Questions about heating cylinder head to pull out the valve guides

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DennisMo

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Hello Everyone,

As this is my first cylinder head rebuild can you advise me on the following to remove and replace the valve guides in my 1973 850?
I have the right tool to do this.

1) There manual says to heat head to 150 to 200 degrees Centigrade. Should I heat to 200 degrees or 175 degrees?

2) Should I put head into a cold oven and turn temperature to the 200 degrees so it heats slowly? Or is it ok to put head into a preheated oven?

3) How long does it take after the oven is 200 degrees for the head to absorb the heat and reach that temperature? ie how long to leave it in the oven?

Thanks Dennis
 
DennisMo said:
Hello Everyone,

As this is my first cylinder head rebuild can you advise me on the following to remove and replace the valve guides in my 1973 850?
I have the right tool to do this.

1) There manual says to heat head to 150 to 200 degrees Centigrade. Should I heat to 200 degrees or 175 degrees?

2) Should I put head into a cold oven and turn temperature to the 200 degrees so it heats slowly? Or is it ok to put head into a preheated oven?

3) How long does it take after the oven is 200 degrees for the head to absorb the heat and reach that temperature? ie how long to leave it in the oven?

Thanks Dennis
Click to expand...
Are you positive you need to change them? It's not a job to do "just because"!

Get a laser thermometer. When the aluminum around the guide is at least 150C and no more than 200C you're fine. Lots of ways to do it. Cold or preheated makes little difference - think about how the engine runs! If it doesn't move, get it hotter rather than hit it harder. Of course, you're supposed to pull them out and in - I never have that tool, but I have the right sized drift so I drift them out and in. However you do it - be sure you start absolutely straight.
 
I won't know until I take out the springs and put a new valve in to check for free play. But the previous owner said he thinks they may need replacing
Thanks Greg
 
It's usually the Norton exhaust guides that wear out first . Watched my machinist put in (supplied) new Black Diamond valves to wiggle check and he said replace both of the exh. guides . The new intake valves ( B.D. ) went in to test with no wiggle room . Intake guide seals fail after many years ( 50 0r so ? ) , replace with J.S. Teflon type . When I went back to the shop there were hundreds of engines being worked on and the huge shop was filled with smoke . Exotic car engines mostly .
 
The valve guide removal section of the workshop manual should be deleted. Heating the head to remove the guides and then drifting them out will ruin the hole in the head. You will remove some little lumps of alloy along with the guide. This is because the hard carbon adhering to the guide will pick up and gall dragging the alloy with it. Even if you blast clean the guide this will happen. The way to ensure that the guide comes out clean is to machine most of the guide away with a sized counterbore, leaving a thin shell of cast iron that then will pull or drift out without galling.
If you do drift the guides out, you WILL be fitting oversize guides because std size guides will just drop in as you will have opened the holes up. Also the holes won't be straight and round so its a trip to an engine reconditioners to get them reamed to suit.
Removing the guides by machining the inside of the guide away leaves the hole in the head intact so refitting std size guides is easy (shrink them in pipe freeze spray) and fit them to the heated head. A big plus is that the valve seats will only need lapping in, no seat cutting required.
Why do you think they sell oversize guides? It is because this 'hole ruining' takes place!
I sometimes think the workshop manual section on valve guide removal was included to create business for engine reconditioners and repeat sales for oversize valve guides.
 
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Guides should be machined out properly, if you can't do that yourself take it to a shop

Questions about heating cylinder head to pull out the valve guides
 
Thanks very much to all of you for your advice. If it needs new guides, it will go to a local shop!
Dennis
 
it looked as though I was going to need to have my inlet guides replaced but after reading all of the advice on offer I found an engine reconditioners who installed the K Line inserts, turned out to be an inexpensive job, had the valve seats recut, valves faced and head skimmed at the same time.
 
I don't claim to be a diva machinist, but I do have some talent, a half decent mill, lots of milling cutters, and a large adjustable angle plate. I found setting up the head to mill the guides out a royal pain in the ass. My mill has some limitations on vertical capacity. Reaching the tool deep enough to complete the boring operation was a challenge. Getting the angle correct enough so I felt confident in leaving a thin guide without compromising the bore was not easy. Halfway through I gave up and resorted to a drift and heat.
I wonder how many shops you took your head to would say they would mill it out for you? And actually do it once you left the shop? Pounding them out or pulling them out would be 10 times faster. Are you really going to pay them 10 times as much to machine them out?
Unless you are doing it in your own shop, with all the right tools and skills, the milling approach sounds kind of unlikely to me.
But then maybe I am just bitter and twisted because I resorted to the caveman approach.
 
Hi all, just to back up the wise words from the experts : my combat head at highly experienced motorsport machine shop for guide change. Discussed workshop manual- 150-175 degrees, correct tool etc . same comments as Dobba et al…Machined out in a heartbeat 👍
 
Stephen Hill said:
I don't claim to be a diva machinist, but I do have some talent, a half decent mill, lots of milling cutters, and a large adjustable angle plate. I found setting up the head to mill the guides out a royal pain in the ass. My mill has some limitations on vertical capacity. Reaching the tool deep enough to complete the boring operation was a challenge. Getting the angle correct enough so I felt confident in leaving a thin guide without compromising the bore was not easy. Halfway through I gave up and resorted to a drift and heat.
I wonder how many shops you took your head to would say they would mill it out for you? And actually do it once you left the shop? Pounding them out or pulling them out would be 10 times faster. Are you really going to pay them 10 times as much to machine them out?
Unless you are doing it in your own shop, with all the right tools and skills, the milling approach sounds kind of unlikely to me.
But then maybe I am just bitter and twisted because I resorted to the caveman approach.
Click to expand...
Who said anything about using a milling machine? I said to machine them out with a sized counter bore. I make these from silver steel, turned up in a few minutes. filed the cutting teeth into the metal then hardened in oil. I then use a pistol drill, by hand, to power the counterbore to machine the cast iron guide away, leaving a thin shell of guide to collapse inwards and come out clean. the photo shows the tool and the removed guides.
Ironically Steve you have all the tools nessesary to do the job this way!
 

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