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Author Topic:   Blind Hole size on Yamaha mounting bracket

dist6b posted 08-28-2012 01:12 PM ET (US)
I am mounting a 2004 Yamaha 90 HP 2 stroke on a 1969 Sakonnet. My bolt pattern on the transom puts the blind hole on the bracket in line with the transom hole. The motor sits on the transom. The top mounting hole is 2" below the top of the transom. This puts the cavitation plate about 1 1/2" above the bottom of boat. Does anyone know the size and pitch of the bolt for the blind hole on the bracket?
Bobs66 posted 08-28-2012 10:00 PM ET (US)
Sorry,I dont know the size.But in 1994 when I repowered my Sakonnet with a new 85 Yamaha (essentially a 90 with a pre-mix fuel system and an aluminum prop)I soon noticed that the lower mounting bolts were not stainless like the top ones.
I complained to the dealer who installed it and he explained how he "really looked around" but was unable to find that bolt in stainless.I contacted Yamaha .They were unable provide or locate the bolt and not very interested to learn that their dealer installed a new engine that way.
I was not pleased but I started to use the boat and I am still using it with no problems.I do put a little grease on the bolt heads occasionally.It is probably significant that 1994 was the year I discontinued mooring the boat in salt water and began exclusively trailering to fresh water lakes.I hope you can find the right bolt.
L H G posted 08-29-2012 02:07 AM ET (US)
Blind hole engine brackets are a really bad idea today, and were yesterday also, something Yamaha copied from OMC. Don't use that system.
On the old Whaler transom, use the engine bolt drilling template so that the top of the bottom set of holes (or slot) is high enough so that the SS bolts can receive a nut on the inside of the splashwell at the bottom. This will determine minimum mounting height of the engine, which will set the anti-vent plate about 1-1/2" above the bottom. Then the upper bolts will go through the 3rd set of holes on the upper mounts. I would use a transom stiffener plate across the top bolts, and the heads of the bolts should be on the inside.
This mounting situation, forced by the old Whaler's shallow splashwell, will require a surfacing prop, such as a 15" pitch Stilletto. The boat should run about 42 MPH.
How do I know all of this? Two years ago I installed an OMC engine on this same hull, and it worked out quite well.
For mounting, you should use 1/2" fine thread SS bolts, with brass locknuts. YOu can buy nice engine bolts kits from Mercury of this description.
jimh posted 08-29-2012 08:50 PM ET (US)
Please read Boston Whaler dealer bulletin 84-10 which deals with precisely this problem:
http://continuouswave.com/whaler/reference/bulletin84-10.html
jimh posted 08-29-2012 08:52 PM ET (US)
Also read the answer to FREQUENTLY ASKED QUESTION 8:
http://continuouswave.com/whaler/reference/FAQ/#Q8
dist6b posted 08-29-2012 10:25 PM ET (US)
My current holes in the transom are: top holes 2" down from top of transom. Bottom holes 6 1/2" below, which puts these holes at the bottom of the splash well. If I mount the motor with the bottom bolts at the top of the slot, that puts the top bolts in the 4th hole down. It would put my anti-vent plate about 3" above the bottom of boat. From top of transom to bottom of boat is 22".
jimh posted 08-29-2012 10:33 PM ET (US)
As mentioned in the FAQ on this topic (see link above), the vertical spacing of the holes is supposed to be 8-inches on-center. If your transom is drilled with a spacing of 6-1/2-inches your lowest mounting height will be the equivalent of the "two-holes-up" position, which is a good engine mounting height for many applications.
dgoodhue posted 08-30-2012 09:17 AM ET (US)
The optimum mounting height is going to either the 3rd or 4th hole hole (meaning the highest or second highest motor height) depending on the prop you have.
quicksilver380 posted 08-30-2012 11:41 AM ET (US)
I recently mounted a 1996 Yamaha 75 with a stock aluminum propeller on my 1970 Katama. I didn't have the money for a new surface prop, and the previous motor was mounted with the blind holes so that is how I mounted mine. I would say there is a good chance the blind holes on our two motors are the same. The blind hole on mine is M12 x 1.25-40 deep. I was unable to find a stainless bolt in this size anywhere so I ended up just buying 2 zinc coated bolts at Lowes and covering them with caulk. I don't remember the exact length I bought. I bought 2 different lengths and returned the one that was too long. Hope this helps
Bobs66 posted 08-30-2012 09:01 PM ET (US)
There is another side to this.The subject boat is 43 years old.My boat is 46 years old .The newest "blue hull"is 40.Over the years the real performance problems with these boats is not related to engine height or aluminum props rather its waterlogged hulls.

