Dax 40mm stroke vs Dax 38mm stroke

Here I'm showing that both the original version Dax 110mm rod GenIV 40mm stroke lower and the newer Dax 114mm rod GT5 platform Gen IV 38mm stroke lower both have the same size transfer ramp openings that allow compressed air/fuel mix to flow from crankcase up into transfers in cylinder jug and then of course ultimately into cylinder for compression power stroke.

The engine to the far left is the 38mm stroke "GT5" platform.

The engine in the middle is the Original GenIV 40mm stroke.

The engine case to the far right is one of my older 2010 (Lucky Early Bird/BGF) "Half Breed" engines, notice the transfer ramp size and shape difference compared to the engines we see nowadays, the old LEB engine also has the larger front and rear mounting saddle, only one of these engines I have seen and only one I have, paid $75 for complete engine right to my door back then and wish I would have bought several of them, I do believe this case design is superior to what we see now.

But back to the dax engines, You'll also see that there are however some differences in the two Dax lowers in the next post I will show pix of these differences.

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The purpose of the two pix below is to show the difference in the crankshaft width between the counterweights.

Look at the original 40mm stroke GenIV and you'll notice how wide the gap is between the counterweights, this is why they needed to use copper shims on each side of the con rod to keep it from having a considerable amount of end play on the lower bearing.

Now look at the second pic and you will notice two differences in the crankshaft, the gap between counterweights is narrower which prevents the need to have shims too reduce end play in con rod, but if you look closely you'll also notice that the counterweights on this latest GenIV are wider also, so not only did they eliminate the need for the shims but by making the counterweights wider they also increased primary/crankcase compression a little which is a good thing.

Next post will have close up pix showing the spacing between crankshaft and case near the transfer ramps, these are also different between the two engines.


GenIV 40mm Stroke lower

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GenIV 38mm Stroke lower

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Look closely at the gap between the crank and the case at the transfer ramp area in both engines and you'll notice that the original 40mm stroke engine has less of a gap for the air/fuel mix to be forces between crank and case, on the newer 38mm stroke GenIV, even though the counterweights are thicker, they are moved inward toward con rod to eliminate needing shims and by doing this and using the thicker counterweights the primary compression isn't reduced and this actually allow a slightly wider space between crank and transfer ramp for the air/fuel mix to be compressed up through the jugs transfer ports, to me this is a win win, not only is the primary crankcase compression preserved and shims not needed between con rod and counterweights, but the slightly wider space between crank and transfer ramps is gonna increase flow I would think because the mix is under the same or possibly a little more pressure and there is less of the crank in the way of getting the fuel rammed up through the transfers and into the cylinder for combustion.

Looks like they made some improvements in design with these newer GenIV 38mm stroke engines.

I dont have one running yet but I think I'm liking what I see here so far.

Next two pix will show difference in the lower portion of the con rod and its ability to get lubricated by fuel mix.


Original 40mm Stroke Gen IV

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Newer 38mm Stroke GenIV

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The first pic shows the lower con rod design which has single gaps on one side of the rod to allow fuel/oil mix to get in and lubricate the bearing, obviously this has never created an issue as far as I know, but just wanted to show the difference between the original 40mm stroke engine and the newer version 38mm stroke engine.

You can see in the second pic which is of the lower con rod in the 38mm stroke engine, it has multiple notches on both sides of the rod to allow fuel/oil mix to easily flood into the lower rod bearing.

not that this has been a huge issue either way, but since a found this difference I wanted to share so we all have a better idea of what the 38mm stroke GT5 type GenIV is vs what the original 40mm stroke GenIV is.

40mm Stroke lower con rod pic
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38mm Stroke con rod pic
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allen standley said:

That is critical to know if trying to identify. Thanks much map. As you know I'm building another now. Not at the engine yet, but all kinds of engine stuff to use and work with. The detail and timing of your diagnosis is very helpful. Thanks again Buddy!

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You're welcome its cold as the dickens here in central Texas today, but I got a little cabin fever so I put on the insulated over-hauls and went out in that cold shop for a while and did some studying of these two engines and wanted to get some pix taken so I could share what I have found.

Since I have 4 of the older version and two of the newer version I figured I ould make this thread and it would answer some of the questions that may be in some peoples minds when they see some of us discussing the Dax GenIV engines and the 38mm vs 40mm stroke conversations we have already had to a small extent.

I'm liking what I'm seeing so far with how the newer version is put together, now all I need to do is get one built the way I want it and get it running so I can see for myself what the overall balance of this newer version is compared to the original 40mm stroke engine.

cannonball2 said:

Great pictorial thanks! So this is just another manufacture of a GT5? I mean its the same geometry. So the only half breed out there is the ZAE50 cranked engine? BTW there also should be three oil holes in the rod at the crank pin on the older crank, at least mine has them.

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Yes it seems that the engines with the ZAE50 marked rods are the only "Half Breed" engines.

And yes the original 40mm stroke GenIV does have three holes drilled in the lower portion of the con rod.

