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Pontiac Forum: Engine Mounts
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  #1  
Old January 16th, 2018, 01:11 PM
Randy68 Randy68 is offline
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Engine Mounts

I have a 68 400 bird, I pulled out a 1975 400 engine and putting in a 1971 400 built to a 461 stroker.... All the info that I have read says the blocks are interchangeable that have the 5 mounting locations. The problem is when I put the 461 in place it doesn't line up with the engine mounts and tranny crossmember. I fought with the engine mount bolts lining up and then saw the tranny crossmember was off 3/4"... You can force into place but then the front of the engine is off 3/4" off the center of subframe... I have new SS motor mounts from Ames and checked everything that I know. When I bolt the pass side of the engine and then the tranny to the crossmember, the drivers mount is about 3/8" back from the frame mount... The subframe is also new. Im lost!!!
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Old January 16th, 2018, 01:35 PM
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melsg5 melsg5 is offline
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sounds like you have a mash up of years 68, 75, and 71. Read the info below from Wallace racing and look at the adapters from Butler or contact them.
Starting in 1970, most blocks were cast with 5 motor mount holes with the mounts using 2 bolts in line with 1 higher up in the middle.Some 1977 and later blocks only had 3 holes that used the newer mounts. This means you can put a 455 in about any Pontiac car with that car's motor mounts, but if you want to use a 428 block in your 1970- Firebird you'll have to modify the frame mounts or make an adapter.

https://butlerperformance.com/c-1393...-adaptors.html
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Old January 16th, 2018, 07:08 PM
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Anouther thing that can throw off the power train alignment in a 1st gen Firebird, the Pontiac V8 engine was off set about 3/4 of an inch to the passenger side to clear the steering linkage, the Chevy small block equipped Camaro's had the engine centered in the chassis (I don't remember how the BBC engines sit) So a Firebird V8 trans crossmenber has the transmission mount off set towards the passenger side the same amount. A Camaro or Straight 6 Pontiac engines transmission crossmember have the trans mount centered on the trans crossmember.
You may have the correct motor mounts with the offset, and either a chevy or Pontiac OHC-6 trans crossmember. This will have the engine/trans sitting strangely with the front of the engine pointing to the right (passenger side), and the rear of the transmission pointing to the left (drivers side).
I have seen it several times, usually when a OHC-6 engine is replaced with a Pontiac V-8.
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Old January 17th, 2018, 11:23 AM
Randy68 Randy68 is offline
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I think that is exactly whats going on... With the engine sitting in the frame it appears the engine is pointing to the passenger side and the tranny is pointing toward the drivers side.. The crossmember is new to accept the 400 TH tranny but its centerline mounting of the frame, forcing me to push the tranny to the passenger side. So would a correct crossmember be 3/4" o0ff the centerline towards driver side for the mounting? If so then another question is wouldn't that throw off the entire drivetrain sitting at an angle with the driveshaft / rear differential connection to the tranny? Thanks for all the info so far!!
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Old January 17th, 2018, 03:48 PM
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It the new crossember for a Pontiac 400 TH and is the 400 TH for a BOP and not Chevrolet?
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Old January 17th, 2018, 05:18 PM
Randy68 Randy68 is offline
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The new crossmember was purchased through AMES or Classic Industries ...and was ordered for BOP .. it didn't come with part #s to verify. However the mounts are dead center of the crossmember. It has been a little trying because everyone thinks Camaro and Firebird are the same car.. there are a few differences... I have been speaking to Butler Performance as well and as CAMMERJEFF said the Pontiacs are off 3/4" So it appears as if I have the wrong crossmember. It just bothers me that if this is the case the engine/tranny wont be center line with the drivetrain and the horsepower 522 and 586 torque will tear things up. It almost looks like a better solution would be to modify the drivers engine mount to have it sit in there straight..
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Old January 17th, 2018, 05:20 PM
Randy68 Randy68 is offline
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Also the tranny is a 400 TH BOP... it has the dip in the housing... and is matched to the engine.
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Old January 17th, 2018, 05:53 PM
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you had a 75 400 engine in the car and I assume it was in there properly aligned with it's transmission. What transmission was in the car and what did the crossember look like, was the mounting bracket centered?
Do you have a receipt from the vendor with the part# on it?
In worst case can you relocate the bracket on the cross member?
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Old January 17th, 2018, 06:37 PM
Randy68 Randy68 is offline
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The car had a 75 400 with 350TH all Pontiac... as for the crossmember it had to be changed from the 350 mount to the 400 mount because the mounting is more to the rear of the tranny of the 400. I cant answer if it was straight when I took it apart but it did come apart really easy.. From all my research running all the numbers and I went with this block and tranny out of a GTO and had it built to a 461 stoker.. So the engine and tranny are out of 71 GTO and should be interchangeable. I can go back and look at the receipts... I seem to always get "Firebird and Camaro are interchangeable" Drives me crazy! I thought about relocating the mounting bracket but I just want to make sure that I am not missing something...So I think I am down to two options... I have the wrong motor mounts which there should be a left and a right or I have the wrong crossmember. I really appreciate all the help!!
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Old January 17th, 2018, 07:06 PM
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You are correct that the entire drive train on a 67-69 V8 Firebird is offset 3/4' to the passenger side. It does not seem to effect how the driveshaft to rear axle operates. I know the driveshaft cannot be straight or it will vibrate, all transmission output shafts and rear axle pinion shafts are slightly out of alignment to avoid this vibration.

I agree on all replacement parts being marketed as "fits all chassis" This was not the case a lot of the time before the mid 1980's. I would wager that the new replacement TH400 crossmembers are engineered to fit 67-69 Camaro's, and just assumed to fit the Firebirds.
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1978 Catalina Safari Buick 350 TH350
1978 Sunbird Safari Buick 3.8 2004R
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1973 Lemans Safari 400 4-speed (factory)
1971 Catalina Freeway Enforcer 455 TH400
1969 Firebird Sprint 301 OHC-6 T-5
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