3.375 stroke

[TrackPack]

I noticed something on several bore stroke charts. The 67/68 383 engines are listed with a 3.375 stroke, and in 69 it increased to 3.380. Is this right, or are some of these charts screwed up?

 

[WileE]

I only recall the 3.38 stroke.

 

[TrackPack]

Me too. I was surprised to see the 3.375 in the NHRA blueprints specs, so I started looking around.

 

[Lionized]

Was it increased that slightly with the advent of the magnum?

 

[happyjackdaddy54]

I noticed something on several bore stroke charts. The 67/68 383 engines are listed with a 3.375 stroke, and in 69 it increased to 3.380. Is this right, or are some of these charts screwed up?

.005 thousandth?

 

[BSB67]

I noticed something on several bore stroke charts. The 67/68 383 engines are listed with a 3.375 stroke, and in 69 it increased to 3.380. Is this right, or are some of these charts screwed up?

Or more likely, just change in what was put in print. On any given day in any year, they were probably +/- 0.005" on stroke, or more. Also, the stuff I have around here only has 3.38, not 3.380. What is in the FSM for 69?

 

[beanhead]

Interesting. It's maybe one of those things that we never truly know the answer to... If I was to venture a guess I would say that as the muscle car wars ramped up in '69, 3.38 looked better on the spec sheet without altering the actual cubic inch number (plus they could save money on ink by printing one less number )

 

[TrackPack]

I just noticed that the 361 was also listed as 3.375 too. That was dropped in what...66 or 67?

 

[BSB67]

I just noticed that the 361 was also listed as 3.375 too. That was dropped in what...66 or 67?

Most of my stuff for most years show 3.375"

 

[66Satellite47]

I call that "rounding". If you look at FSM you notice the "common" dimensions are some sort of rounding.

 

[BSB67]

I call that "rounding". If you look at FSM you notice the "common" dimensions are some sort of rounding.

Right. That's my point on 3.38 verses 3.380. From the rules of significant figures and rounding, 3.375 and 3.38 are not necessarily different. Of course, this speculation falls apart if the 69 FSM has 3.380. That's why I asked earlier

Either way, I still guess that it is more of changing print verses any physical production change.

As an example, my FSM and Motors Manual has everything through 68 as 3.375. The same MM has 69 and up Dodge and Plymouth 383s as 3.38, but Chrysler 383s as 3.375 until end of production. I think we know with certainty that they were not grinding different 383 cranks based on car line.

 

[TetanusShot]

Easier to hold tolerance when machining for .xx instead of .***, probably just a running engineering change to reduce costs and scrap.

 

[66Satellite47]

Easier to hold tolerance when machining for .xx instead of .***, probably just a running engineering change to reduce costs and scrap.

Much easier to quote two decimal points. The FSM have the .000 numbers, we just need to pay attention to them. All in fun for discussion.

 

[TrackPack]

Much easier to quote two decimal points. The FSM have the .000 numbers, we just need to pay attention to them. All in fun for discussion.

Unfortunately (or maybe fortunately) these differences in stroke were submitted to the NHRA for blueprint specifications in class racing.

 

[BSB67]

Unfortunately (or maybe fortunately) these differences in stroke were submitted to the NHRA for blueprint specifications in class racing.

Sure, but it doesn't really affect anything.

 

[TrackPack]

Sure, but it doesn't really affect anything.

Rules allow for adding .015 stroke to the crank, so need to use the right specs.

 

[BSB67]

Rules allow for adding .015 stroke to the crank, so need to use the right specs.

Right. Just follow the applicable NHRA rules and specs, and maximize or minimize to your benefit any tolerances they allow.