Hi-Po Cranks (2.5) - Difference between Large & Small bearing

[aalbert]

I keep on seeing cranks with lg. vs. sm. bearing being a feature difference. Besides the size differences what are the major differences between these cranks, and what configurations were they originally installed in.

Thanks,

Andy

[novalves]

V-6 Cranks (pre heavy metal ball bearing top journals)

The large / small main dia. Refers to the top journal only 1.375 small 1.500 large
The two center mains are all 1.375 Dia. on all
The 1.5 top cranks were used on 2.0 135s, 150s and on HP models.
The 1.375 tops were used on most all 2.4 & 2.5 production models.
However there are several important differences worth noting

1.5 tops have a narrow center journal with thrust faces that require plastic caged mains
this crank also has the thrust faces required for bottom guided rods. (top or bottom guided rods can be used in these cranks)


1.375 2.4 cranks have a wider center main journals for use with the wire caged mains and also have the thrust surfaces for the bottom guided rods (top or bottom guided rods can be used in these cranks)

1991 and later 2.0 & 2.5 cranks have the 1.375 on all three main journals with thrust faces for plastic cages with the large rollers but lack thrust faces on the rod journals (top guided rods must be used)

[aalbert]

Thank you for the lesson.... What do you mean by " V-6 Cranks (pre heavy metal ball bearing top journals)" ... I take it that there is a later style that we haven't addressed.

-=Andy

[novalves]

the later cranks have malory metal? (heavier than steel ) inserts in the counter weights.
HP. also went to a two row steel caged C1 spec. ball bearing on top a few years ago.

The absolute worst posible type of ball bearing (as far as Im concerned).

C1 spec= tight clearances ( little alluwance for misallignment, flex or growth) in a continuus state of binding.

Steel cage Too heavy to allow balls to revolve at 1/2 crank speed (alwaise slipping)

That Explains all the fret rust on and around the top bearing dosn't it?

Beyond that when installed as intended by HP. there is no provision for crank growth (when heated or flexed cranks get longer)
I have been told that there is clearence and not to worry.

However the seven powerheads that are in process at my shop DO NOT have room for A .005 feeler gauge when mocked up.

[aalbert]

Again, very educational... what are the cutoff years for the hi-performance cranks with respect to being classified as heavy metal, and just to make sure I read it right hi-performance 2.5s (pre-heavy metal) should have the 1.5" tops (as should 2.0 135s, 150s, and 2.4 hi-performance)?

Thanks,
Andy

[WATERWINGS]

novalves,

you appear to be the man, you are new to this board, but seem to have plenty of knowledge, I still need to call you about the block/front 1/2 that needs to be line bored to fix the tight center main problem I have.

[WATERWINGS]

novalves,

I just looked at the crank I'm about to run, we have already realised that it takes the metal cage center mains, (no shoulders on center main journals), the top main journal and the center mains are all 1.375, but the rod journals have slight shoulders, for I hope the bottom guided rods.

After re-reading your above reply, I started to worry about having the wrong rods for this crank, (I have lots of bottom guided rods, and no top guided at all)

How much more bad luck could I possibly have??

[novalves]

Gimme a minute I have machine runing.

I thought I could post them quick

Until I ran the usb plug over with the wheel on my chair

[WATERWINGS]

cool, I am standing by, or will check back in a few, thanks!

[Dave S]

Hi john, it sounds like you have a 2.4 crank. If it was a 2.5 it would have sholders on the center main, and use plastic cages.Hi Mr .Novalves great discription on cranks. I did not know that newer cranks were only top guided only use. I have put old 2.0 big top style in to 2.5 fishy motors, the center main is the right style. I just use the 2.5 bearings and top from the 2.0:)

[novalves]

10 fingers all work in the shop.

I sit down here and 3 maybe 4 work (on a good day)
Lookout if I get Via Voice

The plug is F#C#ED!!
I tried to use my ATI video capture it's a pain in the ass.
Pictures will be posted tomorow.

ALL The metal caged center main cranks have the larger thrust faces for the bottom guided rods.

Waterwings
Sounds like you have the right one.

I have used many of the little bottom guided rods in some pretty hot motors and NEVER have broken one.

Properly preped they can take alot of abuse (broken rods are the effect of loose bolts)

New bolts MUST be used.
I install them as follows


15'# 30'# Loosen bolt 15'# 30'# + 60 Degrees. (.005 stretch)
With 271 Loctite and Merc. 101 on the bolt face

[novalves]

H2O nut

Is that cubic inches or cubic money??

Its alwase fun to beat cubic money.:) :)

[WATERWINGS]

nosubforcubes??

Do I need a decoder ring?

Or does it have to do with..............

No replacement for displacement?

[H2Onut]

You posts are very enjoyable to read, KEEP ON POSTIN...

