Honda Twins banner

1 - 10 of 10 Posts

·
Registered
Joined
·
58 Posts
Discussion Starter #1 (Edited)
Today I went on with the re-assembly of the top-end. With the homemade piston supports and neighbor's help, we were able to thread the pistons relatively easily into the cylinders, because the chamfer on the lower end of the cylinder liner greatly facilitates this: https://abload.de/img/img_8877lpjko.jpg

Now my question, I hope this is not stupid, but somehow I do not understand it and surely someone can explain it.
The following information is provided in the workshop manual regarding the timing setting camshaft:
Align the crankshaft and camshaft so the crankshaft mark T is at the pointer when the horizontal marks on the cam sprocket are in parallel to the upper surface of the cylinder. This is logical to me. But the shop manual also mentions that the spark advancer knock pin has to face upwards.

Now my problem: Finally, we have turned the engine on the flywheel. After one revolution you can see the following, at the TDC position, the marks on the camshaft gear are horizontal again, BUT the knock pin at the ignition side is now facing down.
Thinking about it, it seemed logical to me, for four-stroke engines is the camshaft speed exactly half as fast as the crankshaft, so its obvious.

Why is it then pointed out in the manual? It can only be facing up or down (because of the sprocket screwed to the camshaft), and after one turn of the crankshaft the pin is at the oppsite side, because the camshaft has made only half a turn.
That's how it looks:
https://abload.de/img/img_88897mjeo.jpg
https://abload.de/img/img_8890t8kn8.jpg
After one turn of the crankshaft it looks like this (the feather on the NW stump is now below, you can not see it because of shadows):
https://abload.de/img/img_889264kik.jpg

What is the meaning, or where is my error of thinking / ignorance?
I would not like to build the engine into the frame and later find a timing error that could have been avoided.
Any help appreciated, I'm a little confused now.

Thanks & Greetings
Klaus
 

·
66Sprint
Joined
·
26,135 Posts
I don't understand your confusion....You are correct.....
The cam to crank timing is originally set on the TDC index, pin up, sprocket lines parallel to top cover gasket surface as you stated...
Requiring TWO full crankshaft revolutions to (four strokes) to return the camshaft to this setting position (which is the TDC nearest the end of the exhaust stroke) IS both correct and normal as the camshaft rotates at half the crankshaft's rotational speed......
It is simply Honda standard practice to assemble with pin up....
 

·
Registered
Joined
·
58 Posts
Discussion Starter #3
Thanks 66 sprint, i got it sorted, it does not matter if the knock pin is facing up or down, but I did it the way the left cylinder fires on compression stroke when the knock pin faces upwards. Will put it back in the frame next week. And hopefully the blow by is gone...
 

·
Registered
Joined
·
58 Posts
Discussion Starter #4 (Edited)
Very frustrated now. Test runs (not in the frame, oil filled in, but no carbs or mufflers) just by cranking the engine with the electric starter showed 120 psi and 95 psi, this is not much better than before the overhaul. Do the rings build up more pressure after a break in period, or did the motor workshop do bad work?
 

·
Super Moderator
Joined
·
9,784 Posts
There shouldn't be that large of an imbalance between the cylinders if properly rebuilt. Rings will seat and things should get better, but starting out after a rebuild with one cylinder having that much lower compression readings isn't good. If you're confident those readings are accurate, I'd be suspicious of what was done
 

·
66Sprint
Joined
·
26,135 Posts
Since you assembled the engine, there are several questions I MUST ask......

1...Are you sure the pistons were assembled into the correct cylinders? (Each piston SHOULD have been fitted to a specific cylinder and so marked)

2...Are the rings installed in the correct order, and with the correct side up?

3...Did you verify ring fit to the cylinder and end gaps before installing on pistons?

4...Are the valves correctly adjusted for "lash" clearance?

5...Was the engine rotated until the compression gauge reached and held at the peak reading?

6...Did you verify valve seal before installing head? (Acetone test)
 

·
Registered
Joined
·
58 Posts
Discussion Starter #7
Hi 66 sprint, here are my answers:
1-yes, the pistons were marked left and right, plus the man in the workshop inserted the inner c-clip at each piston
2- yes, the rings were put on the pistons by the workshop, I checked with magnifying glasses and it was correct
3- no, I assumed that would have done by the workshop, I did not remove a single ring from the pistons. Of course I set them to 120° scheme.
4- yes,I did that and doublechecked it twice
5- yes of course
6- yes, except i usesd fuel instead of acetone, the fuel was at the same level after 2 hours

I will phone the workshop today and keep you informed...

Thank you both
Klaus
 

·
Registered
Joined
·
58 Posts
Discussion Starter #8
I talked to the workshop. They say, after a run-in of about 250km the compression has to rise significantly, it is not unusual to have low pressure values even after re-boring. The difference between the cylinders results of different amount of oil I may have used during assembly... and the pistons will grow more than the cylinders when the engine builts up heat. Although they told me that i can sent pistons and cylinders back for measurement checking. I do not know clearly what the best option is for me now...
 

·
Registered
Joined
·
2,660 Posts
I reckon that you are worrying too much.

Finish fitting the engine and run it for a while, before doing another compression test.

I'd be much more concerned with confirmation that oil is reaching the top end of the motor.
 

·
Registered
Joined
·
58 Posts
Discussion Starter #10
Hello Richard,
your contribution lets me hope for better...
The oil flow to the head is perfect, i checked it at the right rear stud. after removing the nut, there was oil coming immediatly when the engine is cranked via the electric starter.
Still not decided what to do. I have a friend in a big machine building company, he is able to measure the parts very precise. On the other hand, I am not really happy to disassemble the engine again... although this is good practice :)
 
1 - 10 of 10 Posts
Top