shim swap

DevonCbx · Post by **DevonCbx** » Fri Apr 09, 2010 7:17 pm

Several years ago I assembled my 79 CBX out of 3 CBX's. I learned alot during the restore. Though I really donnot know much about the engine internally. What I am wondering is why do shims need to sometimes be replaced? Is it to do with the cam lobes wearing or is it to do with the springs changing tension? How does the feeler gauge slide under the lobe while the lobe is pressing down on the shim? Or am I not looking at the procedure the right way? Is the feeler gauge slid under the pointy end of the lobe, or the rounded end?

Thanks

ericfreeman · Post by **ericfreeman** » Fri Apr 09, 2010 8:20 pm

You measure the clearance between the shim and the rounded part of the cam lobe where no valve lift occurs. Clearances can change over time, usually as the valve and seat wear, allowing the valve to sit deeper in the head. This reduces the valve/cam clearance and you don't notice it since it's not making any noise. Unlike when valves are too loose and they tick/click when running. Fortunately, once the clearances are set they usually stay in spec for a long time.

Eric

alimey4u2 · Post by **alimey4u2** » Fri Apr 16, 2010 5:58 am

To add to Erics excellent advice, the feeler should be a drag fit, not too tight & obviously not too loose... The "pointy bit" should be pointing away from the shim at 90 degrees. Not to the vertical as the valves are at an angle....

EMS · Post by **EMS** » Fri Apr 16, 2010 7:56 am

Shouldn't the cam lobe be as close as possible to the valve centerline pointing away from the valve in order to ensure you measure at the ground of the cam?
(Sorry fot the "shim under bucket" picture, don't mean to confuse anybody!)

broook · Post by **broook** » Fri Apr 16, 2010 8:31 am

alimey4u2 wrote:To add to Erics excellent advice, the feeler should be a drag fit, not too tight & obviously not too loose... The "pointy bit" should be pointing away from the shim at 90 degrees. Not to the vertical as the valves are at an angle....

Hate to jump in here Larry but shouldn`t that be 180deg like Mikes pic??
If not I have been doing it wrong for over 50 years.

alimey4u2 · Post by **alimey4u2** » Fri Apr 16, 2010 9:08 am

Broook you are correct in saying 180 deg rotation but if you measure the angle to the plane ( shim) it's 90 degrees.. Just a matter of wording I think.

broook · Post by **broook** » Fri Apr 16, 2010 9:17 am

OK Larry, gotchya.

alimey4u2 · Post by **alimey4u2** » Fri Apr 16, 2010 9:19 am

You had me thinking there Bob...

EMS · Post by **EMS** » Fri Apr 16, 2010 11:25 am

Cool, Larry.

I was thinking the same as Bob in terms of degrees.....I guess I should have gotten my protractor out before saying something

alimey4u2 · Post by **alimey4u2** » Fri Apr 16, 2010 12:02 pm

To help Duane understand, there are sometimes "quieting ramps" on the base of the cam profile. These gently release the valve on closure & also gently open the valves. The reason you measure with the cam lobe at 90 degrees to the shim is to remove their effect when doing your measurements...

And Mike, I got one back for all those times you've proven me to be wrong...

broook · Post by **broook** » Fri Apr 16, 2010 1:05 pm

alimey4u2 wrote:To help Duane understand, there are sometimes "quieting ramps" on the base of the cam profile. :

Ah yes. I remember back in the mid fifties Triumph used them on the T110. They stamped a special mark next to the engine # to indicate the fact.

alimey4u2 · Post by **alimey4u2** » Fri Apr 16, 2010 1:11 pm

I bet you remember E3134's too Bob...

broook · Post by **broook** » Fri Apr 16, 2010 9:15 pm

alimey4u2 wrote:I bet you remember E3134's too Bob...

Had them in my 61 Bonnie. :)
Sometimes its good to remember things from days gone bye but on the other hand it reminds you how old you are getting.