Cam Tower Shims - Lash Pad Compensation

Engine, Transmission and related drivetrain.
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D52E
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Cam Tower Shims - Lash Pad Compensation

Post by D52E » 10 Jun 2013 11:10

I had a head-gasket blow between #2 and #3 recently. The head appeared to be ok so I just put a new gasket on. There may be a possibility that I will have to re-surface the head if the gasket goes again. Due to the head already being machined extensively I may have to shim the cam towers to maintain the timing chain tension (I already slotted the guides and tensioner).

My question is regarding cam tower shims. I see that there are aftermarket shims in 0.020" thickness. I'm assuming that I will have to compensate the cam tower spacing with new lash pads in order to maintain the correct valve train geometry.

If I instal one 0.020" shim per tower - What will be the required lash pad thickness increase to maintain the same geometry?

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Re: Cam Tower Shims - Lash Pad Compensation

Post by 510rob » 10 Jun 2013 13:17

It seems reasonable that...

...if {(the rocker ratio is 1.5:1) and (you jack up the cam by 0.020")}, then (you will need 0.030" thicker lash pads).

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Sealik
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Re: Cam Tower Shims - Lash Pad Compensation

Post by Sealik » 10 Jun 2013 21:06

510rob wrote:It seems reasonable that...

...if {(the rocker ratio is 1.5:1) and (you jack up the cam by 0.020")}, then (you will need 0.030" thicker lash pads).

Correct.....assuming the wipe pattern was centered in the first place.... :)

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Re: Cam Tower Shims - Lash Pad Compensation

Post by 510rob » 10 Jun 2013 23:04

Sealik wrote:
510rob wrote:It seems reasonable that...

...if {(the rocker ratio is 1.5:1) and (you jack up the cam by 0.020")}, then (you will need 0.030" thicker lash pads).

Correct.....assuming the wipe pattern was centered in the first place.... :)
agreed! :D

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D52E
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Re: Cam Tower Shims - Lash Pad Compensation

Post by D52E » 11 Jun 2013 05:04

Thanks for the information. The wipe pattern is mostly centered on the pads but very slightly biased to the driver side of the pads. The valve train has been run this way for some time and cam lobe wear looks ok.

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Re: Cam Tower Shims - Lash Pad Compensation

Post by Three B's Racing » 11 Jun 2013 09:53

All great answers but in the end "Recheck your Geometry" for each valve. Things always look good on paper, then you build it and :shock:

Lou "Bent than, been there"
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Re: Cam Tower Shims - Lash Pad Compensation

Post by vgwagon » 14 Jun 2013 00:18

The cam tower shims are only required if you have the top of the head machined, which should happen anytime you warp a head, machining just the head surface could put your cam in a bind situation, so whatever comes off the top of the head should be replaced with the shims to keep the correct geometry.
Cam tower shims have nothing to do with timing chain tension.....that's the tensioners job.
it's funny how the haynes manual says to never remove the towers, but to correctly machine the head they should come off.
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James
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Re: Cam Tower Shims - Lash Pad Compensation

Post by James » 14 Jun 2013 04:37

The distance between sprockets is altered no matter what side of the head material is taken from. The geometry between the valves and cam will only be modified when machining the top. So what you are saying is the shims are only necessary for compensating for the valve train - not making up the difference as the sprockets get closer to each other (which will have an affect on cam timing - no matter what the tensioner is doing....).
Never have had necessity to use shims, just curious about the (valve) train of thought.....
Finished is better than perfect......

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Re: Cam Tower Shims - Lash Pad Compensation

Post by vgwagon » 14 Jun 2013 10:20

James wrote:The distance between sprockets is altered no matter what side of the head material is taken from. The geometry between the valves and cam will only be modified when machining the top. So what you are saying is the shims are only necessary for compensating for the valve train - not making up the difference as the sprockets get closer to each other (which will have an affect on cam timing - no matter what the tensioner is doing....).
Never have had necessity to use shims, just curious about the (valve) train of thought.....
Correct....sprocket timing gets compensated with the 3 different positions on the cam gear, chain tension gets compensated by the tensioner.
Common misconception, but once you sit and think about it it makes sense......if you only machined the head surface rocker geometry isn't touched.....if you added shims then the geometry would change.
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Re: Cam Tower Shims - Lash Pad Compensation

Post by D52E » 14 Jun 2013 11:25

My original intention was to try and restore some of the timing chain tension due to an excessively machined cylinder head. I have previously slotted the tensioner and guides to try and compensate but I believe more work is required since the piston on the hydraulic tensioner is still sticking out quite a bit. If I have to machine the cylinder head again it will only get worse. I understand that there is an aftermarket solution (Kameari twin idler) but it is somewhat cost prohibitive for me at the moment. I want to try accomplish the task with one or two shims under the cam towers and some new lash pads.

Cam timing is a secondary concern that I am not too worried about since I will have to degree the cam again anyway and using my 8 position NISMO sprocket.

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