1974 Mercury 500 Thunderbolt lower unit swap

I have a 1974 I believe 50hp Mercury 500 Thunderbolt Serial #4073302. The Lower unit I want to use is from a 50s model 50hp mercury. Everything looks the same except for the replacement doesn't seem to have the Reverse shift cam or base on the shift rod just the bushing that has the slot thru it. My question is do I need to take the one off of the original lower unit or do some not use this cam and I cannot use this unit. Thanks in advance for any help.

Re: 1974 Mercury 500 Thunderbolt lower unit swap

If you do not have trim tilt you need the cam to lock down the engine in reverse. Just make sure both units are in the same gear and remove replace in the exact same position, Also as long as the splined area had the same amount of splines it shouldn,t be a problem, Some have a different configuration for the cam and you may find the right one on ebay

Re: 1974 Mercury 500 Thunderbolt lower unit swap

Maxz695, Thanks for your reply. Would it be possible to just remove the reverse lock cam from the bad lower unit and use it on the replacement. If so how hard would it be. Thanks again for your help it is really appreciated.

Re: 1974 Mercury 500 Thunderbolt lower unit swap

yes as long as the shift shaft splines will accept the cam thats fine. Be aware of the cam position and what gear the units are in. you cannot just put it on any way you feel it must be put in correct position for F N and Reverse. If not it wont lock the unit down in reverse. Pictures always help. if you can snap a picture of the replacment unit (Good one) and then a shot of the (Bad untit) where the cam is it would help some. I,m no pro at older newer models just basic information. There are some here who ca tell you if it is just an easy swap or if you need other things such as different shifter shaft. Drive shaft splines. (Must have te same length and number of splines and such. You,ll have to measure the shafts and count the splines to make sure they are compatable. The driveshafts may have the same number of splines as the prop shaft may have a different number of splines as they do change over the years. I,m just giving you basic info on the cam here. You have to do the homework

Re: 1974 Mercury 500 Thunderbolt lower unit swap

I have done some homework and think I can accomplish this. The issue I have come up with but can't find any info on is that it seems there is an insert in the cavity that the shift rod goes into. The insert makes the slot round whereas the original utilizes the slots on either side. I don't know if the insert is removable or not. I paid $100.00 on ebay for the replacement used unit. I did not realize the reverse lock cam was different (live and learn). I will post pics of the original and replacement. I appreciate the help greatly.

Re: 1974 Mercury 500 Thunderbolt lower unit swap

You stated in your first post that the replacement LU was a model from the '50s. Are you sure about this?

The early Mercs had a two piece metal cam setup as opposed to the newer single piece which your '74 would be.

Your pictures are too dark to show anything clearly, any chance of re-posting?

See items 16 and 17 in this link for the earlier two piece metal cam:

Mercury & Mercruiser Parts, GEAR HOUSING ASSEMBLY, COMPLETE (PAGE 1), MERC 500

Gerry

Re: 1974 Mercury 500 Thunderbolt lower unit swap

Carholm is very knowledgeable when it comes to parts engine types years etc. I have seen the newer type shift shafts and the cam that rides on them. many splines. I have also recently helped someone re adjust there cam and saw theres was more of a sqaure pattern, but the cam looked the same. I,ll look back and see what engine that was on. I think it was a 650. As Gerry said your pictures are to dark. and reposting a brighter image would be a big help. Also did you measure the drive shaft for compatibility and count the splines to make sure it will fit into the cranckshaft? Ebay according to the diagram search NOS Mercury model 350 and 400 lower reverse locking cam part 52918 Link may not work--->
http://www.ebay.com/itm/NOS-Mercury..._Accessories_Gear&hash=item2ec34cd08e&vxp=mtr View attachment 174836 View attachment 174837 I don,t know tht the hight of the cam would be the same as the old one but you can get plastic ploy washers at any lowes home depot to raise the cam if nessecary. Mine wore and I had to file it smooth. Then I PB welded the washer that fit right to the bottom of the cam and never had a problem with it. The alternative is swaping the shift shaft but you don,t know if there compatible and may be opening a can of worms there. There are not easy to get back in once removed from the lower shift shaft cam that changes the gears View attachment 174838