You technical knowledge is VERY deep.

SO how many cubic inches in volume should the perfect merc 2.4 case have. I.E whats the equation between case volume and cubic inch displacement, (cylinders) or for that matter is ther even such animal on on a reed motor ?:confused:

[novalves]

Speaking of reeds ( Im in it deep now those things are valves)
I meant no (poppet )valves

What if the transfers were scavenged well enough to reduce the crankcase pressure to an ammount less than atmospheric pressure just after BDC?

would those little petals reopen?

I think one biggest reasons that our little mercs have the HP to cubes ratios that they do is there high primary compression ratio

The other big one is the chest / tuner exhaust system.

Ok two systems each with its own job to do.

The chest area

(manages pressure pulses from the other cyls.on the same bank)

The tuner & headpipe

I call the nearly paralell exh. passage in the adapter plate headpipe
(evacuates the chest area) somewhat indipendant of the pressure spikes in the chest area

[WATERWINGS]

novalves,

Did you ever get your wire fixed so you can post the photos?

Or do I need to post a photo of my crank?

(you got me worried, because the crank I have has 3 journals @ 1.375)

[novalves]

Post a pic if you want.
I think you got the right stuff

1.375 Dia journals.
no thrust face on the center two (for wire caged bearings)
Thrust faces on the rod journals ( for bottom guided rods)

Sounds like a 2.4 200 Ect. crank to me

[novalves]

The icky looking stuff is maxima chain wax (great for storage)

[novalves]

The icky looking stuff is maxima chain wax (great for storage)

[novalves]

The icky looking stuff is maxima chain wax (great for storage)

[WATERWINGS]

I think I'm ok too,

I will post photos tonight when I get home.

Thanks!

John

[Mark75H]

novalves you are correct, a properly tuned exhuast system can pull the reeds open just after BDC. I have something here somewhere that mentions that, probably an old motorcycle magazine article. I think Suzuki made a motor that had a short circuit from the carb to the transfer ports to increase the strength of the pulse so it didn't need to pull through the crankcase. Lon Stevens used to mount crabs with reeds directly on the transfer port covers of the old inline 6 Mercs in F. Its not legal in FE on the 6's, but anything goes on a 44 in FE. I've considered building a 44 with lots of carbs..........some on the front, some on transfer port covers, maybe more on piston port intake ports drilled into the valley behind the fuel pumps on a 44.

What do you think?

[espen]

There is a SAE paper of a 2 stroke bike engine fitted with a glass window at the reeds, capturing the reed motion with a high speed camera. They proved that the reeds reopened from the suction puls from the expansion camber.

[B.Leonard]

The biggest reason Mercs make the HP for their size is the rpm they are able to turn.

Hp is a function of rpm, the more rpm the more HP even though your torque can drop off. HP is a measure of work being done not engine strength, the more rpm the motor turns the more work is being done. Engine strength is measured by torque.

In a hot boat that is running on the surface of the water, high HP/rpm is needed more than high torque. The guys runnning the indoor/outdoor drives :p that are burried need incredible amounts of torque to get the same speed. Of course other factors apply.

Cranks...

Don't the late model ('91 and up) 2.5 cranks also have a missing sealing ring groove on #4? Where the 2.4 cranks have all the ring grooves?

-BL

[novalves]

High speed photography is a really cool thing The only one I have seen personally was a dyno run on a inline bp and the crank whiped around like a wet noodle.

There were also dial indicators on the top of the block which was orbiting about .150.

[Mark75H]

BL, there's a little more to it than that, but you are right: shorter stroke and more cylinders gives the same piston accelleration at higher RPM than a longer stroke motor with fewer cylinders.

But, having more, smaller cylinders also gives more port area and tolerates higher compression ratio on the same fuel.

[B.Leonard]

Yeah I was being brief. All that stuff i.e. compression, exhaust timing, porting all enables turning those high Rs. But HP always goes up with rpm. Torque does not.

-BL

[Mark75H]

You are confusing the newbies and lurkers:

Port timing and compression directly affect torque.

[novalves]

2 Stroke horsepower more closely follows piston area than displacement.

Horsepower is not measured it is calculated .

Torque is measured in '#s.
Horsepower is a calculated amount based upon torque over time.

I usually (over explain things ) in my posts this is intended to help the bystanders and never to belittle the Experienced.
Sometimes I find it necessary to point out the obvious no harm intended

[B.Leonard]

"2 Stroke horsepower more closely follows piston area than displacement."

There's an interesting thought. Maybe due to the bigger bore allowing more port area/ better config? Kinda like a big bore 4 stroke allowing bigger valves that aren't shrouded as much?

Good point on the measure/calculate.

-BL

[Mark75H]

It makes sense to me :) :) :) :)