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Decesaed (R.I.P.)
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Discussion Starter #1
Ok, so the manual states not to put a D600 jug on a D400 or failure will occur. I'm building a D400 right now so this has me curious. It says the D600 is a high compression design.

I've got a D400 and a D600 jug sitting here in front of me trying to tell the difference. I can't see any, anyone ever solved this mystery?

I'm thinking of using a D600 SP crank case with a D400 jug. There's no mention of that being a problem. Just the other way around.
 

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Just another Lawn Boy nut
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I don't know the difference between them.

However, years ago before I knew they were different I rebuilt a 8229 with a d600 jug and it is still running fine 15 years later.
 

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Decesaed (R.I.P.)
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Discussion Starter #3
It just talks about the newer high compression D600 cylinder, "Likely" piston siezure will occur due to excessive heat.

I can't see any difference except for the H cast below cooling fins.
 

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Decesaed (R.I.P.)
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Discussion Starter #5
Fairly significant difference in cylinder depth. Just measured each in the same spot with the depth gauge on my caliper.

3.247" D-600
3.348" D-400

That would make a compression difference
 

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5K Poster!!!
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So is the con rod on the D-6 and D-4 different? I thought they were the same, at least the later d-4 and 6 anyway.

I guess I don't see why you can't run a D-6 jug on a D-4 frame..
 

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Now I know for a fact the crankcases can be swapped. I've done that before.

Howz the weather down there?? Bout a 105 here again....
 

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Only thing I see different comparing a D600 Sp to a D400 Sp is the crank has a different part number.

Yeah, it's plenty hot down here, says 105f w/ heat index as well
Due to the point ignition system on the D-4. Same dimentions etc just that the D-4 uses all that flyweight cam and stuff.

Wish the humidity would drop about 30%!!
 

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Senior MTF Poster
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"Now I know for a fact the crankcases can be swapped. I've done that before."

Late model "C" series crankcases interchanging also.

"I'm thinking of using a D600 SP crank case with a D400 jug. "

DO NOT do this. The manual says not to for a reason. IF you check the transition between the cooling fins of the cylinder and the cooling fins of the head area of a D400, you will see a single re-enforcing web on D400s on the D600s the web will form a capital "H" for high compression. Now I can not tell you the exact difference but I do know a D600 jug on a D400 will over heat and seize. There are not enough cooling fins on the D400 flywheel to keep the D600 jug cooled, check both flywheels. I learned the hard way.

Walt Conner
 

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Decesaed (R.I.P.)
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Discussion Starter #12 (Edited)
I already mentioned the H cast into the jug.

Let's assume putting a D600 jug on a D400 is a no no like the manual says. Swapping out my D400 crank case for a D600 shouldn't be a problem. I'm going opposite the way the manual says not to.

I've still got to find out what makes the D600 a "high compression" jug. So far it's the depth of the cylinder.

So higher compression = more heat. They redisigned the flywheel/cooling on the D600's to accomodate this.

Curious how sj701 got away with it. Richer carb setting to run cooler? Maybe some D400 shroud designs work better than others for cooling? Can you run a D600 flywheel on the D400?
 

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Bore and stroke are the same comparing D-400 & D-600. So the "swept volume" swept by their pistons' movements are identical between those two. But unswept volume above their pistons at Top Dead Center of the crank's rotation is smaller in the D-600 than in the D-400. Compression ratio is just the ratio between atmospheric pressure and total compressed pressure within a combustion chamber at TDC. Smaller unswept volumes at TDC translates into higher compression ratios. Larger unswept volumes at TDC translate into lower compression ratios.

The fact that 2-cycle reed-valve induction engines use a 2-stage compression system rather than a single-stage compression system as is found in conventional 4-stroke engines, adds some complexity to estimating how much total compression any specific 2-stroke design will generate. But physical testing easily reveals their functional ratio. For an expanded explanation of this relationship, read my post in this thread:
http://www.mytractorforum.com/showthread.php?t=86587

If you were talking about DuraForce Lawn-Boy engines which have a parting line between their head and cylinder, you could reduce their compression by adding a spacer block between their head and cylinder, which would increase unswept volume without changing piston-controlled port timing. But both the D-400 and D-600 head and cylinder were cast as a piece before the cold cylinder liner was pressed into the hot cylinder allowing them to lock together. So if we were to add a spacer between their cylinder and block to increase unswept volume to lower compression ratio, that spacer would simultaneously change all the piston-controlled port's timings compared to crank rotation.

Walt and the factory manual are right. While they could be modified to work together, the cost-effort/benefit ratio would make no sense.
Just my opinion,
John
 

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Decesaed (R.I.P.)
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Discussion Starter #14
I still say a D400 jug can be put on a D600 with no problems at all, that part is not being addressed. Is anyone arguing that?

I also think there are certain D400 shroud styles that would cool the D600 jug. Something like the brick top not being one of them I would think.

I think you're exactly right, there's less volume in a D600 cylinder a TDC than in the D400 cylinder. That increasing compression, that increasing heat.
 

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Lawn-Boys and VW's
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You hit part of it with the older 400 cooling system being not a efficient as a 600 is. I know someone who put a 600 cylinder on a 400 and it ran all summer but you could see that it was failing by looking into the exhaust ports. I would stay with the factory and not swap. They made the tech bulletin for a reason.
 

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The thing that puzzles me is that both the D-400 and D-600 engines are both rated at 3.5 HP @ 3200 RPM. If the power output remained unchanged, what advantage did a higher compression ratio deliver?
 

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Lawn-Boys and VW's
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Possibly more torque
 

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Just another Lawn Boy nut
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I don't know why ,but it worked for me for a long time on my 8229 (Mag 21 style shroud). I know it bolts right up to a standard d400 bottom end, piston and rod, I did it, I didn't know of the Warning in the service manual when I put this together.

Here are the conditions it ran under: 89 octane and Lawnboy oil, regular maintenance of the engine, and my lawn at the time took 30 minutes to mow so it never ran a really long time. I also either side discharge or baged with the mower at the time so the load wasn't real heavy. The mower finally failed when the mower was mistakenly straight gassed by a "helpful" friend who tried to mow my lawn while I was away.:banghead3 It was running fine for 15 years till then.

I'm now running a proper d400 cylinder on the mower, I don't notice any real difference in the power between the two of them, but the d600 cylinder may have been a little tired.

Thanks
SJ
 

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Decesaed (R.I.P.)
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Discussion Starter #20
I don't know why ,but it worked for me for a long time on my 8229 (Mag 21 style shroud). I know it bolts right up to a standard d400 bottom end, piston and rod, I did it, I didn't know of the Warning in the service manual when I put this together.

Here are the conditions it ran under: 89 octane and Lawnboy oil, regular maintenance of the engine, and my lawn at the time took 30 minutes to mow so it never ran a really long time. I also either side discharge or baged with the mower at the time so the load wasn't real heavy. The mower finally failed afew years back when the mower was mistakenly straight gassed by a "helpful" friend who tried to mow my lawn while I was away.:banghead3

I'm now running a proper d400 cylinder on the mower, I don't notice any real difference in the power between the two of them, but the d600 cylinder may have been a little tired.

Thanks
SJ
Thanks for the details on that.
 
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