Ok, another (possible) piston replacement thread '90/90hp

[Bigprairie1]

Ok, I'm underway with getting started on replacing the centre piston (or more?) on my '90/90hp based on Frank and Jerry's inf0o.
Quick background, this was a great running motor for several years then late last season it suddenly dropped in power a bit along with a rough idle. A quick compression test showed the centre piston was down to about 60-70psi. Then removing the head I saw the tell tale broken piston nub. There is very little time on the engine in the 'broken' ring condition so the bore doesn't appear to be scored too much.
Now its time to get this done and so I'm underway.
The heads off (pretty easy) and now I'm working on the other side of the motor. I've got most of the intake bolts off (I think) and the carbs/linkage/etc are still all attached to the intake. The gas/choke/throttle are disconnected so far....so i'm getting close to looking for the next needed advice here.
I was hoping to keep the carbs/intake intact (as previously mentioned) but it looks like the lower carb(s) might have to come off to get at those 2 little hidden/lower intake bolts?.....does this sound right?
Now in terms of intake bolts the ones I have been removing are both the direct/straight ones and the angled ones.....does this sound right? (question:...what is the idea behind the angled bolts?...just extra? or is there something else in behind?)
So guys any tips you can give me today or shortly would be greatly appreciated. I don't want to mess this up...and so far it seems like I'm on track!!!
More to come...and hopefully some pics once there is more to see.
all Good & many thanks!!
BP

 

[scottmm73]

Re: Ok, another (possible) piston replacement thread '90/90hp

The angle bolts hold the reed plate (#21) in place on the intake manifold. They are not necessary to remove to remove the intake manifold/reed plate adapter. Having gone through rebuilding my own just recently, it would be much more easier to remove the carbs from the intake than the keeping them installed on the intake manifold. The carbs are held in place by just two nuts and the amount of manipulating and forcing the removal of carb+intake isn't worth it. Take the extras three minutes and just remove the carbs, IMO. If you are very careful, you will not damage the carb to intake gasket and will be able to reuse it.

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Good info/advice Scott...thanks, I took it. I removed the carb bolts and took them off separately and then was able to get at those difficult intake manifold bolts. That lower long bolt holding the 'long tall intake air cleaner shroud thingy' was a bit of an annoyance to get at and out.
Regardless, so far so good.
Now....the good news.....I can see the reed valves!!! It looks like the only thing holding them in place at this point is the intake gasket?....does this sound correct?
So, next I'm going to see if I can get that centre reed valve out of there and take a look inside at the conn. rod bolt access.
Next question....so please wade in guys if you have some advice:
Is there any trick to retaining the bearings in the conn. rod cap when loosening and removing the bolts? If so, I'd love some advice on this!!!! I would rather make sure I'm doing things correctly and that I'm on track to maybe 'tap' that piston outta' there this afternoon?
If you guys have any input please let me know!!
Again, many thanks....wish me luck.....more to come on this
All Good
BP

 

[Frank Acampora]

Re: Ok, another (possible) piston replacement thread '90/90hp

Keep the engine vertical. it is a given that at least some of the rollers will fall out and into the crankcase. Be careful retrieving them so you don't drop one or two down the exhaust ports--they will fall right to the bottom of the engine. A long piece of welding wire or #14 house romex with insullation stripped off really helps to get into there and get them. Account for all 16 rollers.

