4200 Turbo >> '51 GMC

Re: 4200 Turbo >> '51 GMC

Postby ca434sbc4 » Sun May 07, 2017 11:41 pm

I finally got all the rest of the projects cleared out of the way - if the weather is good I'm finally going to pull the 4200 out of the truck and fix the damage the old turbo caused and a few other items.

One item of interest is the the GM exhaust manifolds are cast steel - which welds quite nicely using a TIG.
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Re: 4200 Turbo >> '51 GMC

Postby ca434sbc4 » Sat May 20, 2017 8:04 pm

Engine is out and surprisingly showing no damage from the exploded compressor wheel.

The only visible sign is the cylinder walls are scored on the thrust side and pistons even more so. I only pulled #1 as it was the one hole leaking 20%. I'll change the piston and put new rings on #1.

The rod bearing looks like NEW - some of you may recall the issues I had with the late model crank and tearing rod bearings up after 1000 miles.

The 2 changes that I made were:
1) provide direct oil from the filter to the main gallery externally via an AN-8 hose.
2) change to an 2005 service crank that was cross-drilled from GM.

Apparently GM ran off a bunch of early cranks at the end of the 2005 engine run to test the cross drill set up to get ready for the 2006 build.

The 02 to 05 model cranks are fully counter weighted, in 06 it lost 2 counterweights. Based on the article on Ron Bonin I have to believe that the dynamic rod bearing load is a good part of the issue.

More to follow - I need to change out a couple exhaust valves that were not perfect when the head went together last time (2012).

The engine has 60+ dyno pulls + 15 runs at the track + 20000kms on this build.
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Re: 4200 Turbo >> '51 GMC

Postby ca434sbc4 » Mon May 29, 2017 11:56 pm

One issue I am having to deal with is head studs. When the engine was originally built I installed 1/2 ARP head studs, with the deck drilled, reamed and roll threaded to 1/2-13. The short coming is the ARP studs only have a threaded engagement length of 1 1/4" into the block. Upon tear down I found most of the head studs bound up in block.
What is happening the thread material is moving upward binding the threads.

The holes are tapped 1 3/4" deep, so I am waiting for ARP to get back with a cost on a custom set of studs, with 2 possible options:
1) 1/2 straight stud (1/2 in the block, 1/2 at the head). Down side is the head gasket needs to be modified to enlarge the holes. I've done it but it a pain in the butt to do.
2) 1/2"-13 x 1.75" in the block necked down to 12 mm - with a 12 mm fine thread at the top. No head gasket issues and better clamp load.

I should have costing this week.

Anyone interested in a head studs?

On another note I got hold of Cometic - as it stands now - they do not see enough demand to do a head gasket.
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Joined: Fri Jan 27, 2012 1:58 am


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