The chief causes are poor workmanship and compromised hull integrity due to too many holes in the transom and elsewhere.I have read all those bulletins and FAQs.One recommends drilling additional holes at a "slight" ?? angle to acheive one hole up.This in proximity to the existing holes.Sounds like a sloppy mess and a recipe for problems.

I would locate that metric bolt as described above and use the blind hole,even if the only bolt you can find is less noble than stainless .Aviod drilling any more holes especially those at a "slight angle"
I was out today and grabbed the GPS from the car.My boat is the standard Sakonnet layout (console,RPS and casting platform)also on board water skis,12 gals gas,a small anchor and a Mills Bimini with a stainless frame.I am about 190 lbs.The engine is an 18 year old Yamaha 85 with a 13 1/4X17 Aluminum prop.The engine is all the way down and mounted with the blind hole.38.8 MPH.I can live with that.
contender posted 08-31-2012 05:25 PM ET (US)
LHG is incorrect: I have a 1975 16'7" whaler that I run a 140 Evinrude on the transom...I use the blind holes for mounting and it works just fine...I also had a 4 inch transom bracket(CNC) that I mounted as well...Using the blind holes it work out just fine just had to drill two holes in the bracket. I had to remove the bracket because I went to a Hydraulic steering system and the cylinder would hit the bracket when tilting the engine,(sold the bracket to another member here on CW) so I removed the bracket, but The engine and bracket will work work using the teleflex steering cable. Using the bracket you can slide the engine up and down as you please. I also made an aluminum plate (.125) that covers all 4 of the mounting bolts and notch it out on the bottom for drainage. I even went as far as making my own mounting bracket for lining up the engine bracket holes for drilling the transom (even has the blinds holes as well)...Check you email for some pictures...
contender posted 08-31-2012 05:37 PM ET (US)
dist6b...Your email address dose not seem to be working...
jimh posted 08-31-2012 11:41 PM ET (US)
L H G writes:

quote:
Blind hole engine brackets are a really bad idea today, and were yesterday also, something Yamaha copied from OMC. Don't use that system.
Contender writes:

quote:
LHG is incorrect: I have a 1975 16'7" whaler that I run a 140 Evinrude on the transom...I use the blind holes for mounting and it works just fine.
The real "bad idea" was Boston Whaler's shallow splash well design. I don't see any argument against the blind hole mounting except that you can't adjust the mounting height.
contender posted 09-01-2012 05:56 PM ET (US)
Jim: you are correct about the blind holes once you bolt the engine to the transom thats it, no movement. But like I said I did have a 4 inch bracket that I mounted to the transom using the drilled blind holes, Then mounted the engine to the bracket, the engine then could be raised. The problem I had was after the mounting of the bracket (I had teleflex steering) And I wanted to install the hydr. steering. When you would tilt the engine the steering cylinder would hit the bracket, So I removed the bracket... and back the the blind holes...
dist6b posted 09-02-2012 09:14 PM ET (US)
Thanks for all the feedback. The blind bolt is 12mm and 1.25 pitch. I am going to try the the top hole setup first. This puts the cavitation plate 1 1/2 " above the bottom of the hull.
jimh posted 09-03-2012 08:58 AM ET (US)
Regarding the older OMC engine brackets with an option to mount the engine using a blind hole when the engine was installed in boats with an engine splash well that was too shallow to permit use of the lower engine mounting holes, we have a cart and horse situation. To say that the blind hole mounting option on OMC was a "bad idea" is to put the cart before the horse. I believe that the reason OMC provided the option of using a lower mounting hole with less vertical separation from the upper mounting hole was to accommodate installations on boats where the boat manufacturer had not provided an engine splash well that was sufficiently deep. In this context it seems like one could say the blind mounting hole was actually a good idea as a way to overcome an engine mounting problem.
