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Cam Oil Clearance

Practical Engine Building

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Discussion and questions related to the course Practical Engine Building

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I picked up a set of used cams, and had the journals re-polished. Now I'm measuring to make sure it's all in spec.

All the journals are within spec, except for one is undersized by about 0.02mm.

Theses cams run directly in the head without bearings. So I can't just use a thicker bearing there.

But, it looks like the clearance range is wide enough, that if I get the cam line-bored to the minimum allowed size. That will set the clearance to within spec, even though the journal is technically out of spec.

Detailed numbers if it helps:

Allowed journal tolerance: 24.96 to 24.98mm

Allowed cam bore tolerance: 25.015 to 25.040mm

Allowed oil clearance: 0.035 to 0.080mm

All journals measured 24.96mm. Except one measured 24.94mm.

So, if I get the head line bored to exactly 25.015mm. That will put the clearance for the bad journal to 0.075 and the rest at 0.055. So they'll all be 'in spec'.

Is this a good idea that will work fine? Or is this destined for failure?

Hi Matt can you give me some detail on the engine and what its being used for and components like rb25 drift use, bs272 cams with 90 pounds spring pressure etc this will help decide what clearance you can give it,

You don't seem to have measured what the cam' tunnel diameters actually are.

My personal thoughts are to run it as is, unless there's some major wear issues, as the cam' jourmals are actually relatively lightly loaded and it's always in the same direction.

TBH, I'd be more concerned that you're using the matched followers, as they do bed in to the lobes running against them, or at least new ones with plenty of anti-scuff paste/lube.

@Ross, it's a 2.3 Ford Duratec, for rally use. I'm putting in Cosworth 240hp cams, and Crower valve springs.

@Gord, this motor uses graded followers to set valve lash, so it will be getting new followers by default when I get to that point.

Unfortunately I don't have any way to measure the tunnel diameter. The bore is too large use a hole gauge, and too small for a bore gauge. And even if those tools did fit, there's an extra complication, the bottom edge of the bore is thinned out between the valve buckets, and the cam cap on top is extra wide and with an oil groove all through the center. Which means most tools would be held at an angle just to make contact at the top and bottom so that's an impossible measurement. So an inside micrometer wouldn't be able to reach. I'm sure there's a proper tool out there that can measure it, but I don't have it or even know what it is.

My plan was to measure the cams directly, but then use plastigage to measure the bores. Or to just tell my machinist what size to set it at, and then just trust her.

If you are able to plastigage to check the installed clearance I would do that first, if the cam spins freely and the clearance is within .02 I would most likely run it. this is a tricky one because your unable to actually measure the clearance of both parts and I’m also guessing the machinist would have the same issue. Might pay to ask Them if they are able to do this job as that is a very small amount of material to take out, set up in a line boring bar is very difficult with small diameter holes like this