If the engine has never been rebuilt, the manifold/reed gaskets will be destroyed when you pull off the manifold. If you buy these gaskets, they come as one piece that seals both the manifold and reed valve vee block. It is not necessary to remove the vee blocks, you can just cut out that part of the gasket

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Frank....you're on line...wow, thanks!!
I'm right in the throes of it...but, quick question: What is the type/size of socket is needed to remove those conn. rod bolts? I checked my 'arsenal' and the smallest (torx-type) equivalent size I had was 1/4"...but it was too big for that bolt.
I'm going to head over to the local tool supplier and see what they have...but if you have some insight on this it would be really appreciated..that way I can maybe do it in one trip.
Again, many thanks...I wouldn't be undertaking this repair if it wasn't for this forum and the excellent advice and insight offered by you, JerryJerry and many others.
Thanks again,
BP

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Ok, day two on this task...so far so good. I've got both sides (ends?) off the motor to get this piston (or maybe all three pistons?) out of there so we are getting close.
I quit yesterday after realizing that a 7mm socket (although) close was not going to cut it to get those conn. rod bolts off. I'm going out today to another tool shop to see if I can find that particular socket.
After that, hopefully I will get that piston outta' there today and get it measured up for replacement.
As well, I'm going to have to get a gasket set (sets?) on order.
If anyone has any recommendations on a good source for the gaskets I'd be interested.
And again, any insight on the right socket for getting that conn. rod bolt undone would be great appreciated.
Many thanks in advance guys!!
All Good
BP

 

[Frank Acampora]

Re: Ok, another (possible) piston replacement thread '90/90hp

12 point 1/4 inch socket. 1/4 inch drive with a 6 inch extension bar makes it easier. Adapt to a 3/8 driver. Socket available at Sears and K Mart, and probably most auto stores. When replacing be certain to buy or borrow an inch pound torque wrench (available for 10 bucks at Harbor Freight this week). Torque cap bolts to 190 INCH pounds.

Sometimes the cap bolts are dicked-up a little and you may need to push the socket on quite forcefully.

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Got it....thanks Frank. I'll call ahead to make sure they have this particular socket.
I'm of 2 minds with respect to the other 2 pistons. Part of me says...'if it ain't broke..don't fix it'. The other part of me says I should pull those other 2 pistons out and give them a seriously good clean, inspection and de-carbon to make sure they don't suffer the same fate. Any recommendations?
Note: I'm already doing the gaskets, the thing is pretty much apart...so....what the heck?
All good
BP
Note: I'm looking forward to getting that center piston out to measure it up and see which size it really is....its been difficult to narrow down which type/size it is relative to all the info floating around out there.

 

[Frank Acampora]

Re: Ok, another (possible) piston replacement thread '90/90hp

Don't measure the piston, measure the cylinder bore. Let's see: I think your 90 was a 3.3125 bore stock. The pistons are ground to have correct clearance and are also cam ground with a major and minor diameter. (to account for uneven heating and expansion while running)

This will confuse you. So, measure the bore and see if it is close to stock or .010 to .030 oversized. (BTW: Pistons are usually stamped with oversizes on the baffle top or the crown.)

Now, as far as replacing all three pistons, see my reply to Jiggz about Wiseco pistons on his 4 cylinder engine. Then make your own decision.

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

12 point 1/4 inch socket. 1/4 inch drive with a 6 inch extension bar makes it easier. Adapt to a 3/8 driver. Socket available at Sears and K Mart, and probably most auto stores. When replacing be certain to buy or borrow an inch pound torque wrench (available for 10 bucks at Harbor Freight this week). Torque cap bolts to 190 INCH pounds.

Sometimes the cap bolts are dicked-up a little and you may need to push the socket on quite forcefully.

Well....darn it, looks like things are on hold for a couple of days. I checked with 3-4 tool suppliers locally and either they don't have the 12pt-1/4" socket or they are out of stock.
Hmmmmm.....maybe I'll try a couple of local buddies but I hate to 'borrow tools'....even one item this small.
In the meantime I can go outside and stare at the rod bolts for amusement.
All Good
BP

