Lotus Elan

Valve guide clearances

PostPost by: benymazz » Fri Sep 27, 2019 7:20 pm

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.
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PostPost by: nmauduit » Fri Sep 27, 2019 7:37 pm

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)...
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PostPost by: benymazz » Fri Sep 27, 2019 7:50 pm

nmauduit wrote: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.
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PostPost by: Craven » Sat Sep 28, 2019 12:22 am

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,
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PostPost by: rgh0 » Sat Sep 28, 2019 12:53 am

benymazz wrote: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 .001 to .0015. 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

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