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Hi guys I really could use your help since I have zero knowledge on how to build a motor. I have my vac pay coming in Nov and I think its time I start a build, first step is to send my spare 350 out to see if I can build on it. Assuming the block is good to build on, what jobs do I want the machine shop to do? My thoughts are to have it bored to 0.030 for some better pistons since I have the big dished versions in there now.
Just a tad more information will assist in helping others address suggestions for you. Good luck!
Are you 100% positive the engine is an Oldsmobile 350?
What year engine?
What heads are currently on the engine?
Does the engine currently run?
What is the end goal application for the engine? Daily driver? Sport-minded street demon? Race build?
Are you aware of any previous engine modifications e.g. cam, milling, boring, etc?
What intake manifold is currently on the engine? OEM stock iron? Aluminum? Do you intend to retain the current intake manifold?
EDIT: Sorry, I missed your statement you want to go 0.030
There are some good pistons available in the 4.065" size from Mahle and CP for a hone to fit on a low wear block to save an overbore. Mahle are 10cc to work with early heads, the CP have a 1.3cc dish to work with #8 heads.
Yes it came out of 1972, it has the 7A heads, I bought that motor for 350 bucks because it had a 3711 alum intake on it and it was cheaper to get that than to buy new, Canadian dollars btw, I have that in my car now, I wont be using it on a rebuild, I plan on getting a rpm. The motor on the stand has a spun bearing so no I never ran it or seen it run when I bought it, got it for the intake only lol and if I can build on it its a bonus. I currently have a 350 with the same shitty pistons and same heads, I have done all I can do to it and its a fun car but I want something more, I had mark do my cam up for that low compression, changed the springs, lifters, quick fuel carb, headers, 2.5 exhaust with custom 40 on it, 3711 intake, 3.08 gear, 2500 stall, timing chain etc but I want more and there is no way to do that without getting rid of the pistons lol. I would be thrilled to get 350hp and 400tq on this build, but I'm starting from the ground up..I have a 3000 budget to start with and I hope to get the machine work done with that
Last edited by skyhigh; November 1st, 2020 at 12:19 PM.
There are some good pistons available in the 4.065" size from Mahle and CP for a hone to fit on a low wear block to save an overbore. Mahle are 10cc to work with early heads, the CP have a 1.3cc dish to work with #8 heads.
I think Mark quoted me 699.99 for the pistons and rings a while ago, and converting that to Canadian and shipping will be some really pricy pistons lol but you get what you pay for, I will probably go down that road. I dont need everything at once since this will be a slower build while I still have driver to use, well its time to put her away for the winter lol
What are your performance goals? Do you plan on drag racing? I would up the rear gear to 3.42 or 3.73. And 350HP is not hard to obtain.
Just a driver, no racing, but I wanna be able to plant it if need be lol. I chose 3.08 because I like to cruise, don't want the high revs. I had the 2.78 gear so the 3.08 with a 2500 stall is about where I like it but in hindsight I probably could have got the 3.23, oh well to late now.
Machine work, yeah probably a good chunk of that 3 Grand. I will probably be looking at a couple of grand with balancing but my crank is already ground. A large enough cam, matching springs and opening up the bowls with W31 valves will be needed for 350 hp. Sounds like a 2500 stall and 3.08 should work fine with a larger cam.
The size of the overbore will depend entirely on the condition of the block .
It could be .030 , .040, or even .060 . It all depends on the wear in the cylinders and whether the engine has already been bored oversize .
Take your block to your machinist and let THEM decide what oversize piston to order .
Ok, Well I will do an update when I find out how much it needs cleaned up if its good, It did have a spun bearing that I no of so I hope the crank is ok, than I guess I can talk pistons with Mark and what machining I will need. So you think the bill will be 3k without any parts? is any of that 3K cost any head work?
Absolutely, block wear will dictate the piston oversize. But a lot of the decent quality pistons have odd oversizes due to available ring packs. Mahle and DSS are both this way for the Olds 350. If they just bore it .060 oversize, it may mean cast 24cc dish pistons are the only option and back at square one. Most machine shops won't know what pistons are available they will assume this is a chebby with tons of pistons. Another thing to tell the machine shop is make sure they put .0028" minimum on the center exhaust guides if the crossovers aren't being filled. There have been way too many stuck valves on Olds heads due to tight valve to guide clearance on the center exhaust guides.
If they just bore it .060 oversize, it may mean cast 24cc dish pistons are the only option and back at square one.
That would put it in the 4.125" range for 425 pistons, which I think is one of the sizes Mark worked with Mahle to manufacture. Also a few 425 replacement pistons out there from other manufacturers.
That would put it in the 4.125" range for 425 pistons, which I think is one of the sizes Mark worked with Mahle to manufacture. Also a few 425 replacement pistons out there from other manufacturers.
