Just pulled my engine outta my car last night and realized it has a one piece rear main. As far as I remember GM didn't start putting the one piece rear mains in our cars until '87. It's got '86 style heads on it though. I believe GM changed those as well in '87 but mine are '86. I guess it is possible that the previous owner put a new engine in it and used an '87 block with the '86 heads. I've always had a feeling that this car was previously wrecked... at least in the front. It's got the pre-88 TPI on it too.. with the cold start injector. This is engine is just all mixed up. Any comments are welcome.

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1986 Trans AM
305 TPI
Well over 200,000+ miles (speedo/odometer non-funtional! Odometer reads 142,000)
4 Wheel Discs
9 bolt Borg Warner Rear (2.77's....oh joy) :P
Completely Stock
Current Project: upgrade to a 355 TPI, 6" rods, 9.7:1 Compression, Stock TPI ported, 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech hot coil.

 

It was actually mid-year 86 when they started the one-piece rear seal.

 

Many 86's had the one piece rear main seal, and side-bolt valve covers. 86' seemed to be a mixed up year. Optional L69's that couldn't be ordered, half serpentine/half v-belt, 1 piece or 2 piece rear main seal, computer change halfway through model year, bad PROM chips. I'm not sure what GM was thinking.

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89 Transam GTA, 350TPI, auto, 3.27 rear, dual cats, gray.
Magnaflow cat-back,
K&N open element,
Best E.T. 14.63@95mph

 

Well, that would explain it then. Just to add one thing to your list, GM also changed the transmission midway through the 1986 production year. That was a good change though because the newer 700R4 that they changed to was a stronger transmission. My 1986 305 has the TPI with the cold start injector, one piece rear main, perimeter bolt valve covers, V-belts on the smog pump (eliminated) and the power steering pump, and a serpentine belt on the alternator. Talk about mixed up. Good car though, hasn't done me wrong, just got old. Now I've gotta go buy a new flywheel and starter for my new two-piece rear main 355 that's goin' in there. Yay!


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1986 Trans AM
305 TPI
Well over 200,000+ miles (speedo/odometer non-funtional! Odometer reads 142,000)
4 Wheel Discs
9 bolt Borg Warner Rear (2.77's....oh joy) :P
Completely Stock
Current Project: upgrade to a 355 TPI, 6" rods, 9.7:1 Compression, Stock TPI ported, 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech hot coil.

 

Hello, i have a 86 iroc with a 305tpi. it also has a one piece rear main seal (just rebuilt the whole motor, and put in a new comp cam)late 86 iroc...145 speedo, and the trans has a built date of may of 86. best of luck, tom

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86 iroc 305tpi auto. mild cam

 

In 1986 GM went with the one piece rear main seal. In 1987 they switched to roller cam blocks and center bolt valve covers.

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1988 Formula 5L/5sp 3.45 gears, SLP cold air kit, MAS gutted, TPIS air foil, TPIS AFPR, MAC headers, Mac under drive pulleys, MAC cat back, Relocated MAT.
Best ET street tires 13.85
Best 60 FT street tires 1.930
Best MPH street tires 99.29

 

Some of the 86's still had the two piece rear main though.

 

Supposedly. I'm still waiting to see one.

 

I've never seen one either. That's just what I've heard and read somewhere.