Help with 2.5 powerhead rebuild

Oliver.Ashworth · 31-08-2019, 04:19 PM

Hi All,

Hopefully someone can shed some light on what I'm doing wrong....

In the process of rebuilding a 1998 Merc 2.5 200 carb fed, 5-petal reed powerhead after it had a swim a few years back. It had been run on the muffs for about half an hour afterwards but after going back to it some months later it had seized solid. On strip down it turned out the top and bottom crank bearings obviously hadn't gotten any oil and they'd rusted solid... otherwise no signs of damage.

On to the rebuild- have replaced three pistons with standard good, used replacements as three of the originals had wobbly ring pins. Three originals put back in their original bores and all the right way up!

Bought new big end bolts as recommended. All con rods paired and matched with their corresponding 'other half' and orientated correctly. Big end bolts all torqued to 30 ft/lb + 90deg. Did the Mercury check of running a pencil across the 'join' on the big end to check no misalignment. All good- no 'joins' visible and everything smooth. A small amount of play between all big end bearings and con rods. What I would deem to be 'normal'. Each conrod could be moved from side to side over the bearing between thumb and forefinger with very little resistance.

Two of the crankshaft seals replaced with brand new as two had snapped during removal. Everything coated liberally with 2-stroke oil during reassembly. Made sure centre crank bearings correctly rotated so that 'pins' in crankcase lined up with holes in bearing shells. Everything dropped into place nicely.

Refitted other (front) half of crankcase and all bolts torqued to correct value (and in correct sequence). Stood powerhead upright and loosely fitted flywheel to try turning engine over by hand. Flywheel turns easily by hand (clockwise viewed from above) for around two turns before becoming very tight. Engine turns over anti-clockwise by hand forever and a day quite happily!

On looking through the front of the crankcase, all con rods drop to the lowest possible position on the journals when engine is being rotated clockwise. Reaching through with fingers all conrods are tight on their corresponding bearings and can't be moved by hand. After rotating the engine anti-clockwise, conrods re-centre on the journals and go back to the original amount of 'float'.

Anyone have any ideas what's going on here? Hopefully something stupid I've missed!

Cheers,

Ollie

Frenchbruno · 31-08-2019, 10:40 PM

Do you've spark plugs installed when rotating the flywheel??

Frenchbruno · 31-08-2019, 11:54 PM

hope you did respect the connecting rod cap installation and alignment, if not a big risk of damage on rod, bearings, cranckshaft.

Oliver.Ashworth · 01-09-2019, 07:54 AM

No spark plugs yet- cylinder heads are still off....

Yep I made sure the con rod caps were correctly paired with each con rod and the right way round- very puzzled.

Cheers

Frenchbruno · 01-09-2019, 09:28 AM

So, possibly wrong piston position or broken ring coming in cylinder transfer or cylinder port. Now you've no other option than remove the pistons and check what are the culprits. Always cheaper than decide to start engine with such a problem.

Oliver.Ashworth · 01-09-2019, 12:32 PM

Thanks for the reply

Have re-checked all piston positions/orientations and all are correct. All con rods are the right way up with part number facing the flywheel. All rings good with none broken.

Interestingly with the engine laid down on the exhaust plate and the front of the crankcase fitted, the crank spins perfectly in both directions. You can wiggle the con rods slightly on the journals and everything remains free. (the con rods are centred on the journals).As soon as you stand the engine upright and try the same thing, the con rods seem to drop under their own weight and gall on the crankshaft. After 1/2 dozen clockwise turns of the engine they've all gone tight and dropped to the lowest position possible. If you then spin the engine anti-clockwise the con rods re-centre themselves on the journals and everything frees up.

As per the manual I've checked the surfaces where the crankshaft seals run for burrs and everything's nice and smooth. Also checked that the seal ring gaps are all at the 'top' before fitting the front of the crankcase.

It's almost as if there are no guide washers at the small end and the con rods are not staying perpendicular to the pistons... the guide washers are definitely fitted and I don't believe to be excessively worn since the engine ran fine beforehand.

I've checked the big end bearings and both halves look to be identical so I don't think it's an issue with the two halves being the wrong way round?

Frenchbruno · 01-09-2019, 12:57 PM

Interesting,
some more path to go=
Did you check if your rods axis are parallel?
to make it short, may be some rods are slightly twisted?
are you sure of the 3 replaced pistons, reference , size, rings reference for the piston and cylinder diameter?
are you sure your 3 pistons are not oversize?
does your cylinders are clean now after some turns when it's become difficult to turn? any burrs or scarfs is seen now on cylinder surface?
Are you sure the guide rod washers are in good position?
Are you sure you've not lost any needle of the rod bearing during installation?

