Mako 928

I have the cams/heads back on the block and am about to put the timing belt on. The crank is at 45 BTDC. Can I put the cam gears also at 45 BTDC (3 teeth back)and then put the belt on or should I align the cam gears to TDC (the V on the cam gear align with the V on the backing plate) and then turn the crank to TDC and put the belt on?

Big Dave

When cams are installed using the factory settings, they're at the 0 TDC position and the "V" on the gears and back plates should line up. You'll need to rotate the crank from 45 BTDC (clockwise) to 0 TDC to match, then put on the belt.

I made a page showing how this process works....

http://www.928oc.org/928oc_michigan/tbelt10.html

mark kibort

then, why even use the 45BTDC setting to change the belt? I know if the cams are pushed and rotated via certain valve springs, they could contact the pistons, so why not just clamp them down upon the belt removal. Plus this same risk would happen if you have the cams in a resting position and try and rotate the crank 45 degrees forward to TDC. Ive always just put the cam pulleys right at the 3 tooth forward position matching the 45 degree mark on the crank. then, you are right on as 45 degrees crank, is 22.5 degrees cam, which is 3 teeth (7.5degree each). I think we kept scots car, which as a higher lift cam than euro stock, and i remember things kind of hitting, as i was rotating the cams to line up. I had to rotate the crank to reduce the resistance and get the cams in position.

I think the 45degree mark way is the way to go. whats the worst that can happen, you have to move a pulley one tooth or something? not a big deal.

imho

BY THE WAY BIG DAVE, that write up was great and saved me more than a few times during my cam change process. EXCELLENT!!!!

Mk

dprantl

I guess the only reason why the procedure is that way is that with the crank at 45 degrees you can spin the cams whichever way you please and nothing gets damaged.

Also, if you put it to TDC and locked the cams to take the belt off, then put a new belt on and let the cams go and needed to do some small adjustments, you would have to fight the valve springs for it.

Dan
'91 928GT S/C

Big Dave

Mark...thanks for the compliment.

I assume, perhaps incorrectly, that anyone that is tearing down parts of an engine parks the crank at 45 BTDC to eliminate the risk of piston/valve contact. When cams are installed using the factory gages, they're set at the O TDC reference point, with the "V"s lined up. Unless Mako is installing the cams some other way, he have to rotate each of the cams by hand (under spring pressure) to bring them to the 45 BTDC mark to match the crank. It just seems easier to move the crank 45 degrees instead, as the factory manuals recommend.

AO

At 45 you can spin the cams however you want without worry.

Edit: Dan's explanation is much better (and faster).

Big Dave

At 45, they'll be under spring pressure, so it's tricky to keep them parked at the precise spot while you loop the belt around everything. I don't recall there being any significant pressure to spin the cams after bolting them in at 0 TDC.

Just my $.02.

AO

I'll tell ya... that was a head scratcher for me. I did n't realize I was lining them up at 0, so I couldn't figure out why I was having trouble getting everything timed.

I must have read that page in the manual 20 times. I don't recall it saying to put the engine @ TDC or to rotate the cams. I ended up rotating the cams. Everything lined up, so I was happy, but there was a lot of followed by, with an eventual .

mark kibort

I used a little unconventional method. I use large vicegrips, as you dont want them coming off, as a wrench can. then, i bungie cord the ends against the way the spring pressure wants to move them. then, i loop around the driver pulley and pull tight and see how the marks line up. Then, the otherside is easy. pulling from the top side to keep the tension that has been kept by the driver pulley, i fit the belt on. piece of cake! the driver side vice grip can be pulled counter clockwise, to make sure you have max tension on the oil pump and ultimately, the crank gear. after that, its all off the waterpump and passenger pulley. good for the first time alignment and all while using the crank locked at 45. now, you can torque the crank bolt as well so, if you have any issues, you can still do it at TDC after the crank lock is removed and dont have to reinstall it to torque the crank bolt after moving to TDC belt assembly.

mk

Big Dave

Yes...much much easier than just moving the crank to 0 TDC

mark kibort

Hey!! you still have to put the belt on! I just thought that moving it to 0 is an extra step, as you have to re-set the flywheel lock, a step i dont need to do.
(unless im missing something)

your way sounds good. I probably have done it both ways, but i dont recall. (for awhile there, i didnt know about that 45 degree thing!)

mk

Mako 928

Big Dave
I have been using your write up along with the WSM and I believe I have the cams installed correctly. I used special tools 9226/3,9226/4 to align the cams before and after installing them in the heads. When the cam gears align to the "V" marks on the backing plates the "noses" on the cam shafts right in front of the chain point to the exhaust side as per WSM. On the right side head, cylinder #2 intake valves are just about to open fully and exhaust valves #4 are in the same position.Is this correct for the valves to be in this position with the "V" notches aligned to the backing plates? So here is where I am confused. When the cams are installed using the special tools and the noses facing up the cam gears are not lined up to the V notches.

Mrmerlin

when I do the Timing belts on the 32 valvers I mark the position of the cams at 45 degrees also lining up the crank.at 45 deg.
This 45 deg position makes it easier to remove the belt as at TDC the cams are preloaded and both must be held with a wrench to keep them in a position of not snapping one way or the other , and thus risking damage to the valves.
So if you marked the cams at 45 deg then you can usually install the belt without any worries of damage to the valves, or any drama about a pulley slipping and snapping a valve into a piston.
Once the belt is on then you snug the tensioner and rotate the crank to check the alignment, once this is done then turn the crank 2 times and set the tension

If you have lost the 45 deg position for the cams then you can install the belt at TDC but you had better have 2 helpers to hold each camshaft with a wrench then you can turn the crank to TDC and install the belt whilst the helpers hold the cams in position.
45 deg position is easier to do IMHO

Mako 928

Thanks to everyone!
I ended up keeping the crank at 45 BTDC and rotated the cam gears to 45 BTDC ( 3 teeth back) I am using new cam gears so there are not any marks for 45 BTDC on them. I finished putting the belt on, filled the tensioner and rotated 2 times checked and the V marks lined up to the back plates along with the crank at TDC. Rotated it again 2 times retensioned the tensioner and all line up! Crank at TDC, cam gears align V to backing plate and noses on the cam shafts point to exhaust. I guess I was over thinking this, again thanks to everyone who responded. Now to dail it in with the dail indicator. Big Daves write up and Andrews recent write up will make this go alot faster!

Big Dave

Nice job Greg!