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Mounting Mercury 90 HP on 1978 Montauk
robbyc009	#1  Posted on 10/25/10 - 4:50 PM
Member Posts: 4 Comments: 0 Joined: 10/16/10	I'm new to this site and need some help.I will be installing a 1989 90 hp Mercury on my 1978 Montauk 17. What would be the proper engine mounting hight? Will the mounting holes be the same as my older 1980 70 hp Johnson? Any thoughts on jack plates? I'm looking at a Rite-Hite 6" jack plate it will adjust four inches, two inches up, two inches down. I can use all the help and information I can get on this subject. Thanks, Robert Edited by Tom W Clark on 10/27/10 - 7:19 AM
Finnegan	#2  Posted on 10/25/10 - 10:57 PM
Member Posts: 1926 Comments: 16 Joined: 05/02/08	The bolt pattern on the old OMC 70 should be the same as the 90 Merc, with the exception that Mercs have 5 holes in the pattern, rather then 4, and the bottom sets also have five holes, rather than a 4 hole slot. BUT, because the splashwell is shallow on the 1978 Montauk, and will not accomodate the BIA bolt pattern unless the engine is raised to the third set of holes, I'll bet you 10 to 1 that the OMC was installed using their blind hole system. At the time this seemed like a good idea, but most now consider it the installation from hell! If so, you will probably have to filll the bottom holes, and redril. You may also have to fill and raise the top holes too. When you mount the engine, the vertical bolt hole spacing will be 6-1/2", and the bolts will go through the third holes in the top set, and the top holes in the bottom set. Because the Merc 90 has a longer mid section, it will accomodate the 1-1/2" elevation. I would recommend a Laser II/Quicksilver Lightspeed propeller.
Joe Kriz	#3  Posted on 10/26/10 - 9:30 AM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	See this article: http://www.whalercentral.com/articles...icle_id=82
Tom W Clark	#4  Posted on 10/27/10 - 7:28 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Robert -- Welcome to Whaler Central. Finnegan offers some good advice and Joe's article helps to explain what he is talking about. To make a few minor corrections, you can mount the motor only one hole up if you want. The Montauk splashwell is deep enough to accommodate the lower bolts holes in the second set of holes in the motor. I know, I've owned three of them, a Mercury 90 on my last one. I still think you will get better performance if you do mount the motor two holes up though. You will need to use a good propeller to make use of this height, however. Cheap aluminum props that do not grip well are the reason why motors needed to be mounted all the way down 30 or 40 years ago. You will have to fill the existing "blind holes" in the transom and drill new lower bolt holes. There is no need to change the upper bolt holes; they are fine. Filling two little 1/2" holes in the transom is a very simple chore. Do not let anybody scare you away from doing it. The Mercury 90 does not have a longer midsection; it is like all other 90 HP motors.
robbyc009	#5  Posted on 10/27/10 - 4:21 PM
Member Posts: 4 Comments: 0 Joined: 10/16/10	Thanks for all info it all helps. Everything being said to Zappaddles in this thread I think must work for me as well yes? Edited by Tom W Clark on 10/27/10 - 4:25 PM
Tom W Clark	#6  Posted on 10/27/10 - 4:24 PM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Yes.
robbyc009	#7  Posted on 10/27/10 - 4:26 PM
Member Posts: 4 Comments: 0 Joined: 10/16/10	Any thing on prop pitch?
Tom W Clark	#8  Posted on 10/27/10 - 4:28 PM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	A 13-1/2" x 20" Mercury Laser II or Quicksilver Lightspeed or a 13-1/4" x 19" Stiletto Advantage or Turbo 1 would be the best fit for your boat/motor combination.
robbyc009	#9  Posted on 10/27/10 - 5:06 PM
Member Posts: 4 Comments: 0 Joined: 10/16/10	Thank you for helping me.And giveing me some of your advise.It will come in very handy.Robert