My 55 ¾ ton truck is starting to look like a super heavy duty. I am in the process of refreshing the motor and was just trying to figure out what I had for sure. From the below information I have found that all the major parts are from the year or close era. It is a ¾ ton truck built in Kansas City. The truck appears to be untouched aside from an old repaint that also replaced the lower front fender corners and patched what seem to have been dual antennas on the fenders. The front clip was removed but not painted under. The cab corners look like there is a little putty on them but no welding. The bed looks to have newer wood and stainless bolts. Nothing else seems to have been touched.
While removing the drive line and such to remove the motor, all of the nut locking plates and cotter pins have never been removed and seem to be original. Even the motor mounts were in this condition. The truck is the 6900 GVW model. It has the HD 10 leaf rear springs.
Everything appears correct until I get to pulling the motor. The parking brake is the lever actuated version that the Factory Assembly Manual shows only coming in the 3800 1 ton. The hole through the tranny cover looks stock. The motor and trans is green under all the gunk like it is a Load Master. I had to remove the transmission cover plate to remove engine and trans as a unit. The shop manual says this only needs to be done on the 1 ton and larger trucks. Anyone know if this is because of the HD 4 speed trans or because the trans hole is lower?
The only difference between the Thriftmaster and Loadmaster I know of is the valves and piston rings. I don’t know how I would tell visually on the valves and I have not removed the pistons yet. The valve springs are a mustard yellow color. Not sure if that means anything about original or rebuild status.
Is it possible the Load Master and heavy duty trans/parking brake could have been special ordered or came stock in the 6900 GVW truck?
The 1 ton trucks had a GVW of 6200, 7000 and it was 8800 with dual rear tires.
I also have not been able to find out what the SA means on the block number. Anyone have a more thorough list of those codes?
Thanks, Mike

VIN: J255K018XXX
Load Rating Plate: 6900
Block Casting: GM 10 – 3836233
CON 3 D 1 5
Opposite side: S 233
Block Serial: 0651708F255SA
Head: 3835913
GM 24
Cam: GM 29
3836127
Trans: GM
591958
CON 5
1 E1O(5orS)

3836233 is the casting number of a 1955 235 block - only
3825915 = 1955 HEAD
3836127 cam = can't find that forging number at this time
591958 tans. case = Look again, its 591953 which is a "normal" 4 speed case from 1948 and up for 1/2 tons thru 2 tons.
The 1/2, 3/4 and one ton came with a Thrift master engine only and it was painted the "old" Chevrolet gray.
Transmission case was not painted.
The loadmaster was optional in the 1 1/2 ton and standard in the 2 ton.
The Loadmaster would have exhaust valve rotators. They COULD be installed on all modes. If the exhaust valves are original and it is a Loadmaster the letters XCR will be found on the valves (facing the piston)...That was the better valve. There again they could be used on a Thriftmaster. When I was in the parts Dept I stocked the XCR valves only as they were of the best quality.
Yellow valve springs were used on all 235 solid lifter engines in 1955.

Will be doing more research tomorrow.

I was wrong on the casting number on the trans. I had not scraped and brushed the gunk off enough. The trans case also is not painted. The bell housing and the outer bolts on that are green but while cleaning the trans case a little I have not seen any green, just cast. I can not find anything on the valves. The exhaust valves are pretty crusty but I cleaned a few enough that something should have shown up. If the marking was just a light stamp it might be found after some more wire brushing.
Thanks for all your educated info, Mike

The info in the shop manual that pertains to a 1 ton 4 speed and not a 1/2 or 3/4 ton is because the 4 speed was an option on the smaller trucks.

F255SA - 1955 2nd design 235 engine numbers ended in 55X according to my info. It could be a rebuilt engine. If so the pistons will be oversized and should have like .020 or .030 stamped on top of the pistons

From 1955 truck engineering-
"A new band type of parking brake is standard on 3800 and is included with the optional transmissions on series 3600"

parts book says 3/4 ton with 4 speed has same parking brake lever as all 1 tons.

shows rear spring bushings and shackle part as being the same for 1/2 and 3/4 ton trucks. Standard rear spring as having 9 leaves and are 2" wide. ! ton has 8 leaves but the leaf is wider. From what I gather a 1 ton spring could not be mounted to a 3/4 ton and they are 2 1/2" wide.

The exhaust valve rotators are easy to see. The cap on top of the valve spring is in two pieces.

That helps to know that the brake goes with that trans in the truck. The 10 leaf spring reference I found was in the load rating chart along with the GVWs I stated before.
Load Rating Chart

I will look at the head tomorrow to see if I can notice the turners. This engine looks less and less rebuilt the more I get into it. The locks on the flywheel bolts were intact and were not reused. The pistons are not marked on top. There are no marks that I can see on the rods or anything else. I think the head gasket looks original. The writing on the gasket is imprinted in the block. There is "GM" "This Side Down" and a small round fine print area that I have not tried to read.

Thanks again for all the info and putting up with another color discussion.

Mike

10 leaf 2 stage sprins were an option for the 3/4 ton.

The "GM this side down" was imprinted on the replacement head gaskets. Would not have been unusual to have had the head off and the valves ground by 50,000 miles. Also to have the rings replaced by that milage. If the rings have been replaced the new rings will look different than the production rings.

The 10 leaf 2 stage came with the 5800lb GVW. The 6900 came with a HD 10 leaf single stage. My truck has these springs to match the GVW tag.
Mike

I ordered the restoration pack from GM and found the SA engine code is in the 3600 with RPO 227. RPO 227 is the HD 11" clutch.

Mike