Valve clearances with non-stock cams on L series engines

[AliK]

I want to start a debate - a friendly one please! I say that as with many tech topics people have very strongly held views based on personal experiences / sacred texts(!!) and I'm interested in hearing them all, without a potential virtual punch up!

Having adjusted my clearances a couple of Ikea now on my new engine, I notice that the stock quoted clearances for L28 in Haynes / FSM still sound a little tappety. A number of other owners at Goodwood pointed that out too.

Also I am finding that on my new cam, the lifting bits are shiny but the base circle bits are still totally virginal - i.e. I don't think they are connecting AT ALL with the rockers. Is this how it should be!? I was under the impression there should be some small wear on the base also.

BTW when adjusted and fully tightened, not being an expert, I always use feelers that are .001 over and under to double check it's within +/- a thou. I adjusted on hot and cold engine - while some strongly believe that cold setting is a waste of time, I wanted to play safe and check both scenarios.

I have read that too tight a clearance can increase wear on cam/rockers + burn exhaust valves; and too loose can potentially damage valves / seats due to the valves slamming into seats rather than gently "coming off the ramp".

So what is the wise counsel on this based on your experiences / reading? Does one stick with stock clearances regardless of the cam or get tighter on higher lift cams!?

Also does oil viscosity have an effect on this? Tighter for thinner oils looser for the thick ones!?

 

[Rob Gaskin]

"Having adjusted my clearances a couple of Ikea "

If you have a clearance how can the cam wipe the rocker when not lifting? So I don't think it should be shiny.

Exhaust valves need to have a decent clearance so that they can cool when closed for the designated period.

 

[atomman]

When you built the engine was the wipe pattern checked ? this needs to be done to make sure the right thickness shims are used and the pattern is in the middle of the rocker ,

 

[Peato40]

I agree with Atomman, wipe patterns need to be checked and different thickness lash pads used in order to gain the correct valve geometry first, especially if head is skimmed and cam tower shims used, also if a high lift cam is installed. After I did my wipe pattern checks it was apparent that thicker lash pads were needed, so I purchased then from Schneider racing cams, along with larger retaining cups and a set of performance springs. Although my cam is Kent cam and not Schneider I did cross reference my cam with the closest Schneider offer and used those clearances.

If you click on "More Information" under the cam in question it will suggest what the lash clearances should be.

http://schneidercams.com/dastsunL6_NA.aspx

 

[AliK]

Thanks guys

Rob, Ikea = autocorrect for "times"!! I did think the same in that if there is a clearance the bottom should be clearing it but I also assumed there must be some "bounce" in the rockers so if that is correct logic would dictate there should be "some" wear on the bottom of the lobe when it touches.

I didn't build the engine. The builder did indeed check the wipe pattern and ordered the right sized lash pads, I know this as we had a good long conversation about it at the time when I was learning about lash pads etc.

I too have a Kent cam, thanks for the link, interesting to see the specs of the equivalent Schneider ones. The clearances aren't wildly different.

 

[jonbills]

Where are you measuring the clearance Ali? Between rocker and valve or rocker and cam?

 

[AliK]

Rocker and cam chief. Ala this diagram ...

Is it even possible to do between rocker and valve?

 

[Huw]

Hi Ali

Do you know what cam you have? Clearance should be in the specs for it.
Oh and on the topic of measuring and adjusting clearance when hot or cold, always adjust when cold. You can’t get an accurate measurement on a cooling engine. That’s what I was taught in college!

 

[jonbills]

Rocker and cam chief. Ala this diagram ...

Is it even possible to do between rocker and valve?

What's that pic from? I do mine between rocker and lash pad, much quieter that way. Maybe I'm wrong.

 

[Rob Gaskin]

What's that pic from? I do mine between rocker and lash pad, much quieter that way. Maybe I'm wrong.

Never heard of that method Jon.

On my old trackday car I was told to set the tappets 1 thou larger (not smaller Ali).

 

[AliK]

Thanks again chaps.

All I know is I have a 270 degree Kent fast road cam with .46" lift.

Interesting way to do it Jon - not seen that method on my internet travels. The pic is from the How to Rebuild your Datsun Engine book.

Rob the 1 thou here and there post tightening check is mainly to prove I'm not far out - I don't think it's possible for me to be accurate to a thou by feel. I'm sure those who have done it for 40 years have a better feel for these things than I would.

Huw - I totally agree with the logic of what you are saying on cold setting. The counter argument I read was the engine mainly operates when warm so the clearance there is more important. But as you say, unless you have a garage at 90degrees air temp how will you get a consistent setting!?

 

[Farmer42]

.......Also I am finding that on my new cam, the lifting bits are shiny but the base circle bits are still totally virginal - i.e. I don't think they are connecting AT ALL with the rockers. Is this how it should be!? I was under the impression there should be some small wear on the base also.

As far as I am aware, there should be minimal wear on the base of the cam as there shouldn't be any contact due to the fact that you have the clearance (albeit very very small). The base is where the valve should be closed so the cam shouldn't be touching. As the cam rotates it gradually increases contact with the rocker to open the valve gradually - a sort of wiping motion until reaching the tip (fully open) then gradually releasing again to close the valve. The higher up the side of the cam the contact happens, the louder the tapping sound as the cam is hitting the rocker instead of wiping it gradually. Lash pads affect the amount of adjustment that would be necessary. A high lift cam only affect the amount the valve opens fully letting more in on the inlet and more exhaust out.

Daft question but have you set the right clearances on the right valves i.e. not got exhaust and inlet the wrong way round? I have to admit I did exactly that when I first set my clearances after rebuilding my engine and it sounded like yours when I first ran it. The tolerances are very small so it will run. It won't sound like a lump hammer hitting it but it will be noisy and make the engine run lumpy.

I am with Huw in that clearances should nearly always be done when cold on engines where they are set when motionless otherwise the contraction due to cooling will always affect the clearance.

 

[AliK]

Thanks Paul. Yes clearances checked thrice to be shure to be shure to be shure !!

So here's the question: given that hot to cold settings on an L28 vary by 0.05mm say from 0.30 to 0.25, to allow for heat expansion, why not set it as close as possible, say 0.1mm to allow for the smoothest possible cam action but theoretically still not touching.

Would that open the valve too much and cause interference issues? I'm not sure if 0.1mm at cam / rocker translates to 0.1 at rocker / lash pad or if it has a multiplier effect.

Like timing, the more I get into this the more I realise how important it is to get it right.

 

[Rob Gaskin]

Ali, cams will be designed for the tappet clearance given (lift/duration) and for practical reasons (lack of maintenance).

Have you read about desomodromic valve operation.

 

[AliK]

Ali, cams will be designed for the tappet clearance given (lift/duration) and for practical reasons (lack of maintenance).

Have you read about desomodromic valve operation.

No Rob I haven't but will be now!!