top end swap??? , xs360 and xs400 top ends

[Ndowney26d]

Hi guys
I'm new to this site, so if someone has already covered this I'm sorry.
I race vintage flattrack 0-400cc class.
I was thinking about putting together an xs400 to run in this class. after some looking around on ebay, I found an xs360. I thought it might be cool to get that just because not may people have seen an xs360, let alone see one on a race track. but I don't know how hard it is to find parts for, so .....
my question is, if I have an xs360, can I later on, put a xs400 top end on it???
and also, I see Megacycle makes a cam for the xs400, could I use that cam in the xs360?
I would think I could do both of these since the stroke is the same in both engines.
Correct me if I'm wrong, but I think the bore size is the only difference??
Just looking for some advice guys.
Thanks
Nick

 

[volcomskater77]

Yes, the top end will switch over with no issues just make sure you get an xs400 gasket set other wise the bore on the head gasket will be too small. You are correc on the bore being the only difference. I have an xs360 bottom end if you need parts, as well as wiring bit and other misc stuff for it. I believe the gear ratios are different between the 360 - 400, I could be wrong. Download the PDF manual off this site and make sure. As for the cam, it will work as far as bolting in with no issues, but it will probably require more fine tuning due the the difference in displacement. If it were me, I would check out the gear ratios and if they're different, buy a parts engine and maybe swap some stuff around for a more track oriented gearing, put the 400 top end on it and let the head breathe a little better. Thus what I regret about my build, I never did any head work.

 

[Ndowney26d]

Thanks
That is exactly what I wanted to know

 

[xschris]

Use the xs400 cam. The xs360 one has shorter lobes.

 

[Arrowrl206]

So could I use a 360 head with a 400 cam?

 

[xschris]

It should work but have never done it. Are you also using xs360 block/pistons? If so I am not sure about valve clearance. Most replace the entire xs360 top end with an xs400.

 

[Arrowrl206]

Well I have a 77 360 and an 82 400. Neither run. 360 has really low compression and the 400 I think has a head problem. Loud snapping sound happened every once in a while and only get exhaust out one pipe when turning it over with the electric start. So I thought I could just swap heads and get a new 400 cam and call it a day

 

[xschris]

I know the xs360 head gasket is too small for an xs400 . If the xs360 has low compression then it most likely needs pistons and rings, but could be valves. I would pull both motors apart and see why they will not run. Putting bad parts with bad parts won't get you a good motor.

 

[Arrowrl206]

All very true. I was planning on purchasing a whole top end gasket kit regardless with the possibility of rings and pistols regardless of condition

 

[MerlinDudley]

DON'T just change the camshaft, the lobes are higher, and you run the risk of bending your valve stems, just by turning the engine over! Is it worth ruining your engine just to save a few quid, and a few hours in the workshop?

DON'T DO IT! Swap the top end over, and ALWAYS err, on the safe side!

And good luck with your racing!

Merlin

 

[Arrowrl206]

Well I've got this book (binder really) with all the specs I could want. And from what I gather from it is the only difference between the engines is bore size (66.00-66.02 mm vs. 69.00-69.02 mm), piston oversizes (66.25, 66.50, 66.75, 67.00mm vs. 69.25, 69.50, 69.75, 70.00mm), camshaft intake lobe service height (38.70mm vs. 39.38mm), exhaust lobe service height (38.74mm vs. 39.42mm), intake lobe base circle service limit (32.08mm vs. 32.12mm), and the exhaust lobe base circle service height (31.90mm vs. 31.97mm). Rocker arm bore diameter, shaft diameter, shaft to bore clearance, valve seat width, stem diameters (intake and exhaust), valve to stem clearance (intake and exhaust), springs free length and tilt replacement limits, and even crankshaft specs are all identical. Not sure if any of it really means anything, but could it?