The Stovebolt Site Forums The Shop Area Engine Shop 235 rebuild; cam timing problem

Hello all,

I am currently rebuilding a 56 235. I have installed crank/pistons, cam, lifters, head, and the valve train. I've now discovered a problem.

When I align the timing marks (touching) on the cam and crank gear, the engine is at TDC for #6, and not 1. I had the machine shop install the cam gear, but it and the crank gear are both keyed, so I don't think either of them they can be installed 180 degrees off.

Any ideas? Or should I just button it up and finish with them

But are you on the compression or the exhaust stroke? Another rotation of the crank brings you to where?
watch the valves open and close and imagine the fuel flow path and the firing order and remember there are 2 strokes of the piston to each combustion cycle.

I believe I am at the top of number 1, between intake and exhaust. The exhaust valve just closed, intake is just starting to open. And counting from front to back, #7 and 9 are all the way open.

then it sounds to me like your just starting to take your next breath/intake in and you just finished your exhale so your when #1 comes back to TDC you'll be in firing mode yes?

With the 235 having a correct crankshaft, #1 and #6 will always be at the same position at the same time.
At TDC one will be commencing the power stroke the other will be commencing the intake stroke---1 revolution later the positions will be alternated.
I am not sure we can count on all engines to have the #1 cylinder at TC starting power stroke when the TIMING MARKS ARE ALIGNED.
Some that comes to mind are the Y-Block F--- and the 4 main bearing F--- OHV sixes.

thats the exhaust stroke, compression would be one more revolution, with both valves closed. however you need to be certain if you are at the top of No 1, not just "believe", you can't kind of guess on these matters, you could also check and see where the ball is on the flywheel. It won't be exactly crankshaft TDC, as the ball is for ignition timing, but it should be in the very very near neighborhood. If your gears are new, there is a possibility the keyways in your gears are cut incorrectly, a very, very slim possibility, like almost inconceivable, but that sort of thing has been known to happen.

I know the 1 and 6 pistons are at the top, I'm just not sure at which point in the stroke. Unfortunately, the engine is still on a stand, so no flywheel. And I took the rocker arms apart to clean them and the shaft, so they're probably way outta whack. I have the side cover off, so I can watch the actual lifters.

To confirm, TDC for the number 1 cylinder is when the number 6 cylinder exhaust valve is starting to open, correct?

yep

That's perfectly normal. With the timing marks aligned, #6 will be at TDC on the compression stroke, not #1. With #1 at TDC compression, the #6 INTAKE, not exhaust, is just beginning to open. The exhaust has just closed, and the intake is opening. It's called the "split overlap" position on a racing cam because both valves are open quite some distance. Just time the distributor to fire the correct cylinder and everything will work properly. A Chevy V-8 also aligns the timing marks halfway around the firing order from #1, which is also cylinder #6. In that case, it's:
1-8-4-3
6-5-7-2

Your 235 fires:
1-5-3
6-2-4

In both cases, the cam timing marks align with the #6 cylinder on compression. The first time I discovered that, I spent half a day trying to time it differently. The one that really confuses people is a Ford 9-N tractor. It fires 1-2-4-3 instead of 1-3-4-2.
Jerry

Thanks a ton Jerry, it sounds like everything is fine. I appreciate you taking the time to answer, for some reason I always understand the way you write your answers.

Vince

My headache is a '60 vintage 235 that I'm attempting to replace the babbit beater in my '48 3800. I'd almost bet my next SS check that I'm on # 1 firing position ( #6 intake is starting to move when rocked) That said, I have the BB lined up with the pointer. Looks like some kind soul replaced the cam gear so I'm OK with that. I'm a little puzzled by the marks on the crank gear. Rather than a clear dot on one tooth there are two marks lined up on one tooth. They almost look like peen marks but to clean to be a shade tree job. At this position my gears are roughly 180 out, both marks are about the 10 o'clock position. Should I roll the cam mesh the marks?
The gear marks don't show too well in my photos but if you can expand it you'll get the idea. I also included the flywhl. marking cw and ccw of the BB.

Your picture of the timing gears is not focused well enough to see the timing dot(s) on the crankshaft gear. Roll the crank 360 degrees and see whether or not the dots align- - - -it appears that the cam is one tooth out of time, but the details are not clear enough to be sure. Normally, the timing dots align, with one dot on a male tooth, and either one or possibly two dots on a female tooth. Most stovebolt cams have virtually no overlap at the end of the exhaust stroke- - - -the exhaust will be closed all the way before the intake starts to move.
Jerry

This is how the 235 should look:
Keep in mind it's the aluminum gear vs fiber. Same marks.

It appears Jerry is 100% correct. With a straightedge across the gears it looks like they are 1 maybe 2 teeth off. Also, thanks for the great pics of your gear set. I'll clean mine up then mark the dots with a sharpie for a clearer picture. Thank you both, gentlemen.

I rolled the crank 360 and lined up w/ the flywhl mark. Here's what I get. Looks like the dots will index w/ a few more degrees of rotation. # 1 intake is beginning to open, #6 both closed. I failed to mention in my previous posts that we are looking at my 216 flywheel on this 235. Thanks again for your help.

Since you have this all apart and out of the truck, consider pulling the camshaft and installing an aluminum cam gear. Melling makes one and they're not expensive. The fiber ones are for passenger car use and are quieter BUT when put to truck duty, as in a 3800, eventually the fiber gear will shed teeth and the cam will stop turning. Aside from having to get towed off the road, the crank counter weights will hit some cam lobes and you will be out a camshaft. I speak from experience as this happened to me with a completely rebuilt 235 using a fiber gear. The engine had about 20,000 miles on it when this happened. 100 miles from home, I had a load of furniture on and it was most inconvenient.

Thanks, all good advice. I know the fiber ones are a weak spot. Back in the day, I was able to pick up a '64 Corvair convertible for 25 bucks because the fiber cam gear went south and previous owner had enough.

Put a straightedge between the center holes of the cam and crankshaft, and turn the crank until the dots align. It looks like the cam is timed properly, but I agree completely about using an aluminum gear, NOT fiber! There might be a U/C mark on the flywheel that will align with the pointer when the #1 and #6 pistons are both at TDC.
Jerry

Jerry is right about lining the pointer up to the mark between UC on the flywheel. The BB is NOT top dead center. It is 5 degrees before TDC on GMC engines and I expect it is similar in Chevys.

That U/C mark can also be used to set the cold clearance on six valves at a time during engine assembly. Adjust six valves (forgot which ones), turn the engine 360 degrees so the U/C mark aligns again, and set the other six. Some service manuals outline that procedure and indicate which valves to adjust. Recheck the hot clearance after a break-in run.
Jerry

Thanks to everyone for the help. BTW, consider me convinced. A new gear might be cheaper than a tow bill and motel.