Rod guide

[NORSK]

I have always thought it was very important to run 0,10-0,20mm side clearance on piston guided rods,even less on crank guided rods
Until recently i have seen a few engines built by "well respected" engine Builders with a side clearance of everything between 3-5mm
One of them was a engine made by vw motorsport in Germany
And to my big surprise all these engines has run trouble free for a long time
None of them went beside the crank oilhole ofcourse,but still
I would have thought there would be alot of sideways movement for the rod,especcially on Cold engine when piston clearance are Maximum and piston can rock sideways in the bore

[Kevin Johnson]

Talk to some Volvo experts because they produced a line of piston guided engines (part of the red top series). Well proven technology now.

[Ed Wright]

What kind of power has anybody seen?

[Stef]

Until recently i have seen a few engines built by "well respected" engine Builders with a side clearance of everything between 3-5mm
One of them was a engine made by vw motorsport in Germany
And to my big surprise all these engines has run trouble free for a long time

A lot of recent-ish F3 and Super 1600 engines are piston guided with similar clearances at the crank pin. They have good engine life and these days you really need to be using "all the tricks" in professional motorsport in Europe to be competitive.
A recent Super 1600 engine by a French manufacturer had a very elaborate exhaust manifold which seemed to use motorcycle type link pipes. It also had asymmetrical cam lobes with flattened tips (not from wear, as ground).

[Ed Wright]

What I meant to ask was: What kind of power *gains* has people seen from going to that?

[NORSK]

What I meant to ask was: What kind of power *gains* has people seen from going to that?

I find it hard to see any gains at all by having a rod rattling sideways,but someone can maybe explain why they do it like that

[NORSK]

Talk to some Volvo experts because they produced a line of piston guided engines (part of the red top series). Well proven technology now.

Yes,Volvo red Block has piston guided rods
But i have also seen Volvo engines built by well known People With lots of sideways clearance.
I just don't understand why they do it like that,i see no advantage in running that much clearance

[Kevin Johnson]

Talk to some Volvo experts because they produced a line of piston guided engines (part of the red top series). Well proven technology now.

Yes,Volvo red Block has piston guided rods
But i have also seen Volvo engines built by well known People With lots of sideways clearance.
I just don't understand why they do it like that,i see no advantage in running that much clearance

Sorry about "block" versus "top". I get a lot of emails describing different color versions of motors and have to try to sort out the jumble too. I had just dealt with one using silver top versus black top.

My guess is that it (larger side clearance) began gaining steam when people started widely substituting in rods from different engines in performance builds. If you are stroking an engine by turning down the journal diameter (offset grinding) the width of the alternate rod often (usually?) comes with a narrower axial beam width.

What was previously likely an attempt to save money by not having to use additional welding and/or machining operations was found to reduce the reciprocating and rotating mass. It also avoids an issue in bearing cooling where if the cheek of the journal and side of the rod meet, the oil that is seeking a path to freedom will naturally follow a path of lessor resistance on the opposite side. This will lead to higher temps on that portion of the bearing receiving a lower amount of flow.

With the Volvo engines the engineering was already in place for the crankshaft, assuming journal centerlines that correspond to bore centerlines. This assumption is not always valid, however, as with at least some versions of the Porsche 944 and 928.

Greater precision/uniformity in machining in the automotive field will also encourage trends like this (a rod journal that is highly tapered would shortly cause the destruction of the engine).

Narrower bearings -- other things being held the same -- create less hydrodynamic drag, of course; assuming the area is still great enough to support the load.

[NORSK]

Yes,that makes sense to use a narrow rod/bearing
But i still don't understand why they avoid using some spacers between rod and pin boss

[Kevin Johnson]

Yes,that makes sense to use a narrow rod/bearing
But i still don't understand why they avoid using some spacers between rod and pin boss

http://www.wiseco.com/Catalogs/SportCom ... swagen.pdf At least some aftermarket pistons do use washers/spacers.

P - Piston guided rod, washer required.

If you are the OEM designing a piston-guided assembly why would you add an additional component? For an aftermarket company with a smaller demand for parts it does make economic sense.

By the way, there are threads on Speedtalk showing the spacers some members have created. In some cases the images are missing but they can be found using Google image search.

Example: viewtopic.php?f=15&t=26934#p310495

imagesM52QT1FU.jpg

images.jpg

[NORSK]

Yes,most aftermarket piston Suppliers do offer washers and can machine the pin boss to desired Width and so the washers fit in there
However,many engine Builders either seems to forget them or just feel there is no need to use them (which i find very strange)

I have ordered many custom pistons over the years for all kind of engines,depending on what forgings and bore size it is often needed to use spacers because the pin boss is too wide to start With.
OEM stuff do not use spacers,they either use a very wide rod or narrow pin boss or most 4 bangers use crank guided rods

In my opinion,if the crank rod journals lines up perfect to center of the cylinder it will be best to guide the rod in both ends,there better not be much end play on the crank though

[PackardV8]

Same question but different; I've seen some custom pistons with extremely short compression height ordered with tight rod small end side clearance, but that the piston would seem to offer very little sideways "guiding" resistance to rod big end movement.

Assuming the rod is free to move sideways on the crank journal, how does one determine what sort of piston design is necessary to resist cocking in the bore?

jack vines

[Warp Speed]

Same question but different; I've seen some custom pistons with extremely short compression height ordered with tight rod small end side clearance, but that the piston would seem to offer very little sideways "guiding" resistance to rod big end movement.

Assuming the rod is free to move sideways on the crank journal, how does one determine what sort of piston design is necessary to resist cocking in the bore?

jack vines

Very similar to what we run in our engines with no problems. Just about any piston design will work.

[PackardV8]

So, coming full circle, as the OP was asking, does the inside of the piston boss really guide/resist the rod lateral motion and to what degree?

Or stated another way, what bad things happen when both the big and small end have identical side clearance and can float?

[NORSK]

So, coming full circle, as the OP was asking, does the inside of the piston boss really guide/resist the rod lateral motion and to what degree?

Or stated another way, what bad things happen when both the big and small end have identical side clearance and can float?

In theory it would be better if both big and small end had identical side clearance,then the rod will come to a stop at both ends at the same time
But in the real world it is just one of the ends which are being guided.
However on the engines i have seen lately,it has been just a huge side clearance,and not the same clearance at both ends.
If for example the crank rod journal is slight tapered,it will push the rod quite badly sideways and bending/binding the rod