3a rebuild to 4a

How-to's and repair secrets for your 4WD can be found here. Have a question? Ask it in here!
Post Reply
jamiec
Newbie
Posts: 23
Joined: Wed Nov 15, 2006 3:51 am
Location: Tasmania Australia

3a rebuild to 4a

Post by jamiec »

Why not rebuild your old 3a engines to 4a size?
i have done just that lots cheaper then paying for
a 4a that cost alot more.you can sometimes get
3a engines for free.just have it bored out to fit 4a
pistons makes cheap 4a and still have the standed engine
block :D the first 4a engines were just overbored 3a blocks
anyway, thats what toyota did.the wreckers in Australia charge
lots for old 4a motors :evil: bored ours out +40 thousents over 4a
make it about a 1620cc :D
keith
Highest Ranking Member
Posts: 1249
Joined: Sat Sep 30, 2006 1:04 pm
Location: Tennessee

Post by keith »

That would be an option, there is a set of .030 over pistons and rings on ebay pretty cheap ($10 + shipping). But I just bid on a reman 4A.

I was not aware that Toyota's could take an overbore. Toyota engines generally have about the thinnest walls around.
Typrus
Highest Ranking Member
Posts: 3049
Joined: Sat Oct 23, 2004 4:43 pm
Location: Colorado

Post by Typrus »

Ummmmmmm.....

3AC- BorexStroke - 3.05x3.03 inch
4AC- BorexStroke - 3.19x3.03 inch
4AGE= Same bore and stroke as 4AC, and it reads 3.19x3.03.

Is .14 inch overbore really that good of an idea? Thats over an eighth of an inch.... We aren't talking 14 thousandths, we're talking 14 hundreths. 140 thousandths.
Just examined the block in the garage. I have this to say- If you do this, I will call you foolish. The cylinder wall can't be much over 1/8 inch thick. So by boring out a little over 1/8 inch, you'd effectively halve the wall thickness. If you look at the lip on the deck of the head, its much thicker, but you have to notice that the lip tapers off along the cylinder wall.
You know, 1/16 inch of cylinder wall might put up with 70 horses, but I doubt it'd go a whole long time. That, and none of us who does the swap wants to just settle with a simple swap. We want the carb. Some want headers, cams, porting jobs....

Just hit up your local scrappers they can't charge much over $100 for an old 4AGE or 4AC block, or maybe even whole engine.


77.5mm vs. 81mm 2.5mm. Doubt if thats safe.

Unless you could find 1mm undersized pistons. Doubt it though.
RIP 10-07- 1984 Toyota Tercel SR5 4wd Wagen 6 speed

RIP 04-05- 1986 Toyota Tercel SR5 4wd Wagen 6 speed

1st Terc- 1987 Tercel SR5 4wd Wagon 6-speed, Sadly cubed

1985 Tercel Standard 4wd Wagon w/ 3-speed auto, Living a happy life in Boulder last I knew
jamiec
Newbie
Posts: 23
Joined: Wed Nov 15, 2006 3:51 am
Location: Tasmania Australia

Post by jamiec »

well the overbore works very well i have done it with
bigger cam modifide head and exhaust. this motor
has been in every day service for two years then we
sold it.had done lots of offroad work and towing trailers
no problems.like i said before toyota did this to make
there first lot of 4a motors they were the same block.
i was concernd about the cylinder wall thickness to.
i had about 4 spare 3a blocks so could try boreing 1
cylinder to see. the engine rebuilder said it was fine.
it was bored out +40 thuogh = 1mm over the standed 4a bore size.
that motor was better than my standed 4a powered tercel. :D
yes i know the wall tapers off and i guess i must be foolish
but it works fine.
Typrus
Highest Ranking Member
Posts: 3049
Joined: Sat Oct 23, 2004 4:43 pm
Location: Colorado

Post by Typrus »

Yes, I would accuse you of being foolish for trying. If it worked, kudos to you, I must say.
So I suppose let me put in my caution. If you are going to try this, MAKE ABSOLUTELY SURE that there are no visible casting flaws in the cylinder wall outer surface. Any casting "dent" could cause some serious compromise.

Jeeze... I must say I admire your balls for trying a roughly 3.5mm overbore. Wow.

So then here's a thought- do the hyper overbore on the 3AC, then attach a 4AC head? Slightly better flow and the likes. Would need to be a later head though to make better power.

Well, I suppose I can see that.... 83-85 versions of the 4AC making 70HP and 85ft/lb versus later 74/86. Not much difference, but I'd imagine a "simple overbore" would be a little less efficient.

I suppose it might weigh a little less than a 4AC.

I dunno... It just makes me a little nervous to overbore 3.5mm.... "shrug" I guess if you kept it fairly conservative and kept it well cooled and lubed.....

Oil cooler, high volume oil pump, and that fancy electronic water pump in order? Heh.
RIP 10-07- 1984 Toyota Tercel SR5 4wd Wagen 6 speed

RIP 04-05- 1986 Toyota Tercel SR5 4wd Wagen 6 speed

1st Terc- 1987 Tercel SR5 4wd Wagon 6-speed, Sadly cubed

1985 Tercel Standard 4wd Wagon w/ 3-speed auto, Living a happy life in Boulder last I knew
ColSwede
Advanced Member
Posts: 61
Joined: Tue Dec 25, 2018 9:11 pm
My tercel:: 83 Tercel Wagon 4wd

Re: 3a rebuild to 4a

Post by ColSwede »

Does the 4AC or 4AGE bolt up to 6 speed manual 4x4 Trans? Same motor mount placements?
User avatar
BaileySims
Highest Ranking Member
Posts: 626
Joined: Sat Aug 30, 2014 6:52 pm
My tercel:: 1983 Tercel SR5 4WD
Location: Texas
Contact:

Re: 3a rebuild to 4a

Post by BaileySims »

4ac will bolt up no problem with no modifications, the 4age requires a ton of mods.
WAGON MAFIA.
Blood, sweat and beers.
The Professor
Highest Ranking Member
Posts: 650
Joined: Wed May 12, 2010 6:38 pm
My tercel:: 1987 Tercel SR5 4WD wagon
Location: Western Washington
Contact:

Re: 3a rebuild to 4a

Post by The Professor »

I would be interested to know the difference in thickness (if any) between a 3A and 4A cylinder walls. My guess is that larger liners were installed in blocks destined to become 4As, making the OP's mod that much less appealing.
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon & hardtop
1987 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan
2019 Subaru Outback 2.5i Limited
User avatar
Petros
Highest Ranking Member
Posts: 11930
Joined: Sun Jan 28, 2007 6:31 pm
My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: 3a rebuild to 4a

Post by Petros »

there was a technical service bulletin out warning dealers to watch for wall break through when overboring the 3a engine, the "fix" was to sleeve the engine bores. they made no service bulletins for the 4a block. so it appears some of the 3a at least will not tolerate an overbore.

though I do know of someone that overbored a 3ac to 4ac, but not recommended because of the risk. Also, the 4ac head, almost identical (same valve train and externals) has an improved water jacket for better coolant flow around the combustion chambers. Also, if you over bore that much and still us the same head, the compression ratio will get raised to above 12 to 1, which may be too much for pump gas.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
Post Reply