250 Cam timing.

InlineUK

Well-known member
Hello All, I need some help with my cam timing. I have a Clay Smith 274 112* cam from Classic Inlines. This is fitted in a 250 which will be turbo charged. I also have a new stock timing set from Mustang Unlimited.

This is my first time degreeing a cam and it’s not as easy as I thought it was going to be!

I’ve calculated that the lobe centres of this cam should be In 112* ATDC and Ex 112* BTDC. CI recommends installing 4* advanced, so that should make it In 108* ATDC and Ex 116* BTDC. Is this correct?

I installed the cam straight up with the timing dots aligned. I measured/calculated the lobe centres to be In 116* ATDC and Ex 108* BTDC. So by my calculations the cam is 4* from straight up and 8* retarded from the recommended position.

I tried moving the cam around one tooth and measured again. This time I measured In 100* ATDC: 8* advanced over the recommendation (12* adv over straight up).

So which would be better for a turbo engine, 4* retarded or 12* advanced? Has anybody else had similar problems? If so, what have you done to correct this?

There isn’t (as far as I know) an adjustable timing set for the 250. The only other option I can see is to take the cam sprocket to the machine shop and get them to make me a new keyway, that is 4* advanced to give me 8* at the crank and therefore get the In at 108*. What do you think of this idea?

Any help is gratefully received!
Thanks
Nathan
 
Nathan, just use #1 cylinder intake lobe. After you have found TDC, rotate the engine back till the dial indicator is .050" less the full lift on the opening ramp, record that #, then rotate the cam past maxium lift till you are .050" on the closing side, record that # on the degree wheel. Add both the #'s @ .050 together & divide by 2 that will be your intake lobe center. You just use the intake not the exhaust. If you get 110 your are 2 degrees advanced if 108 you are 4 degrees advanced.
See attached link from comp cams. Bill
http://www.compcams.com/Technical/Instr ... es/145.pdf
 
Thats sort of what i did. I didn't use 0.05" from max lift, but used a fixed increment. Got the two degrees and calculated the lobe centre.
First time it was 116* so thats 4* retarded. Moving a tooth made it 100*, so thats 12* advanced.

Any further advice?
 
I don't know how small an increment a stock timing chain set will allow you advance/retard. The set I got from Classic Inlines allows 4 degree increments.
 
I'd think that an offset key would now be the solution.

For another motor I had the key programmed into a CAD setup, so any offset within reason could be readily milled. Quite cheap, too.
 
Nathan, make sure you have a solid mechanical "0" for TDC on your degree wheel. Don't go by the crankshaft pulley.
Check your readings again to make sure.
Most of clay smith cams are right on or even 2 degrees advanced.
After triple checking if you still get a retarded situation, you have 2 choices either offset key or modify the cam gear slot to get you at least 6 degrees.
As i said even get a second opinion on your readings. Bill
 
InlineUK,

I have used small block Chevy offset crank keys with success. They need to be fitted. You can do that with a bench grinder. As I remember they are a touch tall. They are not as long, but its no issue. Just make sure the crank bolt is torqued to specs. That's what really holds everything together, not the key.

I've also filed the cam dowel pin and shimmed using an old feeler gauge. That takes a lot of trial and error but its cheap.

Also, make absolutely sure there's a cam spacer behind the dowel pin on the cam. This gets forgotten on half the builds.
 
Thanks for the replies guys.

From the sounds of it the best way to proceed is to double check everything and then modify the cam gear.

Not sure about the offset keys, all the ones i've seen on the net don't look like they would work with the dowel set-up. Can anyone point me in the right direction of one that would work?

I think i'll take the gear to a machine shop tomorrow to see if they can cut a new keyway. 42 teeth, so thats 8.57 deg per tooth. So in theory if I get them to cut the keyway half a tooth adv (approx 4.25 deg) that should give me 8.5 deg adv at the crank. While they are at it i may get them to cut a third keyway at 2 deg adv to give me a little more flexibility.

drag-200stang":1ovt359a said:
Also, make absolutely sure there's a cam spacer behind the dowel pin on the cam. This gets forgotten on half the builds.

I did that the first time around and wondered why the cam wouldn't turn when torqued to spec! :LOL:
 
The offset keys go into the crank, not the cam. I think the mopar ones are the correct width, but I can't recall. Hopefully someone can verify. If not, the Small block chevy ones are also available.

dcc-4286500.jpg
 
I may have found part of the problem.
The cam timing gear for a 69 year has a different part # than one for a 75 engine.
Could the later cam gear be retarded 6 degrees compared to the 69 year engine.
The lower gear is the same part #S415
69 cam gear S414
75 cam gear S442
This may be part of your problem.
Maybe CZLN6 could chime in. I know because of emissions the later engines retarded the camshaft.
What year did you order the timing set for?? Bill
 
The cam gear has S414 stamped on it, i think it was advertised as 69-72.

It may be a dumb question....but how do those offset keys fit into the crank? :unsure: :)
 
I explained this on another tread, try a cam gear from a 1975 engine if your using a 1" wide chain or a 77-78 if you want the 5/8" wide chain that should get you close, should be 6 deg. diff between the 69-70 cam gear and the 75-78 cam gears
 
another thing, depending on what deg kit you have, make sure you rotate the engine from the flywheel side of the motor, this will ensure you do not move the deg wheel on the front
 
InlineUK":2bhne501 said:
The cam gear has S414 stamped on it, i think it was advertised as 69-72.

It may be a dumb question....but how do those offset keys fit into the crank? :unsure: :)

The crank keys hold the crank timing gear and the damper in place. They fit into the groove on the crank and those components slide over it. By offsetting the key, they are either advanced or retarding the crank gear, and consequently the cam gear.

The later timing sets are 4 degree retarded according to one reference I have. The early ones are cut to install the cam straight up.
 
I took the gear into the machine shop, he'll cut me two more keyways at 2 deg and 4 deg adv. Should be done by the weekend so i shall see how that turns out. I'll let you know how it goes! :D

All of your other suggestions should give me plans B, C and D if this doesn't solve the problem! :D

Cheers.
Nathan
 
Just make sure the machine shop doesn't break the gear!

I had a shop break one when they tried to broach it.

After buying another gear, they used the mill and make the groove. I just have to use a hand file to square it up.

Will
 
Well i got the gear back from the machine shop:
Camsprocket.jpg

2 new keyways and no breakages!

I fitted it and degreed the cam again tonight. My intake lobe centre is now 109* so I'm a happy bunny! :D
 
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