Refitting cylinder head - valve timing problems

If you need technical information or help with your BOSCH ignition squarecase Ducati 900SS - Darmah engine - post your FAQs, comments & questions here.
Post Reply
PJDucati
Cucciolo - the Lil Pup
Posts: 7
Joined: Tue Nov 19, 2013 4:46 am

Refitting cylinder head - valve timing problems

Post by PJDucati » Wed Nov 20, 2013 9:11 am

My issue is with my 1982 MHR. I have noticed burning oil mainly on start up, so have assuming it is oil draining down the valve guides after shutdown.

I have removed the front cylinder head to investigate and found the silicone exhaust valve stem seal loose on the valve stem.(I found that the engine has 88mm pistons and 42mm inlet valves and 38mm exhaust valves)

I rotated the engine until the marks on the two bevels were aligned and then removed the front head. I also checked the vertical cylinder and noted that the dots also aligned. ( Is that correct that a correctly timed engine should have BOTH cylinders with the dot marks not only correctly meshed but with the dots in line with the cylinder ie all the gear teeth with dots are all correctly meshed together and in line?
For some reason, I have come to refit the front cylinder head and the drive end of the bevel shaft is vertical but with the dots correctly positioned in the 'together' position on the rear cylinder the mating drive ( from the idler gear) is in the horizontal position. ( I had not turned the crank.)

Am I making a fundamental mistake? Thanks for any advice.

Phil.

Aussiess
750 GT
Posts: 163
Joined: Sun Oct 31, 2004 6:51 pm
Location: Blue mtns, NSW, OZ

Re: Refitting cylinder head - valve timing problems

Post by Aussiess » Wed Nov 20, 2013 12:59 pm

Could also be rings. My 82 MHR had a similar problem. Was poorly fitted rings from a "rebuild"
Not sure on timing questions, good luck with the "fix".
Regards,
Grant
79 900SS , 82 MHR and 78 NCR Replica

PJDucati
Cucciolo - the Lil Pup
Posts: 7
Joined: Tue Nov 19, 2013 4:46 am

Re: Refitting cylinder head - valve timing problems

Post by PJDucati » Wed Nov 20, 2013 2:54 pm

Hi Grant,
Thanks for the reply. I have refitted the piston and barrel for the front cylinder and the piston rings looked in good condition and the bore in good condition too, if a little polished.
I looked at the new book by Ian Fallon today ( Ducati bevel twins - Authenticity and resoration guide) and in there on Page210 he shows the correct position of the coupling with the "flat" of the coupling in the near horizontal position. However with the dots lined up correctly on the bevels in the cylinder head, the "flat" on the shaft is in the near vertical position. I am baffled how they fit together!
Any advice appreciated! Thanks, Phil.

User avatar
abmartin
SSD900 Darmah
Posts: 358
Joined: Tue Apr 10, 2007 4:08 pm
Location: New Brunswick, Canada

Re: Refitting cylinder head - valve timing problems

Post by abmartin » Wed Nov 20, 2013 3:46 pm

I'm having a hard time understanding what you are doing. If you were only taking off the horizontal cylinder head you should have aligned the dots, removed the head and then realigned the dots when replacing the head. There was no need to worry about the vertical cylinder. Having said that the cylinder head bevel gears only fit one way on the cams. By lining up the dots on (I think - it may be the horizontal head, it has been a while since I did this) the vertical cylinder head with the piston at top dead centre on the compression stroke, with the dots lined up on the horizontal cylinder head it should match up with the drive coupling and the timing will be correct. If you didn't rotate the motor while the cylinder head was off all you need to do is ensure the dots are lined up on the front cylinder provided that were that way when you took the head off.

To check that the valve timing is correct you can rotate the motor with the cylinder heads in place. At top dead centre on the compression stroke of each cylinder the valves should be fully closed and the camshaft lobes for each cylinder should be at their lowest point. If you get the valve timing seriously wrong the motor won't rotate as one of the valves will hit a piston.

Bruce
1979 900SS
Fredericton, NB
Canada

wdietz186
888
Posts: 652
Joined: Sun Sep 10, 2006 4:40 pm

Re: Refitting cylinder head - valve timing problems

Post by wdietz186 » Wed Nov 20, 2013 6:49 pm

If you haven't turned the crank when the horiz. head was off and the dots on the vert. cyl. are still aligned, the front head should slot right on. Keep in mind the intake valve on the front cyl. will be open slightly with the dots in alignment. Also note that the dots will only line up every fourth revolution of the bevel shaft when turning it by hand. Look in the manuals section of this site, all of the square case [860 thru Mille] are timed in the same. If you really want to be sure you can pull the right side[timing] cover and check the dots on the idler and lower bevels too. They are pretty hard to see though unless you remove the bevel support. Yet another thing to remember is the crank turns clockwise[viewed from the left side of the bike] which is opposite from the belt models and most everything else out there.

PJDucati
Cucciolo - the Lil Pup
Posts: 7
Joined: Tue Nov 19, 2013 4:46 am

Re: Refitting cylinder head - valve timing problems

Post by PJDucati » Sun Nov 24, 2013 10:20 am

Hi. Thanks for all the helpful comments put forward. I think I have resolved the problem and it was a mistake by myself. When I was removing the front cylinder head I removed the housing at the bottom bevel, thinking that this was necessary to release the chrome drive shaft tube. ( ie didn't realise the tube was just retained by O rings.) In doing so, I have inadvertently allowed the timing to slip. Fortunately, a previous owner had put a couple of paint dots for the correct alignment, which could be viewed through the lower bevel housing aperture. With the flats on the drive shafts facing outwards from the engine, all the dots aligned and after refitting the cylinder head turned the engine (crankshaft) 6 times to confirm the gears still aligned.
On with the rear cylinder head....

Post Reply

Return to “Squarecase BOSCH Ignition Engines > SD, SSD Darmah, late 900 SS & 900 GTS”