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Discussion Starter #1
So I’m in the middle of a SBC 406 top end upgrade with the engine block still in the car consisting of new AFR heads and a new retro-roller camshaft. The first retro-roller camshaft once installed and indexed would not clear the number two (I believe) connecting rod. It would lock the engine at 310 degrees clockwise rotation and again at 315 degrees counter clockwise rotation from TDC. I spoke with my cam grinder and returned the first camshaft and today received the second camshaft and measured it before trying installation/indexing again, but the base circle on this one measures .008” larger than the first, so after discussing the situation with my cam grinder, he told me the rods/rod bolts need to be nubbed or clearanced for the camshaft.

So I’m looking for guidance, pointers, tips, etc. on this process, as I’ve not done this before. I’m planning on yanking the block this weekend and doing what needs to be done. I guess the one positive thing out of all this is I’ll be able to install a new rear main seal while I have the oil pan off. It has been leaking a bit since I purchased the car. I’ll also be able to install and index the cam, install and torque the cylinder heads and get the timing cover and water pump installed on the block before I put it back in the car saving my back hours of being in a less than comfortable position.

So how about it, what can the Team Camaro community share to help me in my latest endeavor?


Thanks in advance and beers! :beers:
 

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What is the base circle on the current cam? My first thought is send it back and get a smaller base circle, but grinding the rods is no big deal either.

The clearance will also depend on installed position of the cam and duration, as well as lift, so the new cam if different may be ok. Only thing to do is check though.

What rods? Stock style bolted? If so all you need to do is round the edge of the rod that faces the cam, at the shoulder where the cap of the bolt sits (does that make sense?). The best way to do it is hit it very lightly with something like a fine paper sanding wheel on a grinder, you don't need to take much and you don't want to heat the rod and bolt up doing it, I wouldn't use a bench grinder or a stone type wheel.
You only want to take enough to achieve a minimum .060" clearance, since it's tight in there a nice trick for checking is mock the rod up and turn the motor over, sliding a thick paperclip between the cam and rod at it's nearest point. A thick paperclip is about .060".

It's not a big deal though, just take your time and check closely. I wouldn't even consider doing this with the motor in the car, good call on yanking it, and good luck!
 

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I (my machinist) had to grind rods two and 6 (if I remember correctly) when I built the motor last spring/summer...I had a retro-roller Com Cams--XR282HR...

Glad he was looking over my shoulder for the build process...he said it is very common, and not a big deal at all...
 

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Discussion Starter #4
What is the base circle on the current cam? My first thought is send it back and get a smaller base circle, but grinding the rods is no big deal either.

The clearance will also depend on installed position of the cam and duration, as well as lift, so the new cam if different may be ok. Only thing to do is check though.

What rods? Stock style bolted? If so all you need to do is round the edge of the rod that faces the cam, at the shoulder where the cap of the bolt sits (does that make sense?). The best way to do it is hit it very lightly with something like a fine paper sanding wheel on a grinder, you don't need to take much and you don't want to heat the rod and bolt up doing it, I wouldn't use a bench grinder or a stone type wheel.
You only want to take enough to achieve a minimum .060" clearance, since it's tight in there a nice trick for checking is mock the rod up and turn the motor over, sliding a thick paperclip between the cam and rod at it's nearest point. A thick paperclip is about .060".

It's not a big deal though, just take your time and check closely. I wouldn't even consider doing this with the motor in the car, good call on yanking it, and good luck!
Sean, the base circle on the original flat tappet I removed was 1.165" which had of course both a lower lift but higher duration. The first retro roller which my camshaft grinder sent and told me was a small base circle was 1.085" with lift of .547" intake and .510" exhaust. Advertised duration was 292 intake and 294 exhaust (233 and 235 @ .050). That was the cam that locked up between 310 & 315 degrees. This latest camshaft has a base circle of 1.093" with lift of .539" intake (.008" less than the first roller cam and the base circle of this is .008" larger than the first roller cam so the two cancel each other out if you know what I mean) and .510" exhaust. Advertised duration is 290 intake and 292 exhaust (233 and 234 @ .050). Installation of this second camshaft is the same as the first at 104 degrees.

Never had the lower end apart and bought the car with this engine so not sure on the rods or rod bolts though fairly confident due to recently identifying the pistons from the top (United Engine & Machine believes from photos I sent them and measurements I took that they are older Keith Black KB126 pistons) that the rods are 5.7" rods. Once I get the block out and on my engine stand and get the oil pan off I'll know more, hopefully no more surprises that create more challenges. I understand about not heating things up and plan to take the same amount off each rod/rod bolt as I end up taking off of the offending rod/rod bolt to keep the balance as close as possible to what it is now. I actually have a supply of plastic shim stock that I can use to make a custom long length feeler gauge to check my clearance as I go, but that's a neat idea on a paperclip.

I did for a short time consider attempting to do it in the car but quickly came to the conclusion that yanking the block makes the most sense. Thanks for your reply, information, and well wishes, Scott
 

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Discussion Starter #5
I (my machinist) had to grind rods two and 6 (if I remember correctly) when I built the motor last spring/summer...I had a retro-roller Com Cams--XR282HR...

Glad he was looking over my shoulder for the build process...he said it is very common, and not a big deal at all...
From searches I've done on this topic it seems to be much more necessary than not and I now wish I'd just planned on doing it from the beginning. If I had, I would have been done with this project weeks ago. What's that saying, "experience, it's what you get when you don't get what you planned on" or something like that... :eek:
 

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I had a couple that needed a small grind on my 383 - I let my machinist handle the chore. He used plumber's solder - rolled the motor over and the solder gets smashed between the lobe and the rod then measures the thin spot on the solder to determine what the clearance is.

