Top guided vs. bottom guided rods

[silverbullet02]

I've been playing with these mercs for a while now, but never got into building one from scratch, i.e. never touched the rotating assembly. What is a top guided rod and bottom guided rod? I know this is a basic question to some of you, but I just don't know because there aren't many around my area who play with these things. Just looking to understand some of the basics of the rotating assemblies. Thanks!

Brian

[rev.ronnie]

Top guided restricts side to side (up and down on a outboard) movement at the wrist pin area, sometimes with shims that encapsulate the pin bearing where it hangs out the sides, and a bottom guided rod is clearanced tighter to the crank side fillets for lateral movement restriction.

Something has to hold the rod from slopping around from side to side. Top or bottom, but not both.

[luv2gofast]

I think there are two advantages to top guided rods. The first is more clearence on the sides of the big end for lubrication and the most important is less friction because the small end only waggs back and forth as opposed to the big end which would be rubbing all the way around on the crank.

[Dave S]

If ya got losse bores the piston held rod is more prone to go wrong..... splecltly with plastic brearing cages..... Crank held rods are less critical with bore slop.

[silverbullet02]

Thanks guys! That's exactly what I wanted to know, makes perfect sense! So the 225 promax is a top guided motor then?

[malexie]

Yes it is.

[NZ Sidewinder]

This is a point that's not talked about.
It's an old thread but very important.
Wisecos for example collapse in the skirts and if the bore clearence was .009 to start with, which is what you want in a sreaming race motor, then the big end of the con rod is able to walk across (sag) on the crank journal due to gravity.
That causes the rollers to grab at one end.
Plastic cages can't handle this and Aluminium cages can't either.
I caught this issue on the bench.
So my policy is good pistons in a .008 bore can be top guided
Sloppy Wisecos in a .009 bore go bottom guided with 35-40 to 1 premix fuel.

[skialot2]

I get what you are saying but I am not sure about the numbers. On my 2000 2.5 the book calls for a 3.494 piston +/-.001 It calls for a 3.501 bore +.003 allowable taper, OOR, wear. So .007 standard clearance + .001 piston wear + .003 cylinder wear = .011 clearance is still within allowable spec. If standard clearance is .007 and allowable is .011 I don't think you can consider .009 for Wisecos sloppy.

[luv2gofast]

This is a point that's not talked about.
It's an old thread but very important.
Wisecos for example collapse in the skirts and if the bore clearence was .009 to start with, which is what you want in a sreaming race motor, then the big end of the con rod is able to walk across (sag) on the crank journal due to gravity.
That causes the rollers to grab at one end.
Plastic cages can't handle this and Aluminium cages can't either.
I caught this issue on the bench.
So my policy is good pistons in a .008 bore can be top guided
Sloppy Wisecos in a .009 bore go bottom guided with 35-40 to 1 premix fuel.

I'm glad you posted this. It made me think a little deeper into what's happening to the bottom end bearing. I expect that you are right about the extra wear especially at low speed (idle or "on the bench") but I also expect the force against the rollers will square up the big end under load. The journal is pretty wide and clearances are close. For sure something to think about!

[NZ Sidewinder]

I'm not referring to the book.
I'm referring to the motor on the bench that needs to win races and to pistons that start out as one shape and end up another.
The book is a guideline that doesn't allow for motors that are raced.
An engine builder will build an engine based on a combination of what is in the book, what he has in front of him and what's in his toolbox of engineering experience.
With regard Wiseco pistons.
They have a tendency to become ball shaped in the skirts.
This means that the rod has much less support (guiding) from the piston.
I caught this issue on the bench when turning the rotating assembly by hand.
After about 6 rotations the motor which was just sitting on the bench, then started to rotate as a whole, whereas up until that point it would sit still on the bench and allow the rotating assembly to move only.
Then look at the rods through the big open 7 petal front and see that some of the rods had sunk to the bottom of the journal.
Then take a screw driver and flick the rods back up to the middle of the journal.
The rods flick up, don't just slide up.
They were pinched down there.
At speed, centrifugal force holds Then perpendicular to the crank, but it's possibly at low speed and start up where the problem starts.

[skialot2]

Ok I agree with that. Bottom guided rods would definitely support worn out ball shaped pistons better than top guided would. I would also theorize that because a bottom guided rod supports the piston straighter in the cylinder, the piston will have less of a tendency to become ball shaped in the skirts to begin with. I don't build race motors. Just a few recreational motors for myself. I won't reuse a piston with any signs of wear at all. Any piston with ball shaped skirts would be considered junk.

[NZ Sidewinder]

Here's a photo of a ball shaped piston from last season.
It shows very little wear other than on the side of the top rim above the top ring.
Put the mic on this mint looking piston and holy ****, it's a ball.
You are correct, don't use it.

[NICE PAIR]

Here's a photo of a ball shaped piston from last season.
It shows very little wear other than on the side of the top rim above the top ring.
Put the mic on this mint looking piston and holy ****, it's a ball.
You are correct, don't use it.

Great info NZ, can you guestimate the running hours on that piston? For my 2.5 memory banks!

[NZ Sidewinder]

One season racing, maybe 20 races, 5 mins of race time each race, plus another 5 mins per race of 5000rpm milling around, plus testing which would maybe double the race time.
So that's an approx total of 200mins at 7000rpm and 100mins at 5000rpm.
Cooling is stock poppet and stats.
Has diverters.
Bottom guided, which in my opinion saved the motor.
Pistons are plus .020 Wiseco
Bores were fresh 3.523
Piston skirts 3.514

[NZ Sidewinder]

I'm wondering if anyone has tried taking a couple of thou off the top face of a bottom guided rod at the big end.
Or some other mod to the big end that might help oiling.
I have both types of rods and cranks and wristpin spacers etc but right now, based on what I've seen, I've got doubts about top guided stuff with large piston clearences.
This may sound crazy