Nova Rossi 21 backplate scuff???

[Terry Keeley]

The backplate on my Nova 21 keeps getting scuffed by the back side of the rod, wondering if anyone else has this problem? It doesn't really hurt anything except that I really don't like all that aluminum going through the motor. Any ideas?

 

[AndyBrown]

Make a backplate out of 7075-T6 and get it hard anodized.

 

[DaveMarles]

Hey, We three are Beginners now. Not Newbies any more. Do we get a prize?

Dave

 

[Geraghty]

If the motor is being operated in the standard [crank horizontal] position the scuff on the backplate is caused by the crankpin and wristpin holes in the rod are not parallel to each other.

Because the bushings are not parallel the rod is walking back on the crankpin and scuffing the backplate.

When you take any motor apart that has a bushed rod and you see a shiney spot on the bottom of the rod face,the rod is bent and or the bushings are not parallel.

This is definitely has to be a performance robbing condition and is resulting in un-necessary heat in the lower rod bushing.

I made a fixture,probably 20 years ago now,to check for this.This was when I was looking for every little improvement I could find in NAMBA "stock" motors.I have seen rods out of the box at .002" and up to .030" out of parallel and it hasn't been particular to any motor brand.I don't ever recall finding one at zero/zero out of the box.By in large the most consistant rods were RPM.

Every bushed rod that goes through this shop gets checked and straightened to zero-zero parallelism.I don't even know if "parallelism" is a word or not but you get the idea.

OPPS! forgot something.

When I was looking to get the NAMBA "stock" motors to consistantly 2-stage I have to say straighting the rod was the the biggest factor in accomplishing that goal.

 

[crrcboatz]

Makes perfect sense, Rod. You have done it again!! I am quite at ease knowing Rod has put his knowledge to work on 3 of my motors!!!!

Regards Curt

 

[Terry Keeley]

Make a backplate out of 7075-T6 and get it hard anodized.

Ya, was thinking of either annodizing or chrome palting it. Do you know if the cast backplate will take annodize?

 

[Terry Keeley]

Hey Rod:

I thought about something not being straight in the motor and will check it, but, it's not like it's wearing a grooove or anything, just the occasional light scuff. What if everything is in alignment, wouldn't the rod ride back on the pin once and a while and get into the backplate?

I'm thinking about putting a small spacer on the wrist pin to keep it from coming back, waddyathink???

 

[izitbrokeyet?]

Terry,

I've been anodizing my Novarossi back plates and it holds up well without any apparent scuffing. Maybe I lucked out and have some true rods (I have no way of measuring such a small parallelism)

the color comes out a little on the light side because it's cast material as opposed to machined.

 

[AndyBrown]

WOW! Rod's not a Newbie anymore either. B)

Hey Rod,

No doubt the rod bores must be parallel.

However, if what you are saying about the rod holes not being parallel was true than you could simply turn the rod around and it should no longer rub the backplate. ( if the rod is symetrical and allows this.) I have never seen a 21 that didn't show some signs of the rod rubbing unless the backplate surface was very hard.

I believe the reason for the rod pressure on the backplate is because when the piston is at near TDC the high cylinder pressure puts so much force on the crank and bearing that the crankpin deflexes downward. My experience is that this deflection can be as much as .007" on .67 sized engines. This varies with crank design, distance between the two crank bearings (effects deflected crankpin angle) and the quality and clearence of the big bearing. When the big bearing gets loose, the rubbing will get worse because the crank will deflect downward even more.

Detonation will also increase this deflection.

If the bearing bores and the cylinder bores are not square and the angle between them is more than 90.000 degrees the situation becomes worse.

On the other hand it may be a benefit to make the cylinder and crank bores slightly out of square using an angle less thn 90.000 degrees so that when the big force thing happens everything becomes square.

 

[izitbrokeyet?]

Rod,

would it be possible to get some details on the fixture you use to measure/correct this outtaparallelism? seems to be a solid explanation for the .21 sized motors and I'd like to check for it.....but am unable to find any evidence of being out of true in the rods i'm measuring.

 

[Geraghty]

Hey Andrew,how ya doin?? It's been awhile since I have talked to you.How's Donna and the kids?Well I hope.

