re-cut the valve seats.

I say this because I recently screwed up trying to do this to make new guides for a Honda VFR500R, for which OEM guides are no longer available, from those for a CRX450. The valve stems are the same diameter, but the OD needs to be turned down a bit. I didn't take enough care to get the OD concentric, and it moved the valve seat too much to easily re-cut. In the end, I replaced the heads with another set I had as spares, but not before I'd spent way too much money on the tooling to cut the stock seats (which are really tiny!), which I never used. In hindsight, I should have just had KPM make some custom guides (I only needed two) instead. With its larger valves and seats, the Norton would probably not be as sensitive to this as the Honda, but I would still be very careful about it.

Another warning. As Jim said, boring the head out for 850 diameter guides removes some of the support for the intake guides on the manifold side of the guide. Besides the risk of cracking, you will also probably find that the guide is now slightly off angle from the original seat, and will take more than simple lapping to make the seat concentric to the valve again. Then again, just replacing the guides with oversize 750 guides might also require more than just lapping, so you're stuck with doing a valve job whichever way you go.

On the other hand, I believe that Norton delivered some of the last 750 Commando heads with the 850 size guides, so maybe it would work. If it was me, I don't think I'd risk it, but it's your call.

Ken