Brodak 40 Deck Height

In the "Brodak 40 head shims" thread the subject of Deck Height was brought up. I measured 3 of my Brodak 40's and here are the results.

.188
.194
.196

Used two sets of calipers and took two readings with each set to insure my measurements were consistent and accurate. Wow, big difference.. The engine that measures .196 is the one I am running in the Omnium, wished the one in my ARF Nobler wasn't so hard to get to, it is the best running of the two and would really like to know what it is. Anyone know what it is suppose to be????

>In the "Brodak 40 head shims" thread the subject of Deck
>Height was brought up. I measured 3 of my Brodak 40's and
>here are the results.
>
>.188
>.194
>.196
>
>Used two sets of calipers and took two readings with each
>set to insure my measurements were consistent and accurate.
>Wow, big difference.. The engine that measures .196 is the
>one I am running in the Omnium, wished the one in my ARF
>Nobler wasn't so hard to get to, it is the best running of
>the two and would really like to know what it is. Anyone
>know what it is suppose to be????

Well, this verifies what I suspected. Remember, you will see these differences in OS, Foxes, and others. I have seen twice that difference in the FP line and you will see it in the LA line as well. Manufacturing "tolerances".

I have not verified this, but I believe the "innards" on the Brodak were copied from a retimed 40 FP. If so, then the deck height should be .190. This may be something to check on and maybe I will need to go through all of the engines I get in the next shipment and reset the deck height before sending them out. But the difference you have here on your three engines is half a head shim. That's a lot. And that will make a difference in how they run.

>>In the "Brodak 40 head shims" thread the subject of Deck
>>Height was brought up. I measured 3 of my Brodak 40's and
>>here are the results.
>>
>>.188
>>.194
>>.196
>>
>>Used two sets of calipers and took two readings with each
>>set to insure my measurements were consistent and accurate.
>>Wow, big difference.. The engine that measures .196 is the
>>one I am running in the Omnium, wished the one in my ARF
>>Nobler wasn't so hard to get to, it is the best running of
>>the two and would really like to know what it is. Anyone
>>know what it is suppose to be????
>
>Well, this verifies what I suspected. Remember, you will
>see these differences in OS, Foxes, and others. I have seen
>twice that difference in the FP line and you will see it in
>the LA line as well. Manufacturing "tolerances".
>
>I have not verified this, but I believe the "innards" on the
>Brodak were copied from a retimed 40 FP. If so, then the
>deck height should be .190. This may be something to check
>on and maybe I will need to go through all of the engines I
>get in the next shipment and reset the deck height before
>sending them out. But the difference you have here on your
>three engines is half a head shim. That's a lot. And that
>will make a difference in how they run.

I would also guess that the solution may be to shim the liner up or down, instead of change the head shims. .006 variation is very large.

One thing that has amazed me about the PAs and the RO-Jett is the isanely small tolerances involved. Every single part is about as close as I can measure - better than .0005.

LAST EDITED ON Dec-14-03 AT 06:15 PM (CST)
 
Just went down and pulled the head off my NIB B.40. It measures .194" with a genuine depth mike...a Millers Falls, by the way, for you old machinists out there. The one issue, however, is how close to exact TDC did we have our engines, and how much clearance is in the two rod bearings and piston boss?

I did remember to look for cylinder rotation, and it was indeed rotated about 1/8" CCW. Bob, I would fix that anytime, and wouldn't expect any big issues to result as far as breakin or wearing out. If this thing is based on the OS FP, they should have incorporated the pinned cylinder, IMHO. Definitely a move by OS to prevent stupidity on the part of the buyers, but also takes care of the employees, one would hope. Do these things come with a steel or aluminum prop washer? Mine has neither. Steve

Steve,
One thing which was nice about using calipers is that you simply rotate the engine through TDC, and the caliper measures the high point. The downside is that you lose some accuracy from trying to hold the darn thing at 90 degrees. I have seen an accessory which mounts a depth micrometer-like base onto the caliper, which then at least is easier to hold onto the lip.

Brett,
I can understand shimming the liner up but for the life of me I cannot visualize shimming the liner down. Am I just dense tonight?

Best, Hoovie

LAST EDITED ON Dec-14-03 AT 07:30 PM (CST)
 
>Brett,
>I can understand shimming the liner up but for the life of
>me I cannot visualize shimming the liner down. Am I just
>dense tonight?

Of course, it can't be shimmed down, I just misspoke. Moving it down would involve the use of machinery.