Re: 1974 Mercury 500 Thunderbolt lower unit swap

Thanks again for all the help. Well, I checked and everything matches up: drive shaft length, number of splines on drive shaft, shift shaft, and prop shaft. In the new pics it shows mine with the cam base (i removed the cam and made a not of how it should be positioned to go back on) recessed into the slot. The replacement has only the bushing which sit on top. If I just swap the base and cam there would not be enough shaft to connect and it would sit to high. Thanks again for all the help.

Re: 1974 Mercury 500 Thunderbolt lower unit swap

The cam I left the link for above is the cam Carholm left the chart numbers for and is the correct cam for that replacement LU. The cam is not as thick (Or tall) as the one from the oringinal outdrive. I believe if you get that one or another cam with them numbers it should work. See the pictures I left of the cam that belongs with the replacement LU. Then try to compare it to the one your trying to use. We can assist you in placement of the cam being Hi dentent towards the front point of the LU is for rev and must face the point when the LU is in rev. The lowest dentent wil be facing the foward most point of the LU when the LU is in drive. and the middle of the cam facing forward point is neutral. Prop locks both ways in rev spins freely in neutral and clicks one way and locks in the opposite direction in drive. You can use this info to make sure the unit is in the proper gear and set the cam accordingly to those points. On some engines the lock down pin that reaches down to the LU is adjustable but being the shift shaft seems shorter than the original may deem a problem. Measure the length up from the cam base on both and see if there is a difference. Then try to set the unit on the midsection to see if the unit will shift from the engine shift bracket. If the shift shaft does not reach the lower shift shaft without the cam on it then you may need additional parts. I,m thinking the cam is just to high. On the newer LU the shift shaft guide screw is set further down in the unit where the replacment LU you have is above the flush surface. I believe the cam in the link above will be the correct thing to do but i can,t locate the second part of the cam as it,s a 2 piece cam. Are you trying to use the cam base from the original LU with the cam? Try it without the base. You will need a brass or SS shim or thin washer between the shaft screw and the cam. anf test it in reverse to make sure it locks down.

Re: 1974 Mercury 500 Thunderbolt lower unit swap

Better measure the front stud spacing as there are 2 different sizes, if you have wrong size you will need to drill midsection holes larger to accept. Also swapping cams will not work at all as the early models had a totally different reverse lock setup...

Re: 1974 Mercury 500 Thunderbolt lower unit swap

Thanks for the info fatbullet I had no idea about the different lockdown. and I surely didn,t know about the bolt patern being different. So i can see using the old styl cam will not work. He is gonna have to fabricate the one he has to retro fit as it appears from his post to slide on the LU shift shaft. but I think he was trying to use the cam base as well. Thats why I suggested a washer and the cam alone. He will have to make sure it does not bind when putting the unit on. If it does he can remove some meat at the top side of the cam and greese the bottom between the shim washer (Base) and the cam.

Re: 1974 Mercury 500 Thunderbolt lower unit swap

Sorry about the long break in reply's. I got the cam issue straight but the shifter locknut that sits on top would not fit up into the receiving cavity. I found one on ebay for a good price and hope someone can tell me if it would fit. It states that it is from a 40hp Merc 402 ser. # 1643-6038 mines is a i believe 1974 50hp merc 500 ser# 4073302. From the pics (which are a lot more than i had to work with when i bought the wrong one) look the same including the shift rod setup. I have asked the seller the drive shaft length and spline count. I have 13 splines on drive shaft and prop shaft. Also, if they are interchangeable would I be able to just put my prop on the replacement unit. Thanks again for all the help.

Re: 1974 Mercury 500 Thunderbolt lower unit swap

The diamiter of the 40hp may be smaller and not as bulky. Is the thread the same? You may have to do some grinding of the head and possibly the plastic guide to get it to slide all the way up in there. Just don,t take to much from either only enough to get it flush.