OS LA 40 vs. 46 Lighter?? Photos

[Harleyman]

I've read/heard many times that the LA 46 is lighter than the LA 40.
I recently acquired a LA 46, so I thought I'd check it out.  The results are very interesting.
Both motors are equipped with aluminum back plates, venturis and NVA's.  The 40 has a stock OS NVA and the 46 has a Super Tigre NVA.
I weighed them many times and got the same results each time.  
My LA 46 is a bit heavier than my LA 40.  I haven't weighed the NVA's separate yet, but could the ST NVA weigh that much more?
With props and prop nuts removed, the 40 is 242 grams and the 46 is 244 grams.
Like I said, very interesting.  What do you think?

[Paul Taylor]

Good talking with you the other day Chris.

Tell ya what, send me that LA .46 and the Talon and I will let you know what I think.

[Phil Coopy]

I've heard that too but if you think about it, the 46 has a bigger piston and bigger liner in it.  The cranks are the same which is why the 46 vibrates more.  I've been working on rebalancing my 46's but time and other projects interfere.  I ran into a problem using the normal static balance methods because of the imbalance caused by the chunk carved out of the shaft that makes the intake valve.  Once I figure out how much heavy metal to put in the counterweight, I can do it for others.

Phil

[Perry Rose]

The only parts that don't interchange are the piston,liner and head. The liner has to be lighter than the .40 as it's got the same o.d. but a bigger i.d. the piston will be bigger and maybe heavier and the head has to fit the bigger bore. There's room for a couple gram difference. But I would think it would be the other way. Then again it seem you could make a .46 out of a .40.

[Warren Leadbeatter]

The 40 is painted Blue, there's an extra 2 grams. 

[john e. holliday]

Warren he says the .46 is 2 grams heavier.  That is interesting about the weights the scale shows.  2 grams is a lot of metal if it comes to the difference of the needle valve assemblies.  I know the LA 46 I put in my P-39 to replace the os 40 was a full 3 ounces lighter. 

[Larry Cunningham]

I was wondering the same thing; the old FP .40s were lighter than the FP .35s due to the thicker liner..

Hoppy New Year!

L.

"I am not young enough to know everything." -Oscar Wilde

[Harleyman]

Larry says:

the old FP .40s were lighter than the FP .35s due to the thicker liner..

That is the same reason I believed that the 46 would be lighter also.

So maybe the 40 does not have a thicker liner, just a smaller bore in the case with a thin liner, like the 46.

[Phil Coopy]

Just did some measurements on LA40 and LA46

Model                       40                              46

Sleeve                 16.27g                           21.25g
Piston/rod/pin        12.43g                           14.29g
Crank                   53.39g                           53.80g
Head                    26.41g                           28.02g
Sleeve O/D             .903                              .984

Draw your own conclusions.....the 40 sleeve will actually slip inside the 46 sleeve.  I didn't weigh the crankcases but they are obviously different and probably don't weigh the same eithe,r hence the different part number.


Phil

[Perry Rose]

The way I read the parts list the reason for the different part numbers is for the blue or silver case.

[Phil Coopy]

Absolutly right Perry,  p/n for the blue 40 case is 2 4001 001 and the p/n for the natural 40 case is 2 4001 011 and are interchangeable, but the p/n for the 46 case is 2 4001 100 so the 40 and 46 cases are not interchangeable as may folks think. A lot of folks think that the 46 is just a bored out 40 but it's not, the head, crankcase, sleeve, and piston are different.

Phil

[Jim Kraft]

I see also that the crankshaft for the 46 is a bit heavier too, so it must be a little different.

[Phil Coopy]

Yah Jim, checking the cranks out with mics, there isn't any appreciable difference,  it must be in the materials or finishing.

Phil

[Alan Hahn]

I guess this is a case for measurement winning out over hypothesizing!

[Dennis Moritz]

Crankcases have different numbers cast on the side.

[Dennis Moritz]

Just weighed some samples of LA 40s and an LA 46. Engines were weighed without glow plugs, venturis or mufflers.
LA40 silver:  8.2ounces
LA46 powder blue: 8.3 ounces
LA 40 powder blue: 8.4 ounces

I've converted FP35s to FP40s now and then. Gone the other way too. Since the bores are different it was necessary to swap heads, piston pins, head gaskets as well as piston liner/piston sets. Far as I can tell the crankcases worked either way. I have not done the switch back and forth LA40/46. Perhaps there is a greater difference between the crankcases then I thought. As I noted above they would have different parts listings because of the numbers cast into the side of the case. True as well for the FP series. Do the parts listings for the conrods and cranks differ? FP40/35 cranks are the same. In fact I think you can use one in an LA case.

[Phil Coopy]

Dennis,

The crankcases have a different bore to accomodate the different O/D of the sleeves.  The sleeve wall thickness of the LA40 and LA46 is actually about the same. If you were to try to make a 46 out of a 40 by boring the 40 crank case to accomodate the 46 sleeve the head bolts wouldn't line up, not to say what would happen to the transfer port sizing.
Phil

[Dennis Moritz]

Thanks Phil. I didn't know that about the crankcases. Makes sense tho. I believe I heard that the LAs have thinner liners than the FPs, and that it's advantageous to do that. Guess there wasn't enough meat in the LA40 to bore it out, unlike the fp40/35. I need to get new calipers. My good calipers got lost. I  bought a bargain Harbor Freight set. Absolutely useless. The gears have the crunchies.

