First 400ex Rebuild Questions

[300exBorg]

I have a 2003 400ex that I want to rebuild in the upcoming months. Right now I would like to just start gathering parts and information. I would like to go with either a 416 or 426 but am not too sure of the pros n cons of each bore. I am pretty sure I would like to stay around 10.5:1 compression and not go too much more high than stock. I would also like to throw an aftermarket cam to help wake up the motor as well. I am thinking stage 2 hotcam but am open to pretty much any brand and stage. While I have the top end apart what things should I replace so I cut down the chance of having to take it apart again? I would like to cover all angles I guess you could say. I have no previous experience with working on 400ex top ends so I pretty much would like as much input on what I should replace and what brands to use. Also input on compression and if I should go with 416 or 426 would be great!

[Stickman400]

Well you are on the right track. Compression is where you make the power, the difference between 407,416 and 426 won't make enough difference to justify it. So here is a good list to go off of.

Wiseco 86mm(407) piston kit-$130 on ebay

Hotcam Stage 2 cam-$130 on ebay

Cometic MLS top end gasket kit-$30 on ebay

GT Thunder Head studs are good insurance-$50 from GT Thunder

CRF450R timing chain (basically a HD replacement)-$30-$60 depending on where you buy, ebay or motosport

New timing chain guides are probably needed to- $80 on ebay

Also may want to check your timing chain tensioner to make sure it's still working properly. It's $60 for a new one if it's bad.

That should be everything you need for a bullet proof reliable rebuild.

[300exBorg]

Thanks for the response, that sounds like a pretty damn good setup to me. I am going to start searching for all those parts ASAP. What about my valves? Should I do anything with them while I am doing the rest?

Another thing..... Will I notice any difference with say a 407 or 416 piston at 10.5:1 compression over the stock piston and compression? At that compression I would be able to run 93 pump gas just fine too correct?

[CJM]

You can safely run 11:1 compression with 93 octane, I been doing it for eons. Do the 10.5 or 11:1 build on a 407. Trust me, my friends is a 407 and mines a 426 and we have almost the same exact setup and mine isnt really that much faster.

You will notice a slight power difference based on bore size-but its all negligible really. You will notice a HUGE jump from stock to a higher compression. From stock to modded its a night and day difference. the only downside so far is due to heavier piston weights (wiseco mainly but the other makers have some of this issue) is more of a vibration-its noticeable but you get used to it.

You could regrind the valves, I do it everytime I have it apart. Nothing needed besides a valve spring compressor (rocky mtn sells a really nice tusk one), power drill and valve grinding compound you can get from any parts store.