120 Mercruiser engine rebuild...timing... Help!

[nh3]

I'm a bit confused. When I line up the timing marks on the crank and cam gear my pistons are at the top of the cylinder bore. After bolting on the timing gear cover and crank pulley and aligning the timing marks on the pulley with the tab on the cover my #1 piston is in the center of the cylinder bore (along with the other 3 pistons) Shouldn't this be at the top of the bore with both valves closed? If so where is my mistake. Couldn't sleep last night thinking about this!<br /><br />Thanks<br />nh3

 

[DJ]

Re: 120 Mercruiser engine rebuild...timing... Help!

nh3,<br /><br />Yes, the #1 piston should be at the top of the bore with both valves closed (TDC).<br /><br />I suspect that your camshaft gear can be installed 180 degrees out of phase. Not all timing marks are designed to face each other. Some may be designed so both crank and cam point upwards-as an example.<br /><br />Your moving the the crank to TDC on the timing marks (ignition) should not have moved the piston that much.<br /><br />The ONLY TWO pistons that should be at the top of the bore are #'s 1 and 4 at TDC. Possibly you lined up the timing marks when #4 was at TDC. # 4 would be on a different phase of the four strokes than # 1 and visa versa. # 4 would be on the "power" stroke with the valves closed if # 1 was on the compression stroke.<br /><br />See if your timing gear can be installed 180 degrees from where it is now. Also check the crank gear.<br /><br />I hope I didn't confuse you.

 

[nh3]

Re: 120 Mercruiser engine rebuild...timing... Help!

The shop manual is telling me to line up the crank and cam timing marks which gives me #1 & #4 at TDC. It's after throwing a couple of bolts on to hold the timing cover and putting the crank pulley on is where things don't fall into play. With the ignition timing pointing to 0 TDC neither #4 or #1 are at TDC. In fact all the pistons are lined up even in mid stroke in the bores with the pulley and pointer at 0 TDC. I'm heading back to the cellar to investigate. Hell I've rebuilt $100K refrigeration compressors and this GM 4 cylinder timing is killing me. I did not disassemble the motor so that makes it a little more difficult. I can't us my memory on this one. How bout if I get #1 @ TDC and renotch the pulley? I hate to do that cuz that's not the proper way. I just want to reassemble the thing the right way. If you can think of anything else please respond and thanks.<br /><br />nh3

 

[nh3]

Re: 120 Mercruiser engine rebuild...timing... Help!

I rotated the cam 180* and that has put me at 0 tdc. Thanks<br />nh3