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

How about some motor data to help figure out what piston (pistons) sizes are going to work in the repair:
Motor I.D: 906X90C (interesting the manual only refers up to 'B'?)
All pistons are stamped with the following: 3S 4
Top & Bottom pistons are stamped with the letter: 'B'
Centre (broken) piston is stamped with the letter: 'C'
(note: no other letters, stampings, etc are apparent on the top of the pistons)
Approx. measure bore sizes (given the accuracy available to me) are:
#1 Top (vertical direction/parallel to wrist pin): 3.395"/3.396"
(horizontal direction/perp to wrist pin): 3.398"/3.399" (tougher one to measure)
#2 Centre: (vertical direction/parallel to wrist pin): 3.401"/3.402"
(horizontal direction/perp to wrist pin): 3.398"/3.399" (tougher one to measure)
#3 Bottom: (same refs as above: 3.401"/3.402"
(horiz/same ref as above): 3.397"/3.399"

Now as of yet, I still can't measure the pistons themselves until I'm able to remove them....but:
the book states that the standard piston for this motor 1990B-1995 (is 'C' series same as the 'B' series?) should be:
Perp. (90?) to Wrist pin: 3.369"/3.370"
Parallel to Wrist pin: 3.367"/3.368"
Now, I assumed thats the same for all pistons....but? maybe not?

So....I'm trying to figure out if I have a standard bore motor from the above data. It seems close given that what I could measure (more or less) was the bores...and that the stated piston sizes above seem close.

Any feedback or opinion on this is always welcome!!...theres no such thing as too much information.
thanks again guys!!
All Good
BP
......now back to the search for a 12pt-1/4" socket.

 

[Frank Acampora]

Re: Ok, another (possible) piston replacement thread '90/90hp

Ahh! Ok! I though that your 1990 engine was still the small bore. At some point Mercury increased the 90 bore to 3.375

At 3.401 It would seem like a tight .030 overbore was done on the center and bottom cylinder---3.375 plus .030 = 3.405. Top cylinder would appear to be right on the money with a .020 overbore---3.375 plus .020 = 3.395.

It does appear that you are within tolerance of .005 for out of round so a rebore does not seem necessary.

Do you not have a Sears there? Around here, they have a whole wall of sockets in the tool section. I find it difficult to believe that a 12 point socket is out of stock.

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Ahh! Ok! I though that your 1990 engine was still the small bore. At some point Mercury increased the 90 bore to 3.375

At 3.401 It would seem like a tight .030 overbore was done on the center and bottom cylinder---3.375 plus .030 = 3.405. Top cylinder would appear to be right on the money with a .020 overbore---3.375 plus .020 = 3.395.

It does appear that you are within tolerance of .005 for out of round so a rebore does not seem necessary.

Sorry for the delay in response Frank (& the 'Force guys'). I was out of action until I could get that socket. So mission accomplished on the socket and I was able to make quick work out of taking that center piston out.
So I measured it up and here are the results are as follows:
Piston- parallel to wrist pin: 3.398"/3.399"
Piston- 90? Perp to wrist pin: 3.395"/3.396"
(note: that above measurements were done pretty quickly but they are close to the real deal)
So I was trying to figure out which piston I need to order for this? I'm going to start searching around tonight and tomorrow a bit. I wasn't able to identify a piston from the Iboats store.
If anyone has any idea as to what piston (and ring set?) I should order.....please feel free to wade in!!
All Good
BP

 

[Frank Acampora]

Re: Ok, another (possible) piston replacement thread '90/90hp

See, There you go jumping to confusions!

Pistons are cam ground with a major and minor diameter to account for uneven heating during running. HOWEVER: Most manuals do not give piston sizes. Engines are designated by the cylinder bore size--that is standard 3.375.

So forget about the piston size and re-measure the cylinder bore. If it is close to 3.405, then it most likely uses a .030 overbore piston. Remember, there is a tolerance in boring and honing. A good machine shop will be able to hold it to a couple of .0001. A poor machine shop will not. If your cylinder is tighter than 3.405, you may be able to hone it to size. The machine shop I use for boring does a rough bore, a rough hone, then a final hone to size. They are always right on the money with final cylinder diameter.

I like Wiseco pistons because they are Forged, not cast like stock, And they are easier to assemble on the rod.

