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Roller Rocker Arm 1.5 Valve Clearance Inquiry


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#1 KobukKobuk

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Posted 14 January 2022 - 11:54 AM

Hi. Everyone
 
I bought a 1.5 roller rocker arm from a minisport and I'm working on it now and I have a question. (1275 SPI engine)
Cam, shaft, head, etc. are all standard. What is the appropriate valve clearance when changing only the 1.5 roller rocker arm? 
Please advise. thank you.

 

 



#2 Spider

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Posted 14 January 2022 - 07:12 PM

When changing rocker arm ratios, I've always taken the figure for stock rockers, which are near enough 1.3, divided that in to the clearance recommended (unless the cam has been ground for 1.5's or what ever ratio you are using), then multiply that figure by 1.5, viz;-

std clearance = 0.012" / 1.3 = 0.00923

 

0.00923 x 1.5 = 0.0138". In this case, I'd set them at 0.014"



#3 MiniMadRacer

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Posted 15 January 2022 - 08:48 AM

I agree with Spider, the key here is to remeber you are not settign the clearance from the rocker to the valve, but actaully from the CAM follower to the CAM, hence why the setting changes at the feeler guage end to essentially set the same clearance at the CAM follower end



#4 sonscar

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Posted 15 January 2022 - 11:30 AM

Serious question,why do you need to alter the gap?Steve..

#5 nicklouse

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Posted 15 January 2022 - 11:44 AM

Serious question,why do you need to alter the gap?Steve..

As you have changed the arm ratio and as said above what you are actually setting is the clearance between the cam and the follower.

 

if you did not then the cam clearance to follower would be reduced. And may cause bind.



#6 Ethel

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Posted 15 January 2022 - 12:45 PM

just to add the same goes for the valve springs.

 

Their job is to keep  the follower riding on the cam, with all components staying in contact with their neighbours, when the valve is no longer being pushed open. Steeper cams & higher revs require stronger springs to give the faster acceleration of the valve train needed. Higher ratio rockers give more leverage to magnify the spring force on the pushrod & follower side.



#7 Spider

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Posted 15 January 2022 - 05:44 PM

Serious question,why do you need to alter the gap?Steve..

 

If you were to examine the profile of a Cam Lobe, you'll find it has a few different 'parts' to it one of which are Clearance Ramps. It's essential to take up any lash in the valve train on these Ramps, so that the valve train isn't hammered to death. We have clearance in there in the first instance to ensure that as parts grow from temperature, the Valve closes tight under Valve Spring Pressure and also (in these engines) to take all pressure off the Cam Follower to be sure we get some Oil on the Cam Follower Face.

 

<Edit: The clearance also gives a bit of dwell time with no pressure on the rocker again to get some oil in to the valve tip and on the rocket bush.>

 

xgBZ2X8.jpg


Edited by Spider, 15 January 2022 - 09:24 PM.


#8 sonscar

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Posted 15 January 2022 - 06:46 PM

Thanks for the enlightenment,too tight or loose and it opens on the wrong part of the cam?stays open too little or too much?or even holding the valve open?Steve..

#9 Spider

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Posted 15 January 2022 - 09:26 PM

Thanks for the enlightenment,too tight or loose and it opens on the wrong part of the cam?stays open too little or too much?or even holding the valve open?Steve..

 

Pretty much.

 

If you do degree in your cams, next time you do this, move around to the base circle, then keep going, you'll see there's a good many degrees of crank rotation with very little valve lift, then around the 15 - 20 thou mark, the valve lift takes off like a rocket. Similar on closing. It's a bit of an eye opener to actually see it and feel what the rocker is doing in those parts of the cam timing.






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