As Sean said, bolt & nut stock style rods tend to have more clearance issues than capscrew style. My capscrew rods still needed a grind.

If I ever do a cam swap (unlikely) I will pull the motor and do it on a stand - would be leery of what the clearance might be w/o doing so. A small thing to be aware of with the longer stroke.

While you have it on the stand also check rod clearance at the pan rails.
 

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If you're not into disassembly, grinding on rods, re-balancing, then reassembly, then try one of these two cams:

For a single pattern (AFR heads) this will likely work:

http://www.summitracing.com/parts/hrs-110305-08/overview/make/chevrolet

For dual pattern:

http://www.summitracing.com/parts/cca-08-503-8/overview/make/chevrolet

Both will need to be special ordered on a small-base circle. Both are great cams for the street and both will pull 13-14 inches of vacuum in a 400, good for power brakes.

Both have good duration but moderate lifts which will clear everything, or at least they have in the 3-4 engines (400 SBC's) I've built with 5.7" Scat rods.

Good luck!
 

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Like was mentioned above, your cam to rod clearance will change by advancing or retarding the cam. Since you already plan to pull the motor and disassemble to clearance rods, I would do this first(if it were me)

Since the motor will not turn over where the cam is now, I assume you do not know where it is installed in relation to ICL. I would pull all piston rod assemblies except #1. Verify TDC and Degree the camshaft to verify it is installed where your cam card says it should be. Hopefully the cam will clear #1 rod to allow you to do this. If the cam is already where it needs to be then you know you will need to clearance some rod bolts.

If you need to advance or retard to get the suggested ICL, do so and start re installing the rod/pistons and check clearances one at a time. Also if it were me, I would be hesitant to not re-balance. I know it's more money but it's worth the peace of mind.

just my .02

Mike
 

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Sean, the base circle on the original flat tappet I removed was 1.165" which had of course both a lower lift but higher duration. The first retro roller which my camshaft grinder sent and told me was a small base circle was 1.085" with lift of .547" intake and .510" exhaust. Advertised duration was 292 intake and 294 exhaust (233 and 235 @ .050). That was the cam that locked up between 310 & 315 degrees. This latest camshaft has a base circle of 1.093" with lift of .539" intake (.008" less than the first roller cam and the base circle of this is .008" larger than the first roller cam so the two cancel each other out if you know what I mean) and .510" exhaust. Advertised duration is 290 intake and 292 exhaust (233 and 234 @ .050). Installation of this second camshaft is the same as the first at 104 degrees.

Never had the lower end apart and bought the car with this engine so not sure on the rods or rod bolts though fairly confident due to recently identifying the pistons from the top (United Engine & Machine believes from photos I sent them and measurements I took that they are older Keith Black KB126 pistons) that the rods are 5.7" rods. Once I get the block out and on my engine stand and get the oil pan off I'll know more, hopefully no more surprises that create more challenges. I understand about not heating things up and plan to take the same amount off each rod/rod bolt as I end up taking off of the offending rod/rod bolt to keep the balance as close as possible to what it is now. I actually have a supply of plastic shim stock that I can use to make a custom long length feeler gauge to check my clearance as I go, but that's a neat idea on a paperclip.

I did for a short time consider attempting to do it in the car but quickly came to the conclusion that yanking the block makes the most sense. Thanks for your reply, information, and well wishes, Scott
It sounds like you kind of planned to tear into it anyway, but if not I would consider getting a cam on a .900 base circle. I'm not a big fan of small base circles and big spring pressure, but yours would be fine.

It may be a good opportunity to go through the motor and check things out too, depends on what you want to do.

As for rebalancing, I know it seems sketchy but you will not notice a difference on the balance, it's going to change a fractional amount at the most, I know lots of guys that have done this after balancing and never known anyone needing to rebalance or having an issue.
Granted the best approach is buy everything, mock up, and grind the rods and balance, but it won't be a big deal.

Also as mentioned mine were 2 and 6, and I believe I had two others that were close also.
 

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When the machine shop assembled my last engine they ground part of the top of the ARP rod bolt and a touch of the rod journal itself to clear the cam. I drive the car hard and had a problem requiring the engine to be pulled down. The rod that had been ground had cracked down about 1/2" on the journal......so I wouldn't grind rods unless you absolutely had to. I now use 6" Eagle rods intended to be used with the stroker and had a new cam ground (more power this time). Everything works well and no clearance problems.
 

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Make sure you build in the needed rod/cam clearance (.050"?). Just because the engine will turn over by hand doesn't mean she'll live. Once you get some RPM, thermal expansion, and a timing chain that loosens up, all bets are off.
 

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It sounds like you kind of planned to tear into it anyway, but if not I would consider getting a cam on a .900 base circle. I'm not a big fan of small base circles and big spring pressure, but yours would be fine.

It may be a good opportunity to go through the motor and check things out too, depends on what you want to do.

As for rebalancing, I know it seems sketchy but you will not notice a difference on the balance, it's going to change a fractional amount at the most, I know lots of guys that have done this after balancing and never known anyone needing to rebalance or having an issue.
Granted the best approach is buy everything, mock up, and grind the rods and balance, but it won't be a big deal.

Also as mentioned mine were 2 and 6, and I believe I had two others that were close also.
I guess I am one of the anal guys, then. I grind the ones that need it and then grind the others so that the big ends all weigh the same. Then I know the bobweight will be the same from journal to journal.

It may not be a huge deal, but I always prefer the peace of mind, :beers:
 
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