New Crosman 1377 are under power - here's how to fix them.

[barry32]

Ive not been a vistor of the pistol section for a while but im glad I did now intresting read. I have a old 1377c the ones that came in the box instead of the plastic I think its around 4.7ft1bs with heavy pellets. I am at some point going to add a steel breach flat top piston and one of them brass scew cut front sight jobbies to it but the bank balance dont seem to be working out at the minute its always in the minus. Good bit of info thanks all

[Biker_Bob]

I've received a bunch of the GMac brass transfer ports.
They're 1.6mm bore, which is smaller than even the Canadian spec restricted ports.
The 3mm port I've got in is perhaps a little much, the power is up in the high 5's
So I'm going to go to 2.5mm as a starting point and see what happens.
The GMac ports are brass so are easy to drill out. I'll just use my drill press, rather than the lathe.
I'll post when I've got test results.

[airgunpete]

Underpowered or not I just use mine for punching holes in paper and knocking field targets down, I'm very happy with it.

[TonyL]

Will keep a lookout for your findings, Bob.
And I will be in touch at some point re meet up, bit busy with loads of things at the mo'.

[Biker_Bob]

I ended up with the new tuneable transfer port drilled to 2.5mm.
That gives 4.5 ftlbs, I'm going to leave it there. 3.0mm is too big and I don't have a 2.7mm drill bit!

[harvey_s]

I know this is an older thread, but I have been hankering after an ultra carbine for while and so bought myself a 1377 recently and sure enough my one was a Canadian restricted model as well and this thread has served me well in derestricting it.
(If you've got access to a M2.5 tap and screw - you can tap the hole without drilling it out, wind in a suitable screw with some studlock, let it cure, cut it off and carefully dress it flat - after all you're never going to take it out again...unless you emigrate to Canada.)

That said, I do have very different findings regarding the transfer port I thought I would pass on just in case others find them useful....

Firstly, I haven't come across anyone claiming to have a gun without a substantially modified valve anywhere near the legal limit - so I'm pretty confident that just experimenting with the transfer port won't give an over the limit gun without other modifications...

After plugging the hole in the back of the valve, I followed Bob's final drilling advice for the transfer port (2.5mm) and ended up just like he did at around 4.5 ft/lbs....a tad low for my mini-carbine I thought so...

Unlike Bob I do have a 2.8mm drill and so I thought I'd try that....Result! 5.25 ft/lbs

Now this is where it gets odd...I thought I'd open it out a bit more after all the US standard transfer port is 3.8mm and they are not over the UK limit with these fitted - so out comes the drill again and I thought go easy and try 3.5mm first - Bugger! just over 5 ft/lbs....

Next comes a call to GMAC for a full size US spec transfer port and one of his brass restricted ones which I opened back up to 2.8mm...

Result with US 3.8mm port - 4.7 ft/lbs

Result with 2.8mm port - back up to 5.2 ft/lbs again

So, this is where I'm slightly confused by Bob's findings as I haven't found anyone who's come close to 5.7 ft/lbs with a standard gun/transfer port....4.5 - 4.7 ft/lbs seems to be the norm.

Secondly, I know its a generic transfer port and shared with 22xx series of guns, but I'm now of the opinion that the port is optimised for those guns and Crosman just rationalise their stock by just keeping one port to fit all - but my findings indicate that it's too big for the 1377 and a smaller 2.8mm hole actually gives more power in an otherwise standard gun.

Might be worth knowing before you spend any money on that 'Boss' transfer port as it will probably lose you power without making valve mods first....

[Adam77K]

Very interesting and a bit weird findings. I've heard of wider transfer ports being sub-optimal with springers but not with pneumatics. Could it have been the lost volume perhaps? I assume the valve is standard apart from plugging the vent hole?

[harvey_s]

Yep, all standard apart from the plugged vent hole in back of valve and the addition of a small o-ring seal on top of the transfer port (present during all testing)
The results were repeatable, so I'm sure its not a glitch in the measurements...