How big of a sin is it to use an SP1 upper, un-modified on a 601 clone build? I hate to file on a nice Colt upper that I have, but also want it to look good. I know it will pass at a glance, and that MIGHT be ok with me. Guess I'm looking for someone to talk me into going all the way. I would also have to remove the CM forge marks on the side of the handle, then find a friggin sight without the numbers etc,etc,..........crap, I think I already know the answer. I need help

I wouldn't do it if it was a nice part.  I'd wait until I found something that had been damaged or already modified by someone else.

Well there are a lot of SP1s out there so, to me it isn't a big sin. I however would not modify one. These rifles are great, but from what I have observed they don't sell particularly well. Therefore if I were to sell one it would be a part out. I remember seeing a couple extremely nice 601 builds languish on gunbroker because no one would pay the price, you just can't get back what you have into them if you sell as a complete rifle. Part it out and you can get your money back or likely turn a profit.
What I'm getting at is that if and when that upper hits the market again it will be worth more as an unmodified SP1 upper over a fake 601. That's how I look at.

I don't have a complete SP1, just a stripped upper. The Nodak boys aren't making anymore slicksides from what their site says. Are there any other options for an authentic looking upper?

sorry that was erroneous. Nodak is waiting on forgings and will be making more probably Summer this year. Someone told me that they were no going to make any more. Ok so besides SP1's and Nodak are there any other options for a slick sided upper?

Totally agreed.

Fulton Armory made/sold slickside upper receivers.

Agree with M1carbinekid for using pristine authentic pieces. Keep in mind people have been using SP-1 receivers to do exactly what you are describing for 01-02 and even early 604 clone builds for quite some ,so definately no sin.  Most have sold or given away once an authentic piece is obtained.  It's the small stuff that bugs most retroheads regardless of model it's the attention to detail that sets the builds apart.

NDS when available is an excellent option but probably still less expensive initially to use a Colt.  When modifications are included to a SP-1 the NDS is a no brainer for detail accuracy,  plug and play and most end users are less likely to unload these also.  

Cutting or milling for a flat top might be considered a sin depending on attention to detail and particular build desired.  Not much choice if early match style slick side flat top from carry handle upper build is desired.  

Usually best to find a well used piece that is never going to have any real collectors value IMO if looking to modify but we are seeing most all the original slickside receivers bringing daily high pricing increases including arsenal rifinished.  Homebrewed and painted pieces are also climbing just not as fast a pace.  Long gone are the surplus pieces on the down low.  Might see some surplus from time to time but not enough to effect much over any length of time and those acquiring are still wanting top market price unless one has something they want, kinfolk or such.  

Probably gonna be plenty of this stuff available in future as with a market there are usually people willing to invest for some of the share and if retro is what's selling then sellers need retro.  CNC sure has opened up a lot of closed doors.  Patience and retro are usually directly proportional.

I agree with that. I made those mistakes quite a few times for example i could tell some stories that would make most of you retro guys cringe they sure make me cringe

its funny I was looking at one of my post in the achrives where i posted a Colt  C MP Marked 1/12 skinny  barrel assembly flat ring and all  for $155  it took weeks for it to sell and I probably sold it for lower that than . if you saw something like that now it would be gone in 60 seconds It actually took over a month  to sell it for that . Now that same barrel assembly now goes for twice that

I actually took a 604 barrel back then and bored it out for .22 liner to make a dedicated .22 nothing wrong with the barrel other than the previous owner said the front sight was canted a little which is wasn't (maybe it was when it was in his receiver ) i got it for $90 in parts , I kind of cringe when i look at it now with the market full of dedicated .22 uppers .

It's yours.. Do what ever you want

I think I will build on the SP1 for now, as is, leaving the R-arrow and forge marks and finish alone. So it will remain a nice stock colt upper but have an acceptable look. When I find another option that I don't mind cutting on I will switch over. By the way, is the chrome Fulton Armory slick sided BCG a decent candidate, look wise, for the 601? and is anyone reproducing the 601 style rear sight wheel? Sorry about the rookie questions, I've only done an M16A1 and XM177 to date which are easy to make look right.

You should get in contact with John Thomas, at retroarmsworks.com  .

He does a lot of custom work on AR15's/M16 type's.  He does offer a service to Modify your SP1/604 (Slick-Sided) Upper Receivers to 601 style.

And he may even have a Reproduction 601 Sight Wheel available, Or he may know where to get one.

Just go to his web site and look thru the Gallery, then give him a call.  e does GREAT WORK at decent prices.

Good Luck.