MTFCA Forum

I have replaced timing gears. These were really old. The big one was fibre and had very narrow teeth. A 1/8 inch drill fitted nicely between the teeth and I wonder why the fibre gear wasn’t stripped.
I replaced it by a new steel one and aluminium one. The backlash between the gears is currently 0,004 inch.
Now I have a question concerning the lash. According to the Ford manual, the lash should be between 0,003 and 0,005. So the backlash when cold is good. But….the backlash was intended for the original gears, which were -as I read- steel and cast iron. When the engine is at working temperature, the block and gears expand, but because the materials have roughly the same expansion coefficient, the backlash remains about the same. With an aluminium timing gear thats different. Aluminium expands two times more than steel and two and a half times more than cast iron. That means if the engine and oil are 180F, the backlash is reduced by 0,003. That means if hot there is hardly any backlash at all.
I have not found anything about the proper cold backlash when using an aluminium timing gear.

With a nylon timing gear (I don’t like that, especially because I run an alternator) the case is even worse, because nylon expands 13 times more than cast Iron. At working temperature the backlash is reduced by 0,02. If you started cold with 0,003, the gears are almost a press fit and I think it won’t take long to strip the timing gear.

What do you think?

How about using a bronze gear?

Depends a lot on whether or not you have a generator. With a generator, a fiber gear will not last as long. I have successfully used aluminum. I personally think it is a better combination than steel to steel.
Norm

There has been a nylon timing gear, with advanced timing, in our car for the past 21 years, with an alternator by Becker, with absolutely no problems at all. .... looking at the gear through the oil fill , those teeth look the same as when the gear was installed,... A newly rebuilt generator with a new voltage regulator has recently been installed. ... There is minimal backlash between the new generator gear and the timing gear and that I expect no problems.

My house standard spec for backlash is .004/.006". If your front cam bearing is in good condition, and you checked backlash with valve spring pressure applied to the cam you will have no issues. If you have a generator, make sure to check the gen gear backlash as well and shift the generator bracket as needed using gaskets as shims.

Thanks for the thoughts. I know that aluminium and nylon gears survive. The question is what the cold backlash was in those cases when the gears when installed. If the cold backlash was larger than Ford specified, it is logical that these survive. Another question is what is the backlash after a few thousand miles? My car ran without problems with a backlash of 0,125! Because it was a fiber gear, there was not much noise. And if the fitting was too tight at the time of installation, was is the condition of the gears and bearings now? Besides all this, the T is very forgiving for mistakes.
My question is if the cold backlash Ford specified should be corrected for the expansion of the chosen materiel of the gear at the time of installation to do a correct installation. I would like to do the technical correct installation.

"My question is if the cold backlash Ford specified should be corrected for the expansion of the chosen materiel of the gear at the time of installation to do a correct installation. I would like to do the technical correct installation."
YES.

Many years ago the same question arose with an aluminum gear.
So we experimented by measuring the O.D. of the gear before & after a cook in boiling water.
The gear grew a surprising .025". The final gear selected had a clearance of .010". It did get quieter as the motor came up to temp.
The manufacture later changed to a different alloy with less expansion.
It matters the alloy, billet or cast.
Another local fellow used that same gear with a lash of around .005". It knocked out his front cam bearing.

This is why I run Dan's bronze gear. Its quite expansion stable.

Backlash is not a one to one ratio with the expansion . As the gear expands due to heat, the backlash is affected by a less amount depending on the pressure angle of the tooth profile. I do not know what the pressure angle of model T timing gears is though. Possibly someone else can chime in on that. For a 14 1/2 degree pressure angle it is nearly half. A 20 degree angle is 1.374 to one. At least that’s the understanding I have on the matter.

greenacres36 wrote: ↑

Mon Dec 25, 2023 12:07 pm

Backlash is not a one to one ratio with the expansion . As the gear expands due to heat, the backlash is affected by a less amount depending on the pressure angle of the tooth profile. I do not know what the pressure angle of model T timing gears is though. Possibly someone else can chime in on that. For a 14 1/2 degree pressure angle it is nearly half. A 20 degree angle is 1.374 to one. At least that’s the understanding I have on the matter.

You are correct.
In the case of the expanding aluminum gears, the gear peaks & roots are so tight there is negative clearance to push the gears apart when up to running temp. The gear lash was/is of secondary importance.

For a 20 degree pressure angle, the lash changes by .00073 for every .001 inch of gear expansion.

Art Mirtes

Expansion per side

I overlooked the pressure angle and you are right. That reduces the problem. If the backlash is not below the minimum Ford advices, for an aluminium one the gears don’t force each other out of the bearings. But it is critical. For a nylon one its certainly a problem, even with the maximum backlash Ford advices. There you should respect a much larger cold backlash.

speedytinc wrote: ↑

Mon Dec 25, 2023 12:18 pm

greenacres36 wrote: ↑

Mon Dec 25, 2023 12:07 pm

Backlash is not a one to one ratio with the expansion . As the gear expands due to heat, the backlash is affected by a less amount depending on the pressure angle of the tooth profile. I do not know what the pressure angle of model T timing gears is though. Possibly someone else can chime in on that. For a 14 1/2 degree pressure angle it is nearly half. A 20 degree angle is 1.374 to one. At least that’s the understanding I have on the matter.

You are correct.
In the case of the expanding aluminum gears, the gear peaks & roots are so tight there is negative clearance to push the gears apart when up to running temp. The gear lash was/is of secondary importance.