I do think the rod in the newer version will allow better lubrication to that lower bearing but it's not be a big deal because I have run the poop out of my 40mm stroke GenIV engines and never hurt a lower bearing.

YesImLDS said:

Awesome comparison pictures map. The new and improved version should have higher crank case pressure as well as being a shorter stroke should have increased power and rpm capabilities I would assume?

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Very possible that what you have said here is the way it will be, I'm not gonna make any definite claims on what I think it will or wont do, but once I get one together and get it running on a bike I'll know what I think of it compared to the original 40mm stroke version.

typically a shorter stroke engine looses a little grunt power down low compared to the same cc pr CID engine with a longer stroke when all else is comparable, but the short stroke engine will sometimes have the ability to wind up a little quicker from mid range to top rpm's and the shorter stroke can allow for higher rpm's in some cases.

So yes I do have all these things rolling around in my thinking, but I don't want to count my chickens before they hatch and get the cart before the horse, so I'm just looking at all of this with an open mind so I can honestly go about the comparison without any of the placebo affect thinking that gets thrown around on here to often sometimes.

I'm not gonna go into this comparison expecting it to be better than the original since the original GenIV dax is the best running and smoothest running engines I have had, I'm hoping that the newer version will be as smooth and be capable of the same top speeds with the same gearing and mods as I have done to the 40mm stroke engines, the one thing I will have to do to the 38mm stroke GenIV though is I will have to shave the jug deck or I cant get the compression to be comparable since all of the 38mm stroke engine I've seen sit the piston to deep in the cylinder to have optimal compression.

All other mods will be very similar, just basic port clean up, possibly some very slight ramping on transfer area of piston if it needed to expose the ports better @ BDC, I wont know any of this until I start the build up, but I'll be looking at all of this hoping to get the engine to breathe as good as it can and hopefully have good mid to higher rpm potential and power just like my 40mm stroke engines have had.

cannonball2 said:

So as I was saying in an earlier post there must be several versions(at least two?)of the GT5 engines. Daxes and the one that is flooding ebay currently with the FM80 rod. Didn't the GT5 originally refer only to the Grubee engine? The crank with the FM80 rod is slightly different than Daxes. These are pretty smooth. Now it is to see which one is smoother, Daxes or the FM80.
Im still guessing 40mm engines are going away, though I bet there is a good backlog still out there.

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Im thinking the manufacturers of the HB type and what we've been calling the GT5 engine must be making the American vendors a heck of a deal on these engines and that may be why we're seeing more of them.

The HB and the "GT5" been around for a long time, I have a HB geometry engine that I bought back in 2010 so I know it was made at least in 2010 but could have been made a year or so sooner, I think the 40mm stroke engines are still being made as always but for some reason people must be getting a better deal on the 38mm stroke engines and we all know that vendors are gonna sell what they can make the most profit on normally.

Duane told me that he can still get the same well balanced 40mm engines that he's been selling for a while now, but he has had request from customers to carry a well balanced 38mm stroke engine since some claim they rev faster and make better engines for their race bikes, can't testify to anyof that, thats just what I was told and since I truly believe Duane is an honest fella I have to think he's shooting straight with me on that.

He said he is debating on going back to the 40mm stroke engines but just wanted to see how people would like these GT5 type engines since he has had some request for them.

I guess time will tell, all I know is that there are ebay sellers who say they're selling 40mm stroke engines but they're not, I found that out when I bought my last kit.

YesImLDS said:

My previous engine which Map actually has now was a GT5, the mods that were done to it were literally a NT speed carb, SBP expansion chamber, ported intake and exhaust and port match intake as well as exhaust manifolds. No other mods. The motor with a 36T kissed 45mph on flat, but that motor then had a retaining clip pop out and ruin my day and thus was sold to map.

I then bought a PK80 engine off of ebay and did the exact same mods. Port and polish, SBP expansion chamber, NT speed carb, port matched intake and exhaust manifolds, but this motor couldn't hit 39. Now I even milled down the stock head for a little bit more compression and still couldn't get passed 40. That motor now has a bit more done to it. It now has a fred head 6cc, manic mechanic intake, cleaned up transfer ports, AND a 75 gram piston (about 8 grams lighter than when it was stock) and even with all of that done as well with the 36T it was hitting 44mph.

Now maybe it was the fact that the GT5 had MANY more miles on it and was thoroughly broken in or it could be that it was just able to rev higher and easier. Now from my personal experiences I would say the GT5 is an engine that is able to rev higher and thus go faster. These engines are rev limited and it would just make sense with a shorter stroke you would be able to rev higher. Someone needs to get two completely stock engines and a tachometer and do a standing rev test and rev the crap out of them to see what they hit and see what it really does, but the math shows that a shorter stroke with less rotating mass will be able to rev higher.

Now the PK80 I have I will say has been modified even more and should be able to go quite a bit faster than my first GT5, probably because it now has a 32T.

Edit: I should say that the GT5 I got was also off ebay and was nothing special. Actually got it from the same selling as I bought the PK80 from.

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I've been wondering where yo bought the GT5 geometry engine I got from you, which ebay seller did you get it from? if you dont mind me asking.