[SRL790]

Well 2000 miles after the last valve job and once more I'm down on compression so off comes the head again (Can you put these things on with velcro). 

I had noticed a small mark by the exhaust valve seat the last time apart but didn't investigate further.  This time I bead blasted it and it is indeed a crack that I believe is allowing the valve seat to move and causing the valve to burn up.  If I had to guess I would say this was a void in the original casting.

Good news is I have a new (used) correct head on its way from Hitchcock's in UK.

Andy

[Ragmas]

Does the new head have dual spark as well?

[Ice]

The other threaded hole is for the de compressor.

 Lucky are the  AVL owners,,,no worries about tiny bits falling in the cylinder when changing de comp cables.

[Arizoni]

That does look like a casting flaw.

Hopefully Hitchcock did a penetrant inspection on your new (used) cylinder head so it doesn't have flaws in it too. 

[SRL790]

It will get a thorough going over before being installed.  I have new valves, guides and rockers coming with it.

Anyone got any sage words of advice for installing valve guides on the Enfield head, other than clean thoroughly first, heat the head, drift/pull the old one out, drift pull the new one in.

I see that NField Gear has tool set on for doing it that looks like some kind of puller and a drift.  Doesn't look like anything that I can't make in a few minutes.  Anybody used one?

Andy

[Arizoni]

I haven't done this but it seems that an installation tool could be made with a deep socket that would rest against the cylinder head while allowing the body of the valve guide to slip up inside it.

Along with this one would need a length of threaded 1/4-20 (or 28) steel stock** about 6 inches long, a couple of  1/4-20 (or 28) hex nuts  and another socket that would fit inside the valve cavity but would also bear on the flange on the guide.

To use this the threaded rod would be inserted thru the valve guide.
The smaller socket would be located on the valve end of the rod and secured with a hex nut.

The guide would be placed in position with the threaded rod extending thru the cylinder head.
The deep socket would be placed over the protruding threaded rod on the spring side of the head with the other hex nut on top.

Lightly oil the thread and then turn the hex nut to tighten it.  It will compress the valve guide into the head.

Heating the head will help but even heating it to 170 degrees F (which is too hot to handle) will only reduce the interference by about .001 inch.

As a side note, this lubricated 1/4-20 thread will produce about 2000 pounds of force if the torque on the nut wrench is 6 foot/pounds.  That essentially makes this a 1 ton press.

** I'm assuming this will fit thru the inside of the valve guide.  If not, a #10-32 size should.
If a 5/16" bolt will fit thru the valve guide it will take a higher torque than the 1/4" bolt and will raise the compressive force to over 3000 pounds.

[SRL790]

I would rather pull them in and out than beat them like this guy.

http://youtu.be/GEg18pE0PHQ

I have also heard of machining off the part of the guide that protrudes into the port and also boring out most of its thickness before removal so as not to damage the head.

[Ice]

 Send it to a shop that specializes in the rebuilding of motorcycle or airplane heads would be my advice.

 IMHE and $$$, automotive machine shops, even ones specializing in VW and Corvairs, lack the understanding, the techniques and "touch" required to do motorcycle heads correctly.

[Ice]

Eg18pE0PHQ[/url]

I have also heard of machining off the part of the guide that protrudes into the port and also boring out most of its thickness before removal so as not to damage the head.

Correct.

 The valve seats are another part that is machined away too.

 Every removal /installation cycle increases the size of the heads valve seat recess or the heads valve guide bore.

They are never a "drop in" of "prefit" affair.

 Sizes are measured, expansion differences factored and dimensions are adjusted by machining to ensure the parts are proper fitting at running temperatures.

These are parts that head must shrink to fit and precisely so.

 Doing things at less than or greater than the proper temperature range at at other than the right "machined to" dimensions ruins things quickly.
 Guides/seats will not be snug at operating temperature or will be to tight at normal parked temperatures which can ruin a head casting .

[SRL790]

Well I got my new (used) head from UK today and first look over it seems to be in good shape.  Actually the mail man tried to deliver it yesterday but there was no-one here to sign for it.  Since I ordered it on the 18th and it got here on the 23rd I'd say that's pretty good service from the UK!

Anyway, the old guides were already removed so I guess now I need to clean it up and measure the holes and the new guides.  Anyone know how much of an interferance fit I should be looking for?

Andy

[SRL790]

OK.  Looking at my (1954) service manual it gives the valve guide OD as 0.6275"/0.6270" and the head hole ID as 0.626"/0.625", which would give an interferance of .001" - 0.0025".

It does not make any mention of heating the head for guide installation or removal but I still think that this would be wise.

Andy

[SRL790]

For reference about a 0.001" interference worked well.  The inlet had 0.002" and I couldn't get it all the way in.  Had to remove it and shave a little off it.  No problems second time around.

I started yesterday and got all the machining done and new guides inserted, valve seats cut and had the head assembled.

Started this morning with the head on the bench and rode it about 150 miles this afternoon.  I will probably retorque the head tomorrow and hopefully all should be well for a while.  One minor oil leak on the rocker box feed to deal with.

Some pics attached for you gearheads.

My home made guide insertion tool.
Cylinder head after bead blasting.
Skimming the rocker box joint.
Skimming the head surface.

To be continued.......

[SRL790]

I boiled the head in a turkey frier to heat it up before inserting the guides.  I added a healthy dose of purple power to the mix which gave the added bonus of cleaning out all the inner passages and also making sure that all the bead blasting media was removed.

Last picture is the head assembled back on the bike along with new valve springs, rockers, keepers, etc.

Andy

[SRL790]

If a 5/16" bolt will fit thru the valve guide it will take a higher torque than the 1/4" bolt and will raise the compressive force to over 3000 pounds.

FYI the valve stem is 11/32" so a 5/16" stud works well.

Andy

[Ice]

Well done Sir !


Out of curiosity, did you skim the head to raise compression or to tidy things up a bit ?