The easy solution is machining the bottom of the rim of the sleeve. Almost all engines run their tolerances so the deck height is too much, not the other way around. So this becomes an easy solution.

By making sure tolerances result in too much deck height, not too little, you don't get over compressed, or, worst case, have the piston hit the head. Thus I have run into only one engine to date where the deck height was too low. And it was a new 35 FP that was two thousandths off. Not wishing to shim the head up, as Brett suggested (which can be done, by the way) I switched the piston/cylinder with another new engine I had disassembled that was too high, and got both on spec.

But you have the crank pin tolerance, the crank shaft bearing in the case, the rod on both ends, the cross hole in the piston, and, I suppose, even the machining on the top of the case. If these are off in an additive fashion, yes, it will cause a cumulative effect on tolerances.

And you wonder why the added cost of the PA and Jett engines. It is not just metalurgy. It is tolerances and fit as well. This is someting that the mass producers cannot afford at their prices.

When I had mufflers custom made--and this was on CNC machinery--the outside of the muffler was specked to ten thousandths and only the bolt hole spacing was specked to two thousandths. Even this added to the cost. If all of the tolerances on the muffler were specked to two thousandths, it would be higher.

Engine tolerances get worse, but there are some things they can still "fudge" on. If all of the above operations were specked to two thousandths on the engines, for instance, instead of two ten thousandths, it is less expensive to produce, but can result in differences as shown above. And, this will result in differences in how they will run out of the box.

And, remember, all of the mass produced engines will be this way. It is the cost of keeping them competitively priced.

LAST EDITED ON Dec-15-03 AT 06:04 PM (CST)
 
This is quite interesting to me. I work with these tolerances every day as a design engineer. The answer to people like us with the mass produced engines is to do what Len does and that's correcting the deck height "after-the-fact". It is still cheaper to do that on a brand new engine then to control the entire tolerance stack to assure the end result. Still, there is the tolerance regardless of how tight, so engines can never be produced spot-on, even the best need to be corrected at assembly to be wihtin specification.

In order to make a product like our engines cheap enough to be competitive tolerances must be as liberal as possible. As Steve Helmick mentions, the tighter the tolerance the more expensive the part. Mass producers take it a step further by what they call "statistical tolerance stackup". What this means is that once a designer has designed, in this case an engine, that will assemble acceptably every time the statistics are run to predict how often an engine will be unacceptable as the tolerances are loosened. In the end the manufacturer decides how many rejects are acceptable for a give production rate and the tolerances are loosened to the degree that will produce that rate. In the end these parts are still not scrap and in many industries reject assemblies are simply disassebled and returned to the gondolas to be reused. Statistics say that they will not be scrap in the next assembly.

Engine tuners both improve engines as well as simply bring them into specification. Engine builders still have to deal with tolerances but correct the deviation during assembly.

>Brett,
>I can understand shimming the liner up but for the life of
>me I cannot visualize shimming the liner down. Am I just
>dense tonight?
>
>Best, Hoovie
>
>Life is simple. Eat. Sleep. Fly.

True enough, you can't shim down the cylinder, but you could turn a few thou off the bottom of the cylinder flange, face a bit off the top of the case, or look through a few dozen conrods and see if you can find a longer one, or maybe even make one.

As a machinist, my experience is that drilling a hole (center drilling, drilling and reaming) will put the hole within about .002+/-of desired location. If you are dealing with two holes, like a rod, then you could easily have about +.004 to -.004 between the holes. That's why there is tolerance on machined parts, or nothing would get put together. I do my best to make parts at nominal, but industry runs on old machinery, at least in this country. Mine is over 10 years old. Yet, they bring in drawings for parts with .001" TRUE POSITION between holes and/or part edges. If you want to pay $100 for a conrod, you can do that, with the correct machinery, but not ours. If you want .010 true position, from one hole to the other, I can do that. I'd love to see production drawings for OS engine parts, or Nelson parts, for that matter. Anybody got some? Steve

Think I'll pull my engine and give it another 30 minutes on the stand. If I still don't like it, I may donate it for teardown and inspection. Darn thing has a gallon of fuel through it and runs all over the place. Pretty thing, but not reliable, so far.

try mounting the fuel tank well above the engine and on a piece of cardboard. The mixture will change as it runs down a little , requiring the needle to be opened near the end of the tank. I run quite a few engines with no venturi at all and they run rock steady this way. Maybe this will help MM