1979 Mercury 90HP Inline 6 One Cylinder Dead, Needs Rebuild. Please advise.

[Faztbullet]

1) .006 is wrong and .003 is correct. Anything over.003 of factory finished bore sizes is worn. If "Wayne" bored correctly the bore will measure 2.890 as piston skirt will mike 2.887 +/- .0005
2) Nope as bore way past wear limit spec,, will have piston slap..Pro Marine even told ya so.
3) Certified Merc/Mariner /Mercruiser Tech thru Mercury Marine, business owner, build an raced I-6 motors, had Kwik way boring bar and bored blocks, wrenching on Mercs since 1977

 

[elr]

Ok, I took it back apart today and measured best I could with a digital calipers. Pistons measured about what faztbullet said they would. Measured about a quarter inch off the skirt edge and only checked three pistons but were all pretty close. I could only measure into the bore about three eighths of an inch and want to have another machinist measure it on Monday but in the meantime see pictures for results.

 

[elr]

[No message]

 

[elr]

I am guessing the slightly out-of-round piston is intentional...?

 

[jimmbo]

Yes, for 1973, mercury introduced what they called Cam Turned Pistons. Slightly oval when cold, expanding to round when at operating temp

 

[elr]

Had two different machinists measure my problem today... first one says piston measured 2.886 and bores were between .009 and .011 over. That was done an inch or so in the bore.

Second machinist is somewhat of a legend. Two cycle boat motors are not his specialty but he built/builds race cars, motorcycles, has a foundry, builds and flys planes, has a regular machine shop as well and more. He showed 2.886 on piston as well with his (snapon) micrometer (that he verified was calibrated on his $1,000 calibration tree.) He showed .009 on one cylinder and right at .011 on the rest.

Then he wanted to check deeper into the bore and showed more like .007 to .008 down close to the end.

So definitely mr Wayne didn’t do so well with his bore job. I am going to stop at his shop tomorrow and see what he says...

Questions...

1. Should the bore be perfectly parallel all the way in or should it be slightly tapered like it is?

2. With these measurements in mind and in the event that Wayne is not inclined to help in any way, could I get by with letting the bore as is and going to .020 over pistons? That should put it at .004 to .005 over at the open end of bore and real close to .003 in combustion area...

Thanks for your help.

 

[jimmbo]

When fresh, a bore should be the same dia all the way from the top to the bottom, top being where the cylinder head would be, a moot point with your block as there no head to remove..Cylinder wear in 2 Strokes is different than 4 strokes. 4 strokes tend to wear in a taper wider at the top, resulting in a ridge at the top of the ring travel, 2 strokes do that too, to a lesser degree because of lower compression. They also experience wear where the ports are, resulting in a Vase like wear profile

 

[Faztbullet]

Like Jimmbo said should be same size top to bottom...2.890. Sounds like Wayne used a hone instead of a boring bar to do this as only way it could bore tapered. Block will need to go .030 as will need 2 cuts to true cylinder to size and with a boring bar. Specs are to close to bore .020

 

[elr]

Talked to Wayne this morn and he agreed to rebore at no charge to .003” over the new .030” pistons I hope to get. He did not volunteer to pay for new pistons, tho. I just hope pro marine will be able to credit a good amount back for the .015” pistons I currently have.

Wayne says he’s never seen them that tight (.003) and he’s seen plenty running at .010 even .015” clearance, he says. He didn’t have anything to say about how his .006” claimed clearance grew to .010 and .011” when I had it checked by others. I’m thinking his agreeing to rebore was as close as he’ll come to saying he messed up.

Just to make sure it’s clear... if the .015 over pistons were supposed to measure 2.887” then the .030” over should measure 2.902 and the finished bore should stand at 2.905”...

Is that correct?

 

[racerone]

How is he going to bore a " true round " hole this time from top to bottom of the cylinder ?????

 

[elr]

Yes that is a concern... but I’m hoping the knowledge that his work will be checked by others will help sharpen his eye and steady his hand.

I intend to ask him as much once I have other pistons in hand and before I turn the block over to him. It’s just that I would enjoy avoiding another six cylinders worth of blind boring remuneration ...

 

[jimmbo]

Well if he messes this one up, he gets to buy you a replacement block

 

[elr]

If... if... big if I know, but if Mr Wayne pulls thru on this one and if, more big if, if I get it all back together correctly and if it runs and runs correctly.... if all that, will having .030” oversized pistons deliver a noticeable increase in performance?

(can’t help but dream of cruising smartly down the lake someday with a smooth running motor hanging out the back)

 

[racerone]

Really no " kick in the pants " improvement in power.-----You will not notice it.

 

[Faztbullet]

to rebore at no charge to .003” over the new .030” pistons​

Forget all the sizing...the finished hole should be 2.905 all the way from top to bottom. Piston manufacturing is so good nowdays clearances wont be off .0005. Bore and go on a cast piston nowdays..

Wayne says he’s never seen them that tight (.003) and he’s seen plenty running at .010 even .015” clearance, he says.​

He has screwed up a lot of customers blocks then.....I myself wouldn't let him touch it. Have a better machinist do it as send him the bill. Only fair since you have already paid him and he's made no mention of a refund. Ask him if he would have helped out after it was assembled and failed then you found measurement off...bet you would have got a BS excuse like you already have received. Any good machinest will understand what finished bore means...that is you would bore your block to the 2.903-2.904 spec and use a rigid hone to bring out to 2.905.

 

[elr]

Talked to pro marine today... they have received my .015 pistons and will call tomorrow morn to confirm but think they’ll be able to just swap everything out for .030 size. I have nothing but good to say about pro marine so far. Extremely helpful to this newbie since the beginning of this whole escapade.

Also talked to another machinist about reboring... sounds like he knows his stuff. Told me all how it should be machined, how he would do it... using the “dwell” feature when honing to compensate for the dead end hole so that after finished honing it’s a straight hole. Said usually they like to keep stuff within tenth’s of a thousandth, but with blind boring like this they would hold it to within half a thousandth at most. I don’t know but that seems pretty good long as his walk lines up with his talk. He thought his bill would be around 250-300 if he did it but would want to look at the block before saying for sure.

Haven’t decided yet if I will give Wayne another crack at it or not. If I do it will be with the understanding that his work will be checked by at least one other machinist and if it’s not within our previously agreed upon tolerances, then he’ll either buy me another block or maybe a brand spanking new motor or something...

And if he doesn’t agree to all that, then he’ll agree to foot the bill for someone else to do it right.

We’ll see...

 

[elr]

Two questions...

1). What are reasonable tolerances to expect when getting a block machined? I should of had this conversation before the first attempt, I know... But +/- half a thousandth? one thousandth?

2). Are the lock nuts holding the connecting rod to the crank reusable? They appear nylon-ish in nature and I wondered about the longevity thereof.

 

[racerone]

Honing is a " slow process "------A reasonable man should be able to work to half of a thousandth !!!----Or bang on ---as they say.

 

[Faztbullet]

I myself don't like using a honing machine to bore a blind hole block, better off using a bar type an hand feed the last 1/8 inch to ensure its bottomed out. Then use a rigid hone to get finished bore size, which should be 2.905 +/- .0005

 

[racerone]

Honing is done after the boring.----It is a slow process.----A reasonable man should be able to work to +/- 0.0005 "----I do my own blind boring / honing !