# 455 block and head number questions and transmission mount



## gjones (Oct 8, 2012)

I absolutely understand that a th400 is far superior to any 2 speed transmission, and I've had several th400's over the years in Cadillacs, Chevy's, etc..., but remembered the old gearheads actually liked the powerglides for the torque? (maybe I was just dreaming). Having resurrected this Lemans slowly for the past straight year, I guess I am getting lazy, and was hoping to just slap the 455 right in, bolt the existing exhaust manifolds on to avoid flange swaps, and attach the torque converter to flywheel bolts to avoid the trans. mount and driveshaft thing. (already been there,done that-"we do it nice,'cause we do it twice"). What about the driveshaft yoke? Is there a common swap for this part? Also, this 455 was sold to us as a 1975 motor, however, the #'s on the distributor pad say do51, with a large 71 next to the dist. hole.(is this 4/5/71 ?). Near that on the right rear is # 485428. On the front right of the block are #355304 yc. I've tried to look these up, but have had limited success. Heads are 4x with # g203 (7/20/73 ?). Cylinders are 30 over with flat top pistons, with stock cam. What kind of hp roughly would this put out? Thanks very much. More questions to come...


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## 68greengoat (Sep 15, 2005)

You're correct on the date codes. '71 motor and '73 heads. That motor originally came with #66 heads which is comparable to the 4x except the 66's had slightly bigger exhaust valves.

YC	A-body	71	455	*325* Turbo 400	L-75 1x4	*485428* 2	GTO
YC	B-body	71	455	*325* Turbo 400	L-75 1x4	*485428* 2 
YC	B-body	71	455	*325* Turbo 400	L-75 1x4	*485428* 2


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## gjones (Oct 8, 2012)

That's good news for me! Thanks! So the 355304 yc means it was originally was installed in a GTO? What does L-75 1x4 stand for? Is this a HO engine? I've tried to look up the 355304 yc number through old threads-someone else had a similar problem, with very conflicting results. I've tried other sources, with dismal results. THANKS!!!


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## 68greengoat (Sep 15, 2005)

Very good news. '75 hp ratings were in the toilet(and they didn't make a YC in '75..). A YC was installed in GTOs and B bodies(Grand Prixs etc......). 355304 is the unit number which would match the PHS if it was the original motor which it's not. So, it could be either. 1x4 is a single 4 barrel carb. L-75 is the RPO#(Regular Production option). I believe the L-75 stood for a 455, 4 barrel V8. 

It is not an HO. They were WC, WL and YE blocks with casting #483677.


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## gjones (Oct 8, 2012)

excellent info. A '71 455 had a base hp of 325, I believe.(I hope). With 30 over flat top pistons in her, and most all other components stock and at specs, what kind of hp boost would that equate to? It's not like we're going to the drag strip with it every weekend, but want that punch out of it on demand. Only paid $900 for engine & th400. , and helped the guy pull it out of a '50 Merc. Runs great. In the process of stripping paint & priming the block & heads now, then sticking it in the 66 Lemans convertible. The old 326 is tired and in need of a complete rebuild.


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## BearGFR (Aug 25, 2008)

gjones said:


> With 30 over flat top pistons in her, and most all other components stock and at specs, what kind of hp boost would that equate to?


A +0.030 455 works out to 462 inches. Expect the HP boost to be in the single digits - nothing that you'd feel or would show on a time slip.

Bear


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