I am sure what you say is true but I never really took it that far.To be truthfull I really didn't care and there wasn't anything I could about it if I did. All I was chasing was the shiney spot on the bottom of the rod and honestly never really gave much attention to the backplate scuffing.

Come to think about it I'm not sure I have ever seen any .21 without some scruffing on the backplate either.

All I know is that when the rod was straightened that little shiney spot on the bottom the rod seemed to grow very faint if not totally disappear on the motors that I tested with.I did purposely bend rods to see if the shiney spot got worse and......it did.

Now bear in mind I only tested with K&B 3.5's[outboard and inboards] so there never wasn't any major deflection pressures[if any ] on the crank or bearings.

When I first started chasing the "shiney spot",it was with a K&B 3.5 outboard.With the PTO disc being hardened,I never saw any scuffing on the PTO disc anyway.Now I as I recall I think maybe I did see scuffing on the K&B Marine 3.5 backplate I tested with but I can't be absolutely positive,that was a long long time ago.

I guess all I am saying here is I have always just assumed that a parallel rod just has to better than one that isn't.Like I said all I was chasing was the shiney spot. :blink:

I guess what has surprised me most,over the years that I have been checking rods,is the quality of the "parallelism" of the rod seems to have nothing to do with the initial cost or manufacturer of the motor.

Terry,

I sure wouldn't worry about light scuffing on the backplate.I think the rod will get back on the plate every once in a while no matter how straight the rod is.I would just continue to look for a shiney spot on the rod bottom and let it go at that.

I myself wouldn't be putting spacers on the wristpin.I think that would just be a bandaid and not really addressing a cure for the problem if the problem exsists.

I think I would rather just put up with the "shiney spot".

Later guys....

 

[Terry Keeley]

Terry,I've been anodizing my Novarossi back plates and it holds up well without any apparent scuffing. Maybe I lucked out and have some true rods (I have no way of measuring such a small parallelism)

the color comes out a little on the light side because it's cast material as opposed to machined.

I think I like that idea best and am going to try it, thanks for the help.

 

[Terry Keeley]

Terry,I sure wouldn't worry about light scuffing on the backplate.I think the rod will get back on the plate every once in a while no matter how straight the rod is.I would just continue to look for a shiney spot on the rod bottom and let it go at that.

I myself wouldn't be putting spacers on the wristpin.I think that would just be a bandaid and not really addressing a cure for the problem if the problem exsists.

I think I would rather just put up with the "shiney spot".

Later guys....

Ya, on second thought putting weight up there and having something that could come apart isn't the best plan. Going to try the annodize as mentioned.

When you say shiny spot are you talking on the face of the rod where it contacts the backplate?

 

[izitbrokeyet?]

Terry,

I'm setting up to anodize today...It's really not a problem for me to hang an additioinal backplate in ther for you. No charge. (have plenty of spares)

 

[Terry Keeley]

Terry,I'm setting up to anodize today...It's really not a problem for me to hang an additioinal backplate in ther for you. No charge. (have plenty of spares)

Hey Kevin:

Thanks for the offer but I've been doin' my own too for a few years now. Where did you get your dye???

 

[izitbrokeyet?]

I get all my stuff here

anodizing stuff link

great service and prices here

have a great one,

KB

 

[Terry Keeley]

Ah, I've seen that site, I got mine from Caswell...

 

[Geraghty]

Terry,

If you look real close you can see the "shiney spot"[if there is one] on either the back plate or crankface side of the rod.The bottom of rod right where the aluminum beam meets the bushing is where I have seen the "shiney spot" on the crankface side.

Later,Rod

 

[DaveMarles]

On the other hand it may be a benefit to make the cylinder and crank bores slightly out of square using an angle less thn 90.000 degrees so that when the big force thing happens everything becomes square.

And then add something to allow for the heat of the exhaust port distorting the crankcase.

 

[Terry Keeley]

Terry,If you look real close you can see the "shiney spot"[if there is one] on either the back plate or crankface side of the rod.The bottom of rod right where the aluminum beam meets the bushing is where I have seen the "shiney spot" on the crankface side.

Later,Rod

Oh, thanks...