1968 50hp Thunderbolt

[nathanhooper]

So I am curious just how good a motor this is. I currently have a Chrysler. Long story made short is that there are quite a few things wrong with it. Scored piston, scored cylinder, the lower unit was leaking, just found out i am going to have to do some surgery to get the by-pass valve out and then try to clean that mess, so on and so forth.

I work with a guy that is offering me his 1968 thunderbolt, checked by serial number, for $200 bucks. If and should I take it, what would be the things that I should look more closely at, seeing how I was a complete idiot when it came to the current motor I have. Is there anything in particular that I should look at before buying? Does anyone have a manual that i can look at to get acquainted with the motor before buying. If I would have had the manual I do now for my Chrysler then i could have seen where there are certain things to watch out for, ie; discolored heads, ways to check lower unit for water intrusion, compression, by-pass valve, so on and so forth.

Thanks guys, give me your thoughts and idea on it.

 

[nathanhooper]

Re: 1968 50hp Thunderbolt

Bump, no one have any thoughts?

 

[Chinewalker]

Re: 1968 50hp Thunderbolt

Check the wiring harness, both inner and outer, very carefully. Mercury did not use a particularly robust wiring in their motors for a lot of years. The casing would rot and the copper core would fray. New harnesses are available, but at a price.

Also, Mercury at that time was still using carbon steel shafts in the lower unit. They plated them for corrosion protection, but the plating usually flaked or wore off with time and lead to rusting shafts. This was particularly troublesome at the upper gearcase seal below the water pump where water would puddle and eventually rust the shaft enough to allow oil out and water into the housing...

Outside of that, the 500s were solid motors. The 4-cylinder, 44-cubic inch engines are one of the best designs ever, and remained in production into the 1990s.

 

[nathanhooper]

Re: 1968 50hp Thunderbolt

Thanks for the info. He has said that the wiring harness needs replaced. What should the compression run on these?

 

[Aqualift]

Re: 1968 50hp Thunderbolt

I have two Merc 500's (1965 and 1968) and I agree with Chinewalker's comments. Compression numbers of 100lb's or more and your good to go. Close numbers between cylinders is what you want. I prefer 5% or less variation. (Some will accept up to 10%)
Great powerful and solid motor. Only downside, is that the parts are more expensive and harder to find compared to JohnnyRude.

 

[nathanhooper]

Re: 1968 50hp Thunderbolt

Yea, I started looking for parts for it and realized that it was not so easy when you drop below 1970. Does the 1968 parts line up with another year model above 1970?

 

[Aqualift]

Re: 1968 50hp Thunderbolt

There are a lot of different parts that are compatable for the 67-69 motors. Also, some parts from the 65-66 Mercs will interchange too. I swapped a fuel pump , for instance.

 

[CVX20SPRINT]

Re: 1968 50hp Thunderbolt

I've got one that I purchased from the original owner approx. 10 years ago.Never bothered to check the exact year as I really have had no need to.I've never bought any parts for it other than an impeller.I don't use it a lot,but it has never let me down.I was running it on a 15 foot Starcraft and it worked pretty well on it.

 

[tkq23111]

Re: 1968 50hp Thunderbolt

I have a 1985 35 hp johnson on a 16ft aluminun boat .It only goes about 10mph,I did a compreshion test and its good 110 on all.I read a little about it and came up with its got to be carbon build up.i looked in the cylender walls and its caked.I havent been on the water yet but I have done some procedureds i have read online.I have put seafoam into cylender walls for 10 min and turned by hand a couple roatations.then with plugs out i turned it over to blow it out.then i put fresh high grade gas in gas can and added good 2 cycle oil and seafoam into fule.I then put my muffs on and ran on idle for 30 minutes.Im hoping it helps.I would like to know the top speed I should get from the 35 hp,and a 50 hp thunderboalt 1968.I have a thunderboalt i can get cheap and looking for more speed but want to know what to expect before I buy .ty.If I buy the thunderbolt i got a spar that has a bad piston and i foud out the hard and expesive way that son of a _____.he said it was the wrong prop and when i get the right one it would go faster.lol so i will never buy a motor again without checking copresion.

 

[merc850]

Re: 1968 50hp Thunderbolt

You can go to a Mercury dealer and see what's available for your needs paint, seals etc. and if I were you I would replace the timing belt, they get brittle after 40+ years!
; these guys can help with a lot of parts too http://store.eastcoastmarineservice.com/category_s/854.htm. Also ebay.
And on ebay right now is a manual http://cgi.ebay.com/ebaymotors/1966...3274260QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear