Guides - Ream, then hone, or just ream?
 

My ream is cutting my guides nice and straight, to exactly .354". Valves feel great in them, intakes have about .001" clearance (from memory) I feel like I could leave them the way they are.

Bentley says ream guides to .354"-.355". Should I hone them to .355" or leave well enough alone?

If I leave them the clearance is tighter, I have a reamed finish and less clearance.

If I hone them I will have a honed finish and more clearance.

What to do?

Thanks in advance!!

I just did a set of Ferrari 2v 308 heads and set them at .001 clearance and maybe .0015 exhaust and it's just fine. 8mm stems.

Some just hone to size others ream then hone. The final step is to hone for a nice finish on the guide.


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Jonny, what would the purpose of reaming or honing be? Don't you want them as tight as possible, but not so tight that they sieze from thermal expansion?

Dan

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When you install them they will be undersize and slightly smaller where there are squeezed by the head. I have an inside mic that makes it easy and fast to make numerous repeatable measurements and it's surprising how they vary along the length of the guide after installation.They were consistent and straight before install.

Obviously you want there to be some clearance. The manual says to ream to .354" to .355" so that indicates .001-.002" on the intake side, and slightly more on the exhaust side due to the smaller/tapered size of the exhaust valve.

After reaming the guide is straight and parallel. I'd say the honing would be optional but the reaming wouldn't be . I can't see how you could get proper results by honing only as you'd be retaining the shape of the guide and not selectively removing material to straighten it out like you do with the ream.

I'm going to hone a test guide today and see how it goes. If I can hone it for a better finish for oil retention in a few seconds without increasing the clearance too much I will do it to the heads.

Thanks, Craig! I think honing only might be a problem, at least in my case (see post above). Maybe if the press fit is on the lower side (mine were all in the .0025"-.0027" range so relatively tight) there would be less distortion of the guide after install and it would be OK?

With my press fit values I wouldn't say the install was a piece of cake - using an air hammer and a purpose built mandrel they went in consistently but took some effort, even with the heads heated and the guides chilled.

.0025" equates to .0635mm which is on the high side of what wayne's book recommends but on the low side of the Bentley manual.

Looking forward to doing the seats today. I have a LOT of hours into these heads!

Jonny thanks for your reply. I had not considered the ID changing (even more so along the length!) after pressing into the heads. Any possibility that indicates the head fit is too tight? I agree you want the bore straight and consistent. I don't have any idea what honing would give you. There are no rings to seal and it seems honing can only put more oil (insulation) between the heat transfer surfaces.

Dan

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Just a follow up - reaming alone was not enough, some honing was required..... once you ream things are awfully close but I was taking a few tenths off with the hone to make things fit properly.

At some point the measuring stops and "feel" takes over.... guides are reamed and honed, onwards and upwards as they say!

Jonny , seems like you are thinking about the process and doing a great job . I also ike to first ream followed by honing for the correct clearance . Think about this , I did not put valve stem seals on the exhaust valves , they need lubrication and oil is not getting sucked in like the intake valves .

Richey - I have always wondered about the value of exhaust valve stem seals. The guy who showed me how to assemble these motors left them off, though these were all race motors. So I have left them off.

Between adhesion and capillary action, I suppose just a little oil could get into the exhaust port? But that's not like oil in the intake at all.