Correct core ID for a 400 Muffler

[Slakewell]

What is the correct core size for a 400 or open class muffler?

[GMC]

The specs of the stinger can vary with models, pipe specs and porting.
A good rule of thumb is that it should be the same internal size as the stinger tube on the end of the pipe.
I just had a quick look and generally my pipes fall in around 25 - 29mm ID
Some are bigger for 465's etc

[Slakewell]

Thanks for that , I know it's hard to know for sure.
I'm just about finished my 77 KTM 250 only to be told I can now have a 400 engine that's just turned up.
I have all the parts for 250 and a lovely little DG muffler that's 25mm ID. I will now have to make another pipe for the 400 etc so I'm still unsure 77 KTM 400 are rare and wonderful thing thou and having a nice race ready one will be cool.

[Lozza]

Are you trying to achieve a specific goal?

[Slakewell]

Mostly if I can use the DG muffler as I have already modified it to suit the bike, as I haven't made the pipe yet I can make it fit the muffler.