I am not an outboard engine historian, but my understanding is the engine mount design and hole layout that is currently considered to be the standard arrangement is the engine mount and hole layout that OMC developed, but with the blind hole option removed. I don't know the precise epoch that the OMC engine mount became the standard, but perhaps after it did the boat builders began to conform their boat designs to it. As a result the problem of an engine splash well that was too shallow to accommodate the lower mounting bolts of an outboard engine became a thing of the past.
conch posted 09-04-2012 09:37 AM ET (US)
With older OMC V4 engines the motor could be raised even though the blind holes were used. The two brackets were bolted to the transom and the motor slid into the sleeve or pocket formed by the two brackets and could be raised or lowered through several holes of adjustment and the two locking bolts in the bracket reinserted. The locking bolts fastened from the sides of the bracket into the motor, not into the transom. When the motor was removed from the boat the two brackets remained on the hull.
Contender could have used a side mounted hydraulic cylinder and kept his 4in jackplate and not had the clearance problem.
But then I wouldn't be the jackplates new owner.
Chuck
Tom W Clark posted 09-04-2012 10:00 AM ET (US)

quote:
I am going to try the the top hole setup first. This puts the cavitation plate 1 1/2 " above the bottom of the hull.
No, it doesn't. The A/V plate will be about one inch below the bottom of the hull. You do not want the A/V plate one inch below the bottom of the hull.
TC posted 09-07-2012 03:36 PM ET (US)
The last Montauk I owned had a 3 cylinder 90HP Mercury rigged by the dealer with 4 bolts through the top set of holes and it seemed like an easy solution to the splash well problem. My neice now owns this boat and it's still performing well for her. The motor hasn't been off since 1987.

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Author	Topic: Blind Hole size on Yamaha mounting bracket
dist6b	posted 08-28-2012 01:12 PM ET (US) I am mounting a 2004 Yamaha 90 HP 2 stroke on a 1969 Sakonnet. My bolt pattern on the transom puts the blind hole on the bracket in line with the transom hole. The motor sits on the transom. The top mounting hole is 2" below the top of the transom. This puts the cavitation plate about 1 1/2" above the bottom of boat. Does anyone know the size and pitch of the bolt for the blind hole on the bracket?
Bobs66	posted 08-28-2012 10:00 PM ET (US) Sorry,I dont know the size.But in 1994 when I repowered my Sakonnet with a new 85 Yamaha (essentially a 90 with a pre-mix fuel system and an aluminum prop)I soon noticed that the lower mounting bolts were not stainless like the top ones. I complained to the dealer who installed it and he explained how he "really looked around" but was unable to find that bolt in stainless.I contacted Yamaha .They were unable provide or locate the bolt and not very interested to learn that their dealer installed a new engine that way. I was not pleased but I started to use the boat and I am still using it with no problems.I do put a little grease on the bolt heads occasionally.It is probably significant that 1994 was the year I discontinued mooring the boat in salt water and began exclusively trailering to fresh water lakes.I hope you can find the right bolt.
L H G	posted 08-29-2012 02:07 AM ET (US) Blind hole engine brackets are a really bad idea today, and were yesterday also, something Yamaha copied from OMC. Don't use that system. On the old Whaler transom, use the engine bolt drilling template so that the top of the bottom set of holes (or slot) is high enough so that the SS bolts can receive a nut on the inside of the splashwell at the bottom. This will determine minimum mounting height of the engine, which will set the anti-vent plate about 1-1/2" above the bottom. Then the upper bolts will go through the 3rd set of holes on the upper mounts. I would use a transom stiffener plate across the top bolts, and the heads of the bolts should be on the inside. This mounting situation, forced by the old Whaler's shallow splashwell, will require a surfacing prop, such as a 15" pitch Stilletto. The boat should run about 42 MPH. How do I know all of this? Two years ago I installed an OMC engine on this same hull, and it worked out quite well. For mounting, you should use 1/2" fine thread SS bolts, with brass locknuts. YOu can buy nice engine bolts kits from Mercury of this description.
