Just joined forum. Good place for info for sure.
Have a question about timing on my 98 2.5.
Installing rebuilt head on my i4 2.5, when timing marks are all aligned
and #1 piston is at top dead center, TDC, the exhaust valve is pushed
down fully opened. Isn't TDC supposed to be at compression and all valves closed?
Everything's lined up perfectly as far as timing marks are concerned.. Thanks for any help. Mike

 

Welcome to the forum


#1 Piston at TDC is a function of the Crank
Cam controls the Valves which set Compression or exhaust stroke for #1

You will notice that the Cam gear is TWICE as large as the Crank gear, i.e. if crank gear has 10 teeth then Cam gear will have 20 teeth, crank 15 teeth-cam 30 teeth, crank 20 teeth-cam 40 teeth, ect............

This is because it is a 4-"stroke" engine, piston must complete 4 "strokes" to complete 1 full cycle(1 full rotation of the cam)
4 "strokes" requires 2 full rotations of the crank

So there is a 2 to 1 ratio between crank shaft and cam shaft

4 cylinder engines will have 2 matched sets of pistons to balance the engine, V6 3 matched sets, V8 4 matched sets

So on the 2.5l 4 cyl engine #1 and #4 are a matched set, #2 and #3 are the other matched set.
So #1 and #4 will BOTH be at TDC at the same time, but because of cam rotation one will be on compression stroke and the other on exhaust stroke.
If you rotate CRANK one full turn then #1 and #4 will be at TDC again but compression stroke and exhaust stroke will be reversed, so the Matched cylinders alternate back and forth.

So if you set up timing marks and then rotate the crank 1 full turn #1 WILL now be on exhaust stroke, as it should be, rotate crank 1 full turn again and #1 will be back on compression stroke, both valves closed

 

Thanks Ron for your reply. I was not very specific. The issue I am having is when the #1, and #4, piston are at TDC, and I align my cam gear diamond with the diamond on the timing belt cover, the exhaust valve is fully opened. I suppose that is fine if that is the way the timing is lined up with the Ford engine but from past experience TDC was at compression. Thanks.
Mike

 

Something is wrong

The Triangle mark should be used for Cam gear, Diamond mark for AUX gear
They should match the mark on the engine, triangle for cam(about 5:00) and diamond for AUX(12:00)

Cam triangle mark seen here: http://www.therangerstation.com/foru...pictureid=2966

AUX diamond seen here: http://www.therangerstation.com/Maga..._belt/tb-4.jpg