SOHC & cam running clearance, base circles

Competition engines and ancillaries - general discussion
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Andy128
Posts: 10
Joined: March 24th, 2008, 4:07 am

SOHC & cam running clearance, base circles

Post by Andy128 »

Hello Everyone,

First things first, what a really cool forum , loads of really useful information.
Here goes with my first post.

I have a Fiat 128 with a 1500 Strada engine, modified head and 2x 40mm DCNF carbs, I recently removed the head to replace a blown headgasket (which wasnt blown). While I had the head off I thought Id check the cam shim clearances and found there is a range between 0.05 and 0.15 (2-6 thou) not much!!. I dont know the grind on the cam shaft so I have sent it away to be checked out.

Can anyone please explain to me why I need to know the grind of the camshaft in order to workout the clearance, I would have thought the clearance would be a constant only really allowing for the expansion in lenght of the valve as it gets hot or am I miss informed.

Andy Durber
Guy Croft
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Location: Bedford, UK
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Re: SOHC Cam Clearance

Post by Guy Croft »

Andy, hi

good question, readily explained.

Refer to photos - the dial type indicator (dti) shows you what happens as the cam rotates. It's necessary to have a running clearance between the cam and bucket/rocker on units without hydraulic lifters, to allow for expansion. Naturally if there's no clearance the valve won't close. How much there should be depends on the size and slope of the cam flank (the bit that does the useful work of opening the valve), and the ramp that joins it to the non-contact region of the base circle. The base circle as the name suggests is the round bit at the bottom of the cam where you measure the running clearance.

Opening and closing ramps give the valve progressive lift onto the flank (where the accelerative rate is high) and lowering onto the seat during the closing phase. If you don't have decent ramps the cam is noisy and wears fast. if the runinng clearance is too wide the follower skips right from the base circle region straight onto the flank causing rapid wear of the upper cam nose section (the bit below peak lift) and if it's too tight the base circle and ramps get scuffed and worn. Both phenomena can, of course, cause severe damage to valve and seat and springs/collets too..

The diameter of the base circle relative to the overall height of the cam determines the cam lift and it's often the case depending on the layout of the cam bearing housings for competition cams and (ugh..regrinds) to have a smaller base circle than standard. Often but not always. The running clearance for a given design will be determined by the position of the ramps, so yes, it varies from cam to cam, but there tends to be a lot of commonality between cams. Most have somehting between 0.008" and 0.020" and exhaust clearances are often a little wider than inlets.

Hope that makes it a bit clearer,

GC
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base circle.jpg
base circle.jpg (114.28 KiB) Viewed 5158 times
approaching ramp.jpg
approaching ramp.jpg (437.5 KiB) Viewed 5159 times
on ramp.jpg
on ramp.jpg (429.6 KiB) Viewed 5155 times
coming off ramp onto opening flank.jpg
coming off ramp onto opening flank.jpg (428.54 KiB) Viewed 5147 times
early lift phase.jpg
early lift phase.jpg (430.66 KiB) Viewed 5139 times
peak lift.jpg
peak lift.jpg (427.15 KiB) Viewed 5138 times
Phases of cam lift.GIF
Phases of cam lift.GIF (29.28 KiB) Viewed 5132 times
Andy128
Posts: 10
Joined: March 24th, 2008, 4:07 am

Re: SOHC Cam Clearance

Post by Andy128 »

Thanks Guy,

I understand now, cristal clear.

thank you, Andy D
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