M1 Abrams variants
 

General Motors M-1 prototype, 105mm gun, Deisel engine, 6 road wheels.
Chrysler M-1 prototype, 105mm gun, gas turbine engine, 7 road wheels.

"Block I, 1980"
M1 - basic Abrams, 105mm gun, based on Chrysler prototype radical new chobham armor suit giving us today's Abrams look. This is the "Block I," Large numbers built.
M1IP - improved, generally M1A1, but with 105mm in a new universal gun mount, transitional production to the Block II.

"Block II 1985"
M-1A2, 120mm smothbore gun, CITV, radical new digital electronics/communication system, US Army unable to get the bugs out. 330 or so built for Kuaiti Army. some numbers of older base model M-1s upgraded to 'A2 but never successful, withdrawn from service.
M1A1 - 120mm, new NBC system, exactly the same analog electronics as the base model M-1, sort of a M-1A-2 lite always intended to be upgraded to A-2 standards if the US Army ever got the bugs out. 1986.
M1A1(HA) - 1988, turret armor with DU inserts.
M1A1HC - post-ODS, M1A1 with M1A2 turret armor
M1A1D - proposed M1A1 with M1A2 digital electronics thus the "D."
M1A1AIM - Box stock M-1A1XX with digital add-ons to the analog electronics for ease of maintenance, other small improvements.
M1A2SEP - new armor?, looks exactly like M-1A2 but totally new electronics/software, not the same tank as M-1A2.

"Block III" Several prototypes built including:
TTS - 3 men in M-1 hull, unmanned turet w/120mm gun.
"Block III Prototype" new LV-100 engine which greatly changes the rear of the hull, new tracks, new skirts with frills to hide the roadwheel heat and radar signiture, Leopard 2 turret with new armor and 140mm gun.

Other prototypes include M-1A1 with Leo 2 Powerpack, one built and tested.

Re: M1 Abrams variants
 

M1A1HA, HC, D, AIM, M1A2, SEP - is the hull armor also improved compared with M1A1?

Re: M1 Abrams variants
 

The M1IP was an upgraded M1 but not an M1A1 with a 105mm.
M1IP had increased turret armor, strengthened rear hull, new final drives, new wind sensor, improved turret ammo doors, and incorporated the new bustle racks as a production item.

The M1A1, naturally received the 120mm, had increased and different turret armor from the M1IP, new CWS controls, NBC overpressure system, All main gun ammunition separated from the crew, new improved final drives (3rd variation), a new wind sensor (3rd variation), new MRS collimator, fitted to receive NBC warning system and roof built to receive the commanders independent thermal viewer (CITV).

M1A1SA (AIM v.1/2): fully loaded: 63100 kg turret: 24400 kg hull: 38700 kg
M1A2SEP (v.1/2): fully loaded: 63100 kg turret: 24400 kg hull: 38700 kg
So it seems that this variants got similiar armor.
U.S.M.C. M1A1(HC)FEP, similiar to M1A1SA.
The M1A1HA, sometimes refered to as M1A1DU or M1A1 Heavy, were sent to ODS with DU armor.

GENERAL DYNAMICS GREEK TANK DOES NOT CONTAIN DEPLETED URANIUM (January 18, 2001) General Dynamics' offer of main battle tanks, ammunition, and the lease of US Army M1A1 tanks being reviewed by the Hellenic Ministry of National Defense does not contain any depleted uranium (DU). General Dynamics Corporation did not offer the Hellenic Ministry of National Defense any weapons containing DU, and did not use DU munitions in any of the tests. Sales of main battle tanks and ammunition to US Allies - Egypt, Saudi Arabia and Kuwait - have never contained depleted uranium. While the US Army uses depleted uranium munitions and armor, however, it has never offered it for export with its main battle tanks.

Re: M1 Abrams variants
 

The M1IP, or as now known, IPM1, did not have DU armor nor any add on armor. It did have improved armor protection v KE on its turret front as part of its design.

All M1A1AIM are about the same, some have better armor than the others. No you can't tell by looking.
All M1s & M1IPs are "light armor", all USA M1A2 (withdrawn from service) and M1A2SEP are heavy. The exception is the Egyption M1A1s and Kuaiti M1A2s, these are beleived to be light.

The M1IP has heavier and different armor than the M1, and the M1A1 improved on the armor of the MIIP, and of course the M1A1HA improved on the M1A1. From there it gets really confusing because of all the rebuilds on top of rebuilds.

Check - http://collinsj.tripod.com/protect.htm

M1
Turret: 400
Glacis 400

M1IP/M1A1
Mantle & Turret: 450
Glacis: 350-490
Lower front hull:430-470

M1A1HA
Turret: 660-680
Glacis:560-590
Lower front hull:580-630

M1A1HC/M1A1HA+/M1A2
Turret: 880-900
Glacis:560-590
Lower front hull:580-650

M1A2 SEP
Turret: 940-960
Glacis:560-590
Lower front hull:580-650

Side Note on ammo:
M829 January of 1987.
M827 was never DU, was always tungsten, never fielded with US forces.
The M829A2 was designed to offer good performance against K5, the M829A3 is designed for potential future armor threats.

Re: M1 Abrams variants
 

Fabio Prado's Armor Site has a pretty good set of pages on all this as well.

Re: M1 Abrams variants
 

Always thought that was the case but thanks for the confirm.

Re: M1 Abrams variants
 

I was part of the last unit in USAREUR to draw original production M1's, the 2d ACR. Early production IPM1s used the same armor suite as the M1 with later production going to what became the original M1A1 armor package. Externally all the IP's came with turret bustle racks which were also retrofitted to the M1's. You have to look closely and have both IP variants side by side to see it but the later ones have the turret front panels extended forward to accommodate increased armor on the turret front. As far as I know, all M1A1 variants were fitted so as to be able to have a CITV installed later but never were, this became the defining visible characteristic of the M1A2.

Re: M1 Abrams variants
 

I thought that all the was now in service was M1A1AIM for National Guard/Reserve and M1A2SEP versions for active forces, not to count the thousands of older models in storage?

Re: M1 Abrams variants
 

Anyone have info on when the first CROWS was installed on an M1Abrams?

Re: M1 Abrams variants
 

Probably a CROWS was "field mounted" on a "TUSK" Abrams within a week of the CROWS system arriving in theatre *grinz*.