OMC 350 Rebuild Questions
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=40509
Printed Date: October-07-2024 at 2:33am
Topic: OMC 350 Rebuild Questions
Posted By: Benni27
Subject: OMC 350 Rebuild Questions
Date Posted: February-08-2017 at 5:00am
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Hey Guys, I'm thinking about a rebuild of my OMC 350 because of bad Heads, low Compression and ~2500 hours on it. I took of the Heads an all assembly's and had a look at the Block number: GM3970020. Google told me that it is a GM 307 Block (5.0L?!) instead of a 350. Does someone have more information about the OMC 350? Did they build a 350 out of a 307 Block with 350 crankshaft an pistons? Boat is a Ski Nautique from 1979, but I don't know if its the original Engine. Color, Intake, Carb looks like origin... Thanks for the help Ben |
Replies:
Posted By: Duane in Indy
Date Posted: February-08-2017 at 6:01am
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Pretty simple math. With the heads off check the bore and stroke. Pi. (3.1415) X r. X r. X stroke X # clys. Don't have the correct symbols on my keyboard 3.1415 X radius squared X stroke X #cylinders edit: 305 and 350 have 3.48 stroke 305 bore is 3.736 350 bore is 4.000 ------------- Keep it as original as YOU want it 1978 Mustang (modified) |
Posted By: swilliams
Date Posted: February-08-2017 at 7:46am
Ben, you are correct about the block being a 307. If we had your model number, that would be helpful. Omc made a 230 hp that would correlate with your block number. I have a 260 hp 350 ci motor in my 76' Martinique. Most engine parts can be found as it is a GM product. Pics are helpful. I also hav e a service and parts manual that could possibly cover your model.
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Posted By: Benni27
Date Posted: February-08-2017 at 8:29am
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Here are a few Links. Some old pictures and new ones: https://www.dropbox.com/s/lo2xdhdf92o2j3e/IMG_9175.JPG?dl=0" rel="nofollow - pic1 https://www.dropbox.com/s/hgdlwjm22rsvk4k/IMAG0025.jpg?dl=0" rel="nofollow - pic2 https://www.dropbox.com/sh/b1ceg6cvkxejbzi/AADMBLhlOHWKyz5snRtYaWzFa?dl=0" rel="nofollow - folder with pics It should be the 260HP model. This is what is in the papers... Could you send me the Service an Partsmanual :)? |
Posted By: 8122pbrainard
Date Posted: February-08-2017 at 9:16am
http://www.marineengine.com/manuals/omc_inboard/" rel="nofollow - OMC manual ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
swilliams wrote:Posted By: KENO
Date Posted: February-08-2017 at 10:40am
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Duane gave you what you need to know for figuring the displacement. He gave you numbers for a 305 chevy instead of a 307 chevy though.  A 307 is 3.87 inches bore and 3.25 inches stroke. Stick a ruler across the bore for a quick and dirty check. 4 inches would be a 350 3 7/8 inches would be a 307 |
Posted By: Benni27
Date Posted: February-08-2017 at 11:44am
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Quick and Dirty Check was: Stroke ~3.25 Bore ~3.87 So it must be a 307... Heads are 3973487X 76cc from a 350? I'm a little bit confused, maybe I should order the Parts Manual at marineengine. What's the best adress to get parts from? I thought about summit? Thanks :) |
Posted By: swilliams
Date Posted: February-08-2017 at 1:43pm
| I can't send you the manuals but let me see if your model number is included in my service manual. From there I can tell you which manuals to look for. |
Posted By: 8122pbrainard
Date Posted: February-08-2017 at 3:31pm
Steve, Nice gesture but beware. Here's the notation that marineengine.com has on their OMC manuals: "† Please Note OMC created new manuals each year which are specific to various models. When ordering a factory manual you must include the year, horsepower and/or displacement, and most importantly the OMC model number of your engine." Your OMC is a 76 and Ben says his is a 79. ------------- /diaries/details.asp?ID=1622" rel="nofollow - 54 Atom /diaries/details.asp?ID=2179" rel="nofollow - 77 Tique 64 X55 Dunphy Keep it original, Pete < |
Posted By: swilliams
Date Posted: February-08-2017 at 6:38pm
| Pete, my manual shows a long list of model numbers that it covers. If his is there, then we are golden. I believe my parts manual is set up the same way. M service manual does show a 350-4(which his is) and the specs for it. We'll get him there! |
Posted By: swilliams
Date Posted: February-08-2017 at 6:45pm
| Ben, can you provide me a pic of your raw water pump? Just curious if its the same as mine. I know OMC made a change in them after my year but not sure when. Finding parts for mine is tough. Thanks, Steve |
Posted By: KENO
Date Posted: February-08-2017 at 7:22pm
Sounds to me like you should be looking for a 307, since that what his measurements taken earlier today and his block number says too. You even said 307 earlier |
Posted By: TRBenj
Date Posted: February-08-2017 at 7:35pm
| 307 wouldn't have come in a '79... 305 took its place several years earlier. Exh manifolds look correct for late 70's, decals suggest a 307 block took the place of the original 350 at some point in the boats life, which was dressed back up with the original 350 parts. Just a theory. |
Posted By: swilliams
Date Posted: February-08-2017 at 9:27pm
| I agree with Tim. My service manual does have your model number listed but the general specifications say it is a 350. Your block number is definitely not a 350. Do a search on the head casting numbers. Bet they are not for a 350. |
Posted By: KENO
Date Posted: February-08-2017 at 10:28pm
Is that bet just with Benni27 or can I get in on it? Chevy literature says the heads are for a 71-72 350 and they're a pretty good set of heads. Benni has already looked this up by the way. He mentioned that they're 350 heads a few posts back. The block is a 69-73 2 bolt main 307. This info probably pertains to auto engines and we all know that marine conversions can be a mix of different parts The OMC model number says it's a 350 4 barrel like you already pointed out. Model year 1978 according to Marine Engine.com. That's hard to explain. I see from Benni's pictures that it is a RH rotation engine. |
Posted By: mark c
Date Posted: February-09-2017 at 5:16pm
| Heads are a 71-72 350 Chevy Head 76cc combustion chamber with 2.02/1.60 valves. The heads with the same casting number without the X suffix have 1..94/1.50 valves. Originally came on the LT-1 engines. |