jimh	posted 08-29-2012 08:50 PM ET (US) Please read Boston Whaler dealer bulletin 84-10 which deals with precisely this problem: http://continuouswave.com/whaler/reference/bulletin84-10.html
jimh	posted 08-29-2012 08:52 PM ET (US) Also read the answer to FREQUENTLY ASKED QUESTION 8: http://continuouswave.com/whaler/reference/FAQ/#Q8
dist6b	posted 08-29-2012 10:25 PM ET (US) My current holes in the transom are: top holes 2" down from top of transom. Bottom holes 6 1/2" below, which puts these holes at the bottom of the splash well. If I mount the motor with the bottom bolts at the top of the slot, that puts the top bolts in the 4th hole down. It would put my anti-vent plate about 3" above the bottom of boat. From top of transom to bottom of boat is 22".
jimh	posted 08-29-2012 10:33 PM ET (US) As mentioned in the FAQ on this topic (see link above), the vertical spacing of the holes is supposed to be 8-inches on-center. If your transom is drilled with a spacing of 6-1/2-inches your lowest mounting height will be the equivalent of the "two-holes-up" position, which is a good engine mounting height for many applications.
dgoodhue	posted 08-30-2012 09:17 AM ET (US) The optimum mounting height is going to either the 3rd or 4th hole hole (meaning the highest or second highest motor height) depending on the prop you have.
quicksilver380	posted 08-30-2012 11:41 AM ET (US) I recently mounted a 1996 Yamaha 75 with a stock aluminum propeller on my 1970 Katama. I didn't have the money for a new surface prop, and the previous motor was mounted with the blind holes so that is how I mounted mine. I would say there is a good chance the blind holes on our two motors are the same. The blind hole on mine is M12 x 1.25-40 deep. I was unable to find a stainless bolt in this size anywhere so I ended up just buying 2 zinc coated bolts at Lowes and covering them with caulk. I don't remember the exact length I bought. I bought 2 different lengths and returned the one that was too long. Hope this helps
Bobs66	posted 08-30-2012 09:01 PM ET (US) There is another side to this.The subject boat is 43 years old.My boat is 46 years old .The newest "blue hull"is 40.Over the years the real performance problems with these boats is not related to engine height or aluminum props rather its waterlogged hulls. The chief causes are poor workmanship and compromised hull integrity due to too many holes in the transom and elsewhere.I have read all those bulletins and FAQs.One recommends drilling additional holes at a "slight" ?? angle to acheive one hole up.This in proximity to the existing holes.Sounds like a sloppy mess and a recipe for problems. I would locate that metric bolt as described above and use the blind hole,even if the only bolt you can find is less noble than stainless .Aviod drilling any more holes especially those at a "slight angle" I was out today and grabbed the GPS from the car.My boat is the standard Sakonnet layout (console,RPS and casting platform)also on board water skis,12 gals gas,a small anchor and a Mills Bimini with a stainless frame.I am about 190 lbs.The engine is an 18 year old Yamaha 85 with a 13 1/4X17 Aluminum prop.The engine is all the way down and mounted with the blind hole.38.8 MPH.I can live with that.
contender	posted 08-31-2012 05:25 PM ET (US) LHG is incorrect: I have a 1975 16'7" whaler that I run a 140 Evinrude on the transom...I use the blind holes for mounting and it works just fine...I also had a 4 inch transom bracket(CNC) that I mounted as well...Using the blind holes it work out just fine just had to drill two holes in the bracket. I had to remove the bracket because I went to a Hydraulic steering system and the cylinder would hit the bracket when tilting the engine,(sold the bracket to another member here on CW) so I removed the bracket, but The engine and bracket will work work using the teleflex steering cable. Using the bracket you can slide the engine up and down as you please. I also made an aluminum plate (.125) that covers all 4 of the mounting bolts and notch it out on the bottom for drainage. I even went as far as making my own mounting bracket for lining up the engine bracket holes for drilling the transom (even has the blinds holes as well)...Check you email for some pictures...