My Wiseco catalog does NOT show a 1990 90, in fact it does not show any Force 90 before 1991, but I know they were made because I sold a 1990 90 with 3.3125 bore.

Based on your measurements, I believe that your engine would use the 3151 P3 (.030 over) Wiseco piston and W5216 bearing kit for the rod small end. However, Don't take this as gospel and blame me if it doesn't fit--I am just going by measurements you supplied.

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Frank
thanks for the info!!...quick questions:
-On the Wiseco Website the bore references they make are for the Bore size NOT the piston (diameter) measurements...correct? (or no?)
-I was doing some quick referencing/checking on Ebay and wanted to know if these were the ones needed:
http://www.ebay.ca/itm/320891516669?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
(...size wise it looks close to the piston I pulled out...but they call it a .020" oversize).
As well, I will remeasure that bore carefully Frank and post the results to see if they deviate from the first references.
Many thanks for the excellent input and assistance in walking me through this!!!...so far so good!.
BP

 

[Frank Acampora]

Re: Ok, another (possible) piston replacement thread '90/90hp

Read the last sentence of second paragraph of my last reply! I know you want to be absolutely certain but I hate repeating myself.

ebay, right piston, wrong size. More likely you need .030 over. Again repeating myself.

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Read the last sentence of second paragraph of my last reply! I know you want to be absolutely certain but I hate repeating myself.

ebay, right piston, wrong size. More likely you need .030 over. Again repeating myself.

I hear you Frank...understood.
That said, I bought a good complete Force piston/rod assembly last year that I (mistakingly) thought would fit. It's basic dim'ns are 3.363"/3.364" (parallel) and 3.361"/3.362" (Perp). So I guess that one will be going go back on the market somewhere. Its stamped/listed as a 3141PS 'Std' 4232K.
UNLESS....you could use it? or trade?
I'm going to try to get my stuff sorted out this week to get all the new needed parts going so I can hopefully get the motor completed just in time for winter storage....LOL!!
All Good
BP

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Read the last sentence of second paragraph of my last reply! I know you want to be absolutely certain but I hate repeating myself.

ebay, right piston, wrong size. More likely you need .030 over. Again repeating myself.

Frank,
I won't be boring the cylinder out...just honing it to clean it up. Am I still in good shape 'fit wise' with the .030" if I don't bore?
Thanks again.
BP

 

[Frank Acampora]

Re: Ok, another (possible) piston replacement thread '90/90hp

That is difficult to say: It depends upon the original condition of the cylinder and the skill of the person honing. If we are still talking about the cylinder that is 3.401 diameter: Given the size , if your measurements are correct, and given you don't take off too much metal honing, then yes an .030 oversized piston should fit well.

 

[Bigprairie1]

Re: Ok, another (possible) piston replacement thread '90/90hp

Ok, clearly I'm takin' my sweet time with this....but its time for an update.
Well I finally got my ordered piston in which is the Wiseco 3151P3 (.030" over) and the required piston kit.
As well, I picked up a hone and started to clean up the bore a bit.
Now, I have a couple of questions for you guys in the know on this:
1) -What is the best way to get the old piston pin out of that (NON wiseco) piston? Is this a press fit? I tried lightly and loosely to put it out with my bench vise...it moved a bit but I didn't want to force the issue (pardun the pun) until I had a little bit more info on the best method.
The old/damaged piston might be a merc based design or one of the others (sierra?) ...but it definitely is not a wiseco.
Once I get the piston pin out then I can start re-assembling the new piston assembly using the original con rod and the new Wiseco piston/piston kit.
2) Does anyone use a small file or similar small grinder etc to chamfer the inner ports? (noting that Wiseco recommends this and I was considering using a small file to clean them up a bit before the final honing.
Allright I'll leave off there for the moment guys...and please any input or advice on this is more than welcome.
By the way...so far so good...and many thanks to Frank A. on this.
All Good
BP