Xj750 engine swap

I've got a 1981 seca 750 with 85,000 miles and a broken chain guide. Instead of fixing this I'm thinking of buying a used engine.

I found a 1982 xj750 from a Maxim with 10,000 miles. Will the Maxim engine fit on my Seca? Are they the same engines?

Thanks,
Rustee Gold
1981 Seca 750

 

The Maxim engine uses rubber mounts to secure the engine to the frame, while the Seca models did not use bushings (lack of bushngs makes for a bit more vibration transmittal, but helps stiffen up the frame as the engine now becomes an additional "frame member", adding rigidity).

So make sure you get the mount bushings, brackets, bolts, etc.

 

Thanks for the quick reply Chacal,

So if I understand you correctly - the frame of the Maxim and the Seca are identical. However the engines of these two bikes are not. The Maxim engine is narrower and in order to put the Maxim engine into the Seca frame, I HAVE TO have the rubber mounts... Did I understand that correctly?

 

>>>> Did I understand that correctly?

Well, yes and no. The frames are different in a couple of major and subtle ways to account for the differences and design/function of the bike ("touring" vs. "sport" riding). The engines are the same physical size, it's just how they mounted that differs. If you look at the upper engine mounting locations (they're the easiest to see, but the same applies to the other locations), you'll see rather large holes in the Maxim engines, since that's where the rubber insulators fit. A Seca engine will have small holes where a M8 sized bolt just passes thru.

So yes, you will need to have the rubber mounts and associated hardware (bolts, nuts, etc.) since those will be different. I believe that the engine mounting BRACKETS are different, too (the Seca uses the same brackets as the 650 Turbo). On the FRAME side of the upper engine mounting brackets, you'll continue to use the Seca hardware, since it's a different mounting configuration that on a Maxim frame.

 

Also the shifter covers are different but conveniently swap from one to the other

Maxim


Seca

 

The Maxim engine is also missing the starter, the carbs and the shift lever. Can I take those things from my seca engine?

 

Yes that too! Good catch........

Yes, they're the same.

 

To clear up about the motor mount differences: the case castings are the same. The Seca uses pressed in steel motor mounts, and the Maxim uses slip-fit rubber mounts over a steel core. To fit the Maxim engine into a Seca frame you will either, have the Seca mounts pressed out, and swapped over to the Maxim cases, or use the Maxim mounts (the lower mounts will need to be slightly modified to clear the frame lugs). I elected to modify the Maxim mounts, just because I have a angle grinder on hand, and would have had to borrow a press.

 

I've compared several of the engine components between the Maxim (XJ750) and Seca (XJ750R) and most of the engine components seem to be the same (using the diagrams on Partzilla.com). However, I was surprised that the Starter Clutch Outer Assembly was different on these 2 engines. Can anybody enlighten me?

My current plan is to get the Maxim engine which has 10,000 miles (with the frame stays, and mounting dampers and all bolts, nuts and spacers) and probably insert the Maxim engine into my Seca frame... but still open to the idea of cannibalizing the Maxim engine for parts to fix my Seca engine (which has a worn-out starter clutch and broken chain guide and 85,000 miles).

I'm also slightly concerned that many of the engine parts seem to be "unavaiable" but I think Chacal has them all (like the head gasket and piston rings)

 

They are the same part. One has a revised part number is all.
This is one of many inconsistencies in Yamaha's online parts lookup database.