Valve guide clearances

[benymazz]

Getting ready to do valve guide replacement on my "road-use" cylinder head. I know the workshop manual gives a spec for the stem-to-guide clearance, and my impression based on my forum-browsing is that the clearance should be set as near to the tight end of that spec as possible but I want to verify/ask around: when you had your guides replaced, what was the number you gave to your machinist? The spec that I have from my book is:

Intake: 0.0003"-0.0020"
Exhaust: 0.0003"-0.0025"

I will be replacing the existing with colisbro valve guides.

As always any and all counsel is appreciated.

-Ben

P.S. What is the expected mileage from a set of colisbro guides? Everyone seems to say 50,000 miles is the magic number but I have heard accounts of having no symptoms of worn guides (i.e., blue smoke) at 50k.

[nmauduit]

Are you replacing the guides yourself? if a machine shop does it, they should be able able to check if you need oversize guides or not, insert them properly using temperature difference as an aid, and ream the colsibro to the proper tighteness (valve guide clearance would be dependent on material, here colsibro may differ from the original manual)...

[benymazz]

Are you replacing the guides yourself? if a machine shop does it, they should be able able to check if you need oversize guides or not, insert them properly using temperature difference as an aid, and ream the colsibro to the proper tighteness (valve guide clearance would be dependent on material, here colsibro may differ from the original manual)...

I am having a machine shop do it.

[Craven]

Couple of points, use only a machine shop that is fully familiar with Colisbro bronze don?t just accept we know all about bronze valve guides.
Colisbro has a temperature coefficient very similar to aluminium, advantage being less of an interference fit to head, with respect to your question very low clearance to valve stem.
To take full advantage of this low clearance the actual valve stem should be measured and the guide honed to suit, usually a ball hone is used as surface finish is important for maximum life.
On the inlet valve guide only clearance needed is enough to allow the valve to drop freely under its own weight.
On the exhaust valve guide, 0.0002 - 0.0015.
These are clearances I used after quite an extensive search on Colisbro bronze,
FWIW

[rgh0]

Getting ready to do valve guide replacement on my "road-use" cylinder head. I know the workshop manual gives a spec for the stem-to-guide clearance, and my impression based on my forum-browsing is that the clearance should be set as near to the tight end of that spec as possible but I want to verify/ask around: when you had your guides replaced, what was the number you gave to your machinist? The spec that I have from my book is:

Intake: 0.0003"-0.0020"
Exhaust: 0.0003"-0.0025"

I will be replacing the existing with colisbro valve guides.

As always any and all counsel is appreciated.

-Ben

P.S. What is the expected mileage from a set of colisbro guides? Everyone seems to say 50,000 miles is the magic number but I have heard accounts of having no symptoms of worn guides (i.e., blue smoke) at 50k.

I aim at the bottom end of the range for the inlets i.e. around .0005 to .0008 and mid range for the exhaust .0026 to .0028 (edited to correct). The inlet guide clearance is the critical one to minimise oil burning. With Cosilbro guides and modern synthetic oils you can expect well over 50,000 miles. You dont want the exhaust to tight as when the engines working hard and the exhaust valve is getting hot its stem expands more than the guide does unlike the inlet where the guide and stem run at a similar and relatively low temperature

cheers
Rohan

[jabingb]

Rohan, the manual has significantly higher values for the exhaust, .0025" to .0030". I'm not sure what the material (colsilbro?) of the new guides I installed is, asking the vendor presently.

[rgh0]

Rohan, the manual has significantly higher values for the exhaust, .0025" to .0030". I'm not sure what the material (colsilbro?) of the new guides I installed is, asking the vendor presently.

Good question and not sure why I wrote the above . I use midrange for exhausts which is 0.0026 to 0.0028 inch. My only excuse is there is to much good red wine here in Australia to drink while posting

I have edited my post to correct

cheers
Rohan

[benymazz]

Rohan, the manual has significantly higher values for the exhaust, .0025" to .0030". I'm not sure what the material (colsilbro?) of the new guides I installed is, asking the vendor presently.

Good question and not sure why I wrote the above . I use midrange for exhausts which is 0.0026 to 0.0028 inch. My only excuse is there is to much good red wine here in Australia to drink while posting

I have edited my post to correct

cheers
Rohan

I think you may have used the midrange of my values in my original post. My service manual that was purchased with the car (the early type, same manual as for the S1) states .0005” to .0025” for the exhaust. It would be typical Lotus to change tolerances/clearances over the years but this seems like a large change

FWIW, I had my machinist set the clearance to .0005” to .001” intake and .0014” to .0018” exhaust. I’ve had no problems so far. The engine is stock but I have run it very hard on track days with zero issues. 7,000 road miles and 200 track miles now since the rebuild.

-Ben

[2cams70]

FWIW, I had my machinist set the clearance to .0005” to .001” intake and .0014” to .0018” exhaust. I’ve had no problems so far. The engine is stock but I have run it very hard on track days with zero issues. 7,000 road miles and 200 track miles now since the rebuild.

I wouldn't necessarily be confident that your machinist used those figures unless you measured them afterwards yourself to confirm. Most machinists (the best ones at least) know from experience what does and doesn't work. When confronted by customers that think they know everything they will just nod politely rather than start an argument because after all they are the ones likely to be suffering reputational damage and have to bear the cost when things go wrong. If you find it necessary to have to explain to a machinist details such as this my strong suggestion is to go and find another machinist.

From the Ford manual (both inlet and exhaust):
Valve guide inside diameter: 0.3113 to 0.3125" (7.907 to 7.938mm)
Valve stem diameter: 0.310 to 0.311" (7.874 to 7.899mm)
Valve stem to guide clearance: 0.0025 to 0.003" (0.640 to 0.760mm)

That's with standard valves and cast iron guides of course

[elanman999]

As all the Ford/Lotus guide clearance data is with cast iron guides it's not valid for Colsibro. It was Mick Miller that first put me onto the ability to lower the clearance used for the guides. Ben's numbers are the same as I use, although I would not let the inlets go out to .001". I would also say that if using QED valves then you need to stone the markings off the top of the valve otherwise it scores the guide when installing the valve.
NB you may need more clearance for racing.
YMMV.
Cheers.
John.

[2cams70]

What do QED themselves recommend for Colisbro guides seeing as they are the supplier?

Less clearance may not necessarily be better because you can't get as much oil in there. Just because it's seen to work under a particular circumstance doesn't necessarily mean better all round. I'm not saying it's wrong to reduce the clearance but I'm not convinced of the justification or evidence yet that it's better. Also my previous comments about what you told the machinist to set your clearances to and what they actually did may be different. You must measure them yourself after machining to be sure.