[Geoff Goodworth]

I don't have any FP 40s apart at present but I find it intriguing that the OD of a Tower 40 cylinder is 0.928".

Also, regarding the crankshaft, Tower describes the LA 46 crank as FP 35/40.

Cheers, Geoff

[Dennis Moritz]

The OS engines website lists the crank of the LA46/40 as the same as the crank for the fp40/35. The connecting rods for those four engines also have the same listing number and are referred to as FP40/35 connecting rods on the LA46 and LA40 parts lists.

[Dennis Moritz]

Is this a safe conclusion then: crankcases la46/40 have different internal dimensions, not interchangeable. Cranks are interchangeable. FP 40/35 cranks are same as LA 40/46 cranks (?).

[Phil Coopy]

Looking at several parts listings it appears that you are right about the interchangeable parts between the FP's and LA's.  I have not had any FP's around for a number of years now.  My main interest at the present time is to work on the counterbalance in an effort to get less vibration on the LA46.  I noticed  an increase of vibration felt throught the lines when I switched two planes from LA40's to LA46's at the same time. As I mentioned before OS used the same counterweight in the 40 and 46, and with the 46 having more reciprocating weight because of the heavier piston, it vibrates more.  I am working out a method to drill the counterbalance to insert a tungsten slug of the proper weight to get a better static balance. It's more complicated than I first thought because of the extra imbalance caused by the chunk of metal cut out of the shaft for the intake port, and its position in relation to the counterbalance.

Having said all that...I think we done beat this dog to death...

Phil

[Dennis Moritz]

True. We have beat this dog. Nevertheless. Leonard Neuman, I believe, wrote somewhere that the LA cranks are cast, wheras the FP cranks are forged. At least that is what I remember reading. Guess this could be true, even though the parts are interchangeable. Certainly that's true for small block Chevies. Could be you're better off attempting to balance one or the other. If you want a used FP crank on which to experiment, let me know.

[Geoff Goodworth]

Phil, not to disagree but I'm fascinated by the info that has come forth.

I'm a manufacturing engineer with 40 years expereince and, given that the bore and stroke of the LA 40 are the same as those of the FP 40, I'm amazed to find that the LA 40 cylinder seems to be different to the FP 40.

With machine settings established after years of use, is there really a saving to be had from a cylinder that is about 0.010" thinner in the wall. If they make enough, I guess that the answer is probably, Yes.

Cheers, Geoff

[Larry Fulwider]

Just did some measurements on LA40 and LA46

Model                       40                              46

Sleeve                 16.27g                           21.25g
Piston/rod/pin        12.43g                           14.29g
Crank                   53.39g                           53.80g
Head                    26.41g                           28.02g
Sleeve O/D             .903                              .984

Draw your own conclusions.....the 40 sleeve will actually slip inside the 46 sleeve.  I didn't weigh the crankcases but they are obviously different and probably don't weigh the same eithe,r hence the different part number.


Phil

If we're still collecting data, just weighed a new LA 46, set up in the most common way, I imagine --

Front NVA and NV, rear plastic backplate, Thunderbolt plug, front nuts and washers. Supplied venturi was alum .282, I replaced with alum .275.

Weight = 250.3 g ~= 8.83 ozm

Oddly, my records also show exactly 250.3 g for an LA 40 in '08; unfortunately, I didn't record what was in / on it, but it was a plastic blackplate and front NVA.

       That would indicate, using one good data point, and one "manager" data point, that LA 40s and LA 46s probably don't differ much in weight for our purposes. I don't think there is enough data here yet to say definitely one is lighter or heavier than the other, and individual setups probably have more to do with final weight of either than the raw basic parts.

       Larry Fulwider

[Dave Gardner]

Differences I've found.....relative to fits, not weights.....

The LA liners are MUCH thinner than the FP liners.....approx 1mm thick, and the crankcase bores are different for the 40 and 46, to accomodate the different bore sizes.

In the FP series, OS did a 'Fox' and used the same crankcase bore on the 35 and 40, with a thicker liner on the 35.......and yes, you CAN make a 40FP out of a 35FP, with a new piston, liner and head.  The crankshaft and rods are interchangeable between the 4 engines, but that doesn't mean they are identical.  The early FP's, particularly the iron/steel versions, used what appears to be a fully machined crank.  Many of the later ones seem to be forged and machined....and there seem to be production differences over time, as well.

Also, if you want a 'true' ABC FP 35/40, you can use the Tower 40 P/L in either case.....and it is still a 40, not a 35!  As noted in the SSW engine info, the bore/stroke ratio is different and it seems to affect the 'stuntability' of the two engines.  The LA 40/46, without a boost port, seem to have overcome that issue, but the balancing is obviously affected by the heavier piston in the 46.

Just observations from an intinerant tinkerer.....

Dave G