contender	posted 08-31-2012 05:37 PM ET (US) dist6b...Your email address dose not seem to be working...
jimh	posted 08-31-2012 11:41 PM ET (US) L H G writes: quote: Blind hole engine brackets are a really bad idea today, and were yesterday also, something Yamaha copied from OMC. Don't use that system. Contender writes: quote: LHG is incorrect: I have a 1975 16'7" whaler that I run a 140 Evinrude on the transom...I use the blind holes for mounting and it works just fine. The real "bad idea" was Boston Whaler's shallow splash well design. I don't see any argument against the blind hole mounting except that you can't adjust the mounting height.
contender	posted 09-01-2012 05:56 PM ET (US) Jim: you are correct about the blind holes once you bolt the engine to the transom thats it, no movement. But like I said I did have a 4 inch bracket that I mounted to the transom using the drilled blind holes, Then mounted the engine to the bracket, the engine then could be raised. The problem I had was after the mounting of the bracket (I had teleflex steering) And I wanted to install the hydr. steering. When you would tilt the engine the steering cylinder would hit the bracket, So I removed the bracket... and back the the blind holes...
dist6b	posted 09-02-2012 09:14 PM ET (US) Thanks for all the feedback. The blind bolt is 12mm and 1.25 pitch. I am going to try the the top hole setup first. This puts the cavitation plate 1 1/2 " above the bottom of the hull.
jimh	posted 09-03-2012 08:58 AM ET (US) Regarding the older OMC engine brackets with an option to mount the engine using a blind hole when the engine was installed in boats with an engine splash well that was too shallow to permit use of the lower engine mounting holes, we have a cart and horse situation. To say that the blind hole mounting option on OMC was a "bad idea" is to put the cart before the horse. I believe that the reason OMC provided the option of using a lower mounting hole with less vertical separation from the upper mounting hole was to accommodate installations on boats where the boat manufacturer had not provided an engine splash well that was sufficiently deep. In this context it seems like one could say the blind mounting hole was actually a good idea as a way to overcome an engine mounting problem. I am not an outboard engine historian, but my understanding is the engine mount design and hole layout that is currently considered to be the standard arrangement is the engine mount and hole layout that OMC developed, but with the blind hole option removed. I don't know the precise epoch that the OMC engine mount became the standard, but perhaps after it did the boat builders began to conform their boat designs to it. As a result the problem of an engine splash well that was too shallow to accommodate the lower mounting bolts of an outboard engine became a thing of the past.
conch	posted 09-04-2012 09:37 AM ET (US) With older OMC V4 engines the motor could be raised even though the blind holes were used. The two brackets were bolted to the transom and the motor slid into the sleeve or pocket formed by the two brackets and could be raised or lowered through several holes of adjustment and the two locking bolts in the bracket reinserted. The locking bolts fastened from the sides of the bracket into the motor, not into the transom. When the motor was removed from the boat the two brackets remained on the hull. Contender could have used a side mounted hydraulic cylinder and kept his 4in jackplate and not had the clearance problem. But then I wouldn't be the jackplates new owner. Chuck
Tom W Clark	posted 09-04-2012 10:00 AM ET (US) quote: I am going to try the the top hole setup first. This puts the cavitation plate 1 1/2 " above the bottom of the hull. No, it doesn't. The A/V plate will be about one inch below the bottom of the hull. You do not want the A/V plate one inch below the bottom of the hull.
TC	posted 09-07-2012 03:36 PM ET (US) The last Montauk I owned had a 3 cylinder 90HP Mercury rigged by the dealer with 4 bolts through the top set of holes and it seemed like an easy solution to the splash well problem. My neice now owns this boat and it's still performing well for her. The motor hasn't been off since 1987.