Gear tooth roots are generally cut to allow for clearance. The root depth is not normally cut so close that any variation, such as we're discussing, should make any difference.

So to be on the safe site, the cold backlask of 0,003-0,005 as advised by Ford when installing new gears steel/aluminium should be enlarged to 0,005-0,007? Same correctionon the generator side: not 0,002-0,004 but 0,004-0,006?

The normal pressure angle of T timing gears is 14½ degrees. The formula for the change in normal backlash ( 90 degrees to the teeth, not in the plane of rotation) is 2 X tan PA X profile or center dist change. For a .001 change in the center distance ( or a .001 change in the radius of the gear) the change in normal backlash will be
2 X 0.257 X .001 = .00052". This would correspond to a gear DIAMETER change of .002"

Normal backlash is what you would measure with a feeler gauge inserted between the teeth or with a dial indicator set perpendicular to the gear teeth.

A pretty good rule of thumb is the normal backlash will change about one half of what ever the center distance changes.

Jerry VanOoteghem wrote: ↑

Thu Dec 21, 2023 8:45 am

How about using a bronze gear?

I did not react on the question of Jerry. Bronze is better, also in regard to running culture, but still expands more than cast iron.
Regular expansion coefficients in inch/inchK are:
Cast iron 9
Steel 12
Bronze 17,5
Pure aluminium 23
Polyamide (nylon) 120
These figures vary with alloy (or reinforcement in case of plastics), but indicate their relative position.
Regardless the choice, the lash is influenced when the gears are not the original cast iron ones.

Might the engine number only be accurate for the build date of the engine? With assembly plants in various locations, and overseas, the cars were built at various times. I can't imagine there being control over the stamping of build dates co-ordinated daily for each assembly plant.

Allan from down under, where engine numbers normally begin with a C, the chassis were fitted with bodies built in Australia, in different factories, in different states.

Allan wrote: ↑

Thu Dec 28, 2023 3:31 am

Might the engine number only be accurate for the build date of the engine? With assembly plants in various locations, and overseas, the cars were built at various times. I can't imagine there being control over the stamping of build dates co-ordinated daily for each assembly plant.

Allan from down under, where engine numbers normally begin with a C, the chassis were fitted with bodies built in Australia, in different factories, in different states.

Allen, Might be in wrong topic.

Sorry: the expansion coefficients divided by 1.000.000 of course

Lexveen wrote: ↑

Thu Dec 28, 2023 2:04 am

Jerry VanOoteghem wrote: ↑

Thu Dec 21, 2023 8:45 am

How about using a bronze gear?

I did not react on the question of Jerry. Bronze is better, also in regard to running culture, but still expands more than cast iron.
Regular expansion coefficients in inch/inchK are:
Cast iron 9
Steel 12
Bronze 17,5
Pure aluminium 23
Polyamide (nylon) 120
These figures vary with alloy (or reinforcement in case of plastics), but indicate their relative position.
Regardless the choice, the lash is influenced when the gears are not the original cast iron ones.

Well Lex, I guess your only solution is to have a gear custom made or find an excellent, original cast iron gear. In the meanwhile, thousands of Model Ts are running around with aluminum and bronze gears, as well as some nylon ones. I don't mean for that to appear sarcastic but that seems to sum things up.

As I previously wrote: the model T is forgiving, most don’t drive thousands of miles a year and you can accept faster wear of components, but I am a technician and prefer to do things technically right. The solution is simple: a 0,002 or 0,003 inch undersize aluminium gear would do the trick. At this moment only a 0,005 undersize is available and that is a bit too small

Anyway, thanks for all your thoughts and suggestions.

I have .002 and .003" undersize aluminum gears in stock. You can contact me at dmcgearsATyahooDOTcom or send me a PM thru the forum.
Dan

Dan McEachern wrote: ↑

Thu Dec 28, 2023 1:32 pm

I have .002 and .003" undersize aluminum gears in stock. You can contact me at dmcgearsATyahooDOTcom or send me a PM thru the forum.
Dan

I have sent you a PM

Interesting discussion. I would believe it was a concern of the vendor. I would trust that it just worked.

dykker5502 wrote: ↑

Sat Dec 30, 2023 2:56 am

Interesting discussion. I would believe it was a concern of the vendor. I would trust that it just worked.

TRUST???????
As usual trust but verify.
Don't blindly install ANY part.
I guess you haven't been personally burned badly enough by repop parts.

There is also the issue of proper distance between the crank & cam. Your Babbitt machiner could have screwed up also.

The distance between crank and cam is spot on 3.9375" center to center crank to cam. The cold lash between the timing gears is exactly according to the Ford specs 0,003-0,005. The gears are machined to the original dimensions of the cast iron gears and are not adapted to the difference in expansion. It sounds logical to adapt the dimensions of a gear so when the engine is warm, the lash is correct. I am afraid nobody does that.

Richard, I wondered where that post went!

The only time I have ever encountered timing gear lash problems was when the machinist was off a tad when setting the distance between the camshaft and crankshaft when line boring the bearings. The solution was to take .005" off the OD of the alloy timing gear on the camshaft so it didn't bottom out on the crank gear. Probably the backlash was a bit closer than optimum, but the gears got to know themselves and work as a team. That was 28 years ago and the combination is still working in harmony.

Allan from down under.