Cam Timing question.

[DavidApp]

Getting ready to set/check my cam timing on the 1275 Midget engine I am rebuilding. I can not find any information on the valve opening and closing angles. Aligning the dots on the sprockets and hoping seems a bit off.
Going to set a degree wheel on the crank but need valve lift times.

David

 

[DrEntropy]

Getting ready to set/check my cam timing on the 1275 Midget engine I am rebuilding. I can not find any information on the valve opening and closing angles. Aligning the dots on the sprockets and hoping seems a bit off.
Going to set a degree wheel on the crank but need valve lift times.

David

It mostly depends on the cam and if it is the OEM one. If so, the "dots" method should be okay. It'll be within a few degrees of spot-on, If you have doubts, a wheel and careful finding/setting of TDC are step one. If you have a shop manual the cam timing should be in the specifications section. I recall seeing 1275 cam degree info ~someplace~ here on the Forum but can't find it just now.

 

[bobhustead]

Randall Young wrote a good post on cam timing on a TR3, pointing out that you can't get precise no matter how careful you are due to immutable bolt hole, Woodruff key, and tooth positions. If you have your old sprockets and can duplicate the marks, making sure the gears are oriented same side up and with same bolt hole/pin/key slot positions when you do so, you will be fine. And it won't take long before chain stretch and sprocket wear change what you've done anyway.
Bob

 

[DrEntropy]

If you have your old sprockets and can duplicate the marks, making sure the gears are oriented same side up and with same bolt hole/pin/key slot positions when you do so, you will be fine. And it won't take long before chain stretch and sprocket wear change what you've done anyway.

So true. "Don't pet th' sweaty things!" Over-thinking just wastes time you would better be spending by DRIVING the li'l beastie once it's done. Just replace the tensioner and chain.

 

[DavidApp]

The 1275 setup has little to no adjustment besides off set keys. All you could do is jump a tooth on the sprocket.
Also no tensioner.
It was more an intellectual exercise and a it of OCD. I did find the settings in my other book. Everyone said the Bentley book was the word but it seemed to have nothing on that subject.

David

 

[DrEntropy]

IIRC the offset keys are half a tooth? 4° or there about? Unless the engine is being built up as a racing unit with "annual tear-down" in mind, it's picking nits IMHO. An ex-pat Brit pal o' mine once said: "Even half right, these things will run in the bottom of a bucket of s**t!"

The tensioner reference was a brane-fade, thinking of the 1800 timing chain setup. I'll be available for whipping after five PM on Tuesday.

 

[bobhustead]

My 50 cents on offset keys- The work of a key in resisting torque (i.e., keeping the sprocket from spinning on the crankshaft) is in large part a function of the shear modulus of the metal the key is made of. If you have a key that is offset half its thickness, the shearing force needs to overcome only half the modulus needed for a full thickness key. This may be a pointless objection since I have never used one and probably couldn't actually do the math, but...
Bob

 

[bobhustead]

On the offsets, the pulley bolt pinch would obviously help, just don't know how much.
Bob

 

[DrEntropy]

My 50 cents on offset keys- The work of a key in resisting torque (i.e., keeping the sprocket from spinning on the crankshaft) is in large part a function of the shear modulus of the metal the key is made of. If you have a key that is offset half its thickness, the shearing force needs to overcome only half the modulus needed for a full thickness key. This may be a pointless objection since I have never used one and probably couldn't actually do the math, but...
Bob

Agree as to shear strength (or lack of) with an offset key. But again, if the mill is intended as a competition one and torn down annually, the key would be replaced. Along with a number of other bits.

 

[mezy]

cant see the point in buggering about, any adjustment would change as the chain stretches,
even with a chain tensioner the cam timing still alters with stretch.
A much better option would be a re-profiled cam shaft with a propper full strength key.

 

[DavidApp]

The offset key was not an option that I had considered for the obvious reason that it must reduce the strength of the key. It is mainly an exercise in curiosity and getting a better understanding of the timing setup. If I was really into getting it Perfect I would have ordered an adjustable sprocket.

David

 

[DavidApp]

Found this handy site where you can print out a degree wheel with the valve opening and closing points set to your cam. Outside diameter of thr print can be changed as well as the printed center hole.

David

Degree Wheel Templates Valve Timing - Metric

Create adjust and print full scale degree wheel templates to check valve timing - Metric

www.blocklayer.com