Oliver.Ashworth · 01-09-2019, 06:36 PM

Have removed all pistons again and am stripping them all down to re-check number of pins (29) and straightness of con rods.

Three down so far, I've laid the con rods on the bed of my lathe to check for any 'wobble' but the first three have checked out perfect. I'll re-check the diameters and bores tomorrow as my vernier has a flat battery

Just a thought- are the big end bearings supposed to be paired? Manual says not to mix them..... a bit late now!

Thanks for your help

Frenchbruno · 01-09-2019, 07:49 PM

I don't exactly understand what you mean by the big end bearing, as the top is a needle bearing and the lower a roller bearing, so difficult to make a mistake, and they are ordered as a single reference each so not paired.
For the 2 big central needle bearing using split race, in this case yes they are paired and cannot be mixed.
A=do you have any burrs inside the block where the contact where the crankshaft sealing ring are installed, they need to move freely.
B= the connecting rods reference must face the flywheel
C= are you sure the ring references you use on the 3 used piston installed are suitable (perhaps these pistons doesn't use the same ring type)
D= did you check the end gap of ring inside the cylinder with a feeler gauge?
E=piston ring side with letter or dot must be facing up
F=connecting rod and connecting rod caps are matched halves
G= you've 2 dowel pin in the block, be sure there is no dirt inside the dowel hole, this can twist a bit the block
F= one more time be sure the 3 used pistons are ok for your engine , reference position, rings,
G = don't use thick paste on the block and its halve, better to use stuff like loctite 518
H= remember, you can check your rings by the exhaust ports

Oliver.Ashworth · 02-09-2019, 08:10 PM

Hi,

OK maybe you've identified the problem-I've mixed the two halves of the six bearings at the bottom of the photo.... Is there an easy way to re-match them or do I need to order new?

Frenchbruno · 02-09-2019, 09:41 PM

Due to the torque on the rod bolts, you also need to replace all rod bolts qty 12) ref#10-76349 1 - Screw, they are only use to be torqued only one time.
But look accurately on your crankshaft using a magnifying glass, as now I suspect you've perhaps damaged the crankshaft pin surface also.
Same for the rods. using a big compression on the 2 halves you probably damaged the surface contact of the halves, so I suspect you need to replace at least 2 of them.
Safer to install 2 new rods, and replace all rod bolts as you've torqued them many times, they are only used for one time use.
important= Don't forget also to clean the inside hole of connecting rods using crocus cloth or 320 grit carborundum.
Replace any rods that cannot properly be cleaned up. chatter marks or uneven wear = replace the rod.
For the 2 pair of half bearings, better to put 2 new ones and avoid to mix them of course. you can see a difference on the inside chamfer are not the same size to be sure they are correctly installed. 2 Bearing Kit 31-8M0045428
hope cranckshaft and rods are not damaged....Good luck!

Oliver.Ashworth · 03-09-2019, 06:33 PM

A good idea to replace the rod bolts, I'll swap out for brand new ones once the problem is found (I don't want to waste another set of new bolts if it has to be stripped down again, it won't be run without new bolts).

The crankshaft has no damage and the rods are in good order, the rod bolts were only torqued down with the alignment correct as per the manual drawing.

I'm terms of matching the split race bearings, they are all mixed unfortunately. Which chamfer are you referring to? I've looked at the bearings closely under magnification and I can't see any differences to the sizes of the internal chamfer?

Frenchbruno · 03-09-2019, 10:08 PM

I've never mixed each kit, and don't know the result.
the halves act like a guide , needle support.
unfortunately I don't have your rod bearing close to me.
you can check on each halves if there is a different chamfer size, or a dot , or a letter engraved. I doubt but....
On the 3.0liter merc the chamfers are not the same on each side, sure at 100%, even merc on these 3.liters don't specify any position , and but on the other hand they also never wrote you can mix all kit.
So I've never did this, and don't know the result, or if on each kit , each half must be paired.
after reflection, the half are only a sort of separator/ guide for the needles and not an accurate broken crack you can see on each center bearing races, in this case there is no possibility to mix for the center halves race.
Did you check is your crankshaft is not bend on a reference table using a pair of vees, and a dial.
you can also install piston rods bearing by pair, then close the block, then turn the crank, pretty long operation, but can check at what moment which pair of piston starts the turning difficult.

Brett C · 30-09-2019, 06:58 PM

Hi have you checked the measurement of the new pistons. Small ends of rods can measure.712 or .812. So 100 thou difference. Are you new pistons the right measurement? This is difference between a top guided or bottom guided engine. If you have top guided rods in bottom guided pistons the big end will rub on the crank thrust areas.