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Old 08-28-2010, 07:58 PM
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Connecting Rod Cap Hitting Cam

I'm building a 406 SBC with 5.7" 350 rods and Comp Cams XE262H-10 hydraulic flat-tappet cam. The rods were purchase as part of a Strong Arm 406 kit from Speed-O-Motive, with the edges of the rod bolt caps ground for additional clearance. I have ample clearance with the blocks's oil pan rail, but am not clearing the cam. Ive read in several magazines and tech forums of others successfully building the 406 SBC, so what am I missing? Is there someone out there whos actually built this engine for use on the streets and overcome this obstacle? I've read that using a small base circle cam will solve this interference problem, but what exactly is a small base circle? Are they readily available, or are they custom ground? What are the cam's dimensions/specifications ? Is a small base circle cam available as a hydraulic flat tappet or only as a roller? Is it the proven solution? Any examples of small base circle cams with specs similar to the XE262H? Sorry for so many questions but I'm lost and losing faith. Help me Obi-Wan Kenobi, you're my only hope!

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Old 08-28-2010, 08:25 PM
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The 5.7" rod is a bit taller at the bolt head than a stock 400 5.565" rod is, with a longer bolt, and frequently has the problem you describe. If the edge of the rod near the bolt head and the edge of the bolt head won't clear the cam you have you can grind it for clearance, cutting that edge of the rod away along with a slight amount from the bolt head. If this has been done and you still don't have enough clearance you then have two choices.

One is to use stroker clearanced capscew bolt type rods that use a capscrew instead of a bolt and nut, the body of the rod is threaded for the bolt(Scat, Eagle, etc I-beam or H-beam), since there is no bolt head in the way on these types they can be clearanced farther.

The other option is the reduced base circle camshaft, which basically means the whole lobe is just proportioned smaller. The heel(or no lift portion) of the lobe is ground on a smaller radius, allowing the whole lobe to be smaller but still provide the same lift and duration profile. this makes the lobe clear the rod bolt head area of the rod. Many common cam grinds are available as a reduced base circle already on the shelf, some others need to be custom ground, but every cam manufacturer offers them, you just have to know enough to ask for one. They are available in every cam type, flat tappet, hydro roller, and solid roller

If you search "383 stroker" builds here I'm sure you will find plenty of pictures, this question comes up fairly often. Info will be the same for you because you are working with the same 3.75" stroke crank into an SBC using 5.7" rods.
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Old 08-28-2010, 08:29 PM
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Im running a similar combo but with a comp 292 roller and i had the same problem but it was the rod BOLTS that were hitting most likely the same issue you are having i had to tear it back apart and clearance I BELIEVE 1-3-5-7 rod bolts and you need to have .030 clearence and it is a pita to check but should be done
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Old 08-29-2010, 02:32 PM
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Quote:
Originally Posted by SeniorRodder
I'm building a 406 SBC with 5.7" 350 rods and Comp Cams XE262H-10 hydraulic flat-tappet cam. The rods were purchase as part of a Strong Arm 406 kit from Speed-O-Motive, with the edges of the rod bolt caps ground for additional clearance. I have ample clearance with the blocks's oil pan rail, but am not clearing the cam. I�ve read in several magazines and tech forums of others successfully building the 406 SBC, so what am I missing? Is there someone out there who�s actually built this engine for use on the streets and overcome this obstacle? I've read that using a small base circle cam will solve this interference problem, but what exactly is a small base circle? Are they readily available, or are they custom ground? What are the cam's dimensions/specifications ? Is a small base circle cam available as a hydraulic flat tappet or only as a roller? Is it the proven solution? Any examples of small base circle cams with specs similar to the XE262H? Sorry for so many questions but I'm lost and losing faith. Help me Obi-Wan Kenobi, you're my only hope!



Do you have your timing chain on with crank indexed to cam?? Reason I ask is many times if you just rotate the cam to "check' you will have interference.
As for small Base cams, comp cams offers on many grinds with little or no extra cost. I run a 6" rod in my 406 with a small base from comp Cams.
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Old 08-29-2010, 02:41 PM
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Quote:
Originally Posted by 406 bug
Do you have your timing chain on with crank indexed to cam?? Reason I ask is many times if you just rotate the cam to "check' you will have interference.
As for small Base cams, comp cams offers on many grinds with little or no extra cost. I run a 6" rod in my 406 with a small base from comp Cams.
Very true, nice catch. Crank, timing set and cam must be correctly installed to make checks.
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Old 08-29-2010, 03:01 PM
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Are these the stock chevy rods or aftermarket rods? I would have used aftermarket stroker rods because they give extra clearance.
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Old 08-29-2010, 06:21 PM
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I think that you have very serious issues if you are hitting the cam on a small cam like the 262h. I would make sure you have the correct cam, rods, block, crank etc. If this is all correct then I would try to find out if the block has had the mains align bored. If there has been a lot of damage to the main caps or the main caps had been replaced with the wrong aftermarket pieces then they may have been bored excessively. This would move the crank closer to the cam. Was there excessive slack or play in a new timing chain? This would indicate the same problem.
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Old 09-01-2010, 08:13 PM
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Thanks to all who came to my aid. I was at my wit's end. 409 Bug was right. It was the timing set. I had a CompCams timing chain/gear with the 3-keyway crank sprocket. The 3 keyways where not marked, and I installed it in what I thought was the straight-up position. WRONG. I orderded another gear set from Cloyes (these keyways were marked), installed it today, and now the enging turns just perfectly. What a relief! Thanks again. I really appreciate you sharing your insights to possible fixes.
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