True enough, just best to make sure the overbore matches the available pistons.
So I talked to my machine shop and he told me I can drop the motor off but its going to have to be a winter project cause he's busy, that work's for me cause I want to do a slow build anyways, so I told him I was gonna take the heads off etc and he told me to punch a mark on the driver side head, looking at the 7A head I seen a mark of 4A on the drivers head, is that mark unique on the front of the driver and what does that mean? Here what I will be working with.
Ok so 2 pistons have a little cresent mark like something hit it, flipping the heads over you can see what made the mark, the piston came up and hit the head, would this co inside with the 2 spun bearings the guy told me bout, is the crank ruined?
probably a 1972 since it had the 7A heads, I thought I read a while back that serial num on the block covers alot of years, got the rest of it busted down, pretty much every bearing was wiped or on the verge, alot of metal shavings, only where you seen it hit the head had a little scoring on the crank, rest was smooth, it might have even been bored before, I will get it to the machinist next week and wait for him to look at it..
The 398555 2 is the block casting number, which was used from 1968 to 1976. The engine serial number is stamped on a pad beneath the #1 cylinder.
https://oldsjunction.classicoldsmobi...sfaq/ofblk.htm This number should take the form of "31Mxxxxxx" where:
3 = Oldsmobile division.
1 = year of manufacture (8=68, 9=69, 0=70, 1=71, etc.).
M = location of manufacture (M = Lansing, B=Baltimore, X = Kansas City, Z = Fremont, CA, etc).
xxxxxx = last six digits of VIN of car that motor originally came in (original car's sequential production number).
Last edited by Fun71; November 7th, 2020 at 03:01 PM.
It doesn't really matter that much. All of the 68-76 blocks are similar and you know it has 7a heads. The VIN pad will just confirm the year the engine was manufactured.
That engine appears to have been worked on not to long ago. The valley pan area and heads are way to clean. When i opened up my original 72 there was buildup everywhere in those areas as well as plenty of carbonised oil near the exhaust crossovers under the intake
A quick way to tell if it has been bored over and to what size is to buff all the carbon off the top of one piston with a wire brush .
If the pistons are oversize , they will be stamped with size of the overbore . Like .030 or .060 .No marks indicate a standard piston .
Of course , if you have a 3 to 4 in . micrometer handy , you can know the exact size of the piston .
Ok I will have a look at the pistons just because id like to no, even though there junk lol, so its obvious the crank is gonna need to be cleaned up, does that mean standard replacement bearings wont do the job, sorry for all the newb questions but I have never worked on a motor before. Would you guys re use connecting rods? Mains etc or is just better practice to marry new parts together.
If you have a 3 to 4 in . micrometer handy , you can know the exact size of the piston .
Actually a 4-5 mic would be required.
Budget will still determine the parts used moving forward. But remember newer piston/ring styles may be more expensive but also offer HP and longevity benefits.
The piston and rings will be Mahle, I'm this deep into it now, so the modern piston and ring is a no brainer. All though I appreciate Olds Performance giving me other options I want to use Marks parts for my major pieces, that will include cam, lifters, springs etc.. Here a cleaned up piston
Last edited by skyhigh; November 8th, 2020 at 07:13 AM.
They feel smooth for the most part but I honestly have no idea what I'm looking for lol, seems tapered at the top a tad, smooth bore, and a notch at the bottoms
Last edited by skyhigh; November 8th, 2020 at 08:39 AM.
Those might clean up with a hone, should save a few bucks. Get some estimates for machine work from a couple of local shops. Just inform them there is an available piston in a 4.065" oversize, hone to fit. Do you have close up pics of the spun crank journal? Was it rod or mains? That crank looks fine from what I can tell,
Last edited by olds 307 and 403; November 8th, 2020 at 10:06 AM.
Thanks for the update.
if the journal is that bad then the rod is probably toast as well. They’ll need to cut more than a few thousandths off the cap and rod, making it that much shorter than the rest. All the more reason to do the new Scat rod as well.
Ok I had to go back and read the questions again lol, I'm trying to learn, I thought the crank was just a crank, didn't no about the different sections lol. So the bad one is I believe the Rod journal. Feels like I'm learning some **** with doin a tear down and looking at the parts, thanks. Good question Mark, I should have taken pics of everything but I didn't lol..Im not sure what pistons were in that particular spot on the crank but all the bearings looked shot on all of them
Last edited by skyhigh; November 8th, 2020 at 11:54 AM.
Did you keep the rods and caps together? Also did you keep or mark the position of the main caps? They all need to go back together the same way they came apart.
A thing to remember is this is an N crank, make sure if a replacement is needed, use the same, at least. The later light weight cranks are weaker and may be an issue at your power level.
November 1st, 2020, 10:13 AM
