Almost since the first production engine rolled off the line, people
have known that when it comes to power, it’s cubic inches or cubic
dollars. In some cases both. There’s only two ways to get more cubes
bigger bore and longer stroke so engine builders use both to get the
performance they want from the available components.
Traditionally, stroker engines were made by offset grinding or
welding up the throws and offset grinding the existing crank. Welding
was the road to the biggest strokes, but it also resulted in a lot of
failed cranks. I’ve seen a welded stroker done for a race car and can
tell you it is a complex job if you expect it to stay together. Adding
to the difficulties is getting an existing rod and piston combination
to work with the modified crank and existing block. Stroking, for the
longest time, was a combination of skill, art and experience.
Relatively recently, the advent of CNC machining and various
production efficiencies have made it possible for quite a number of
aftermarket suppliers to add complete stroker kits to their product
lines. These are pre-engineered kits designed to fit specific engines
and typically come complete with cranks, rods, pistons and rings. The
350 to 383 Chevrolet stroker kit is now almost legendary, for example.
The idea, of course, was to offer those who did not or could not
machine up their own an opportunity to stroke their engines and join in
on the fun.
Unfortunately, this has led to horror stories from machine shops all
over the country. Those of us who’ve done our own ‘kits’ over the years
know a good many of the potential pitfalls and take the time and know
the solutions to correct or accommodate them. The horror stories come
from encounters with people who know they want more cubes, know that a
stroker kit can do this, but have no real idea about what it takes to
It is common to discover that someone ordered up his kit from one of
the parts warehouse suppliers, has someone prep the block in stock
style, and then discovers that he can’t assemble the short block
because the crank and rods won’t turn. It’s pretty sad to discover how
many of these guys will immediately call the parts supplier or machine
shop and start screaming about how they were ripped off or how they
got the wrong parts. Some of these characters never do get it. There
are lots of aborted stroker kits around because the end user refuses to
admit he screwed up or doesn’t know enough to get the job done.
I was in the Johnson Machine shop in Rapid City, SD for another
project when I noticed that Mike Schortzman was working on a stroker
kit for a big block Chevy. Having done quite a few strokers myself, I
noticed that Mike was doing a methodical and thorough job at checking
and correcting the typical problems. I whipped out the old notebook and
camera and made note of what he was up to with the idea that you might
like a leg up on how to identify and fix the typical interference
points you can run into and at least understand why most strokers are
not a simple bolt-in deal.
Because changing stroke also requires changes to either rod length or
piston pin height or both, it’s important to make sure when it’s all
assembled the top of the piston is at the right height relative to the
block deck. In most cases, you’ll find that these kits are engineered
so you’ll have to remove a little deck height. This is done to
accommodate surfacing previously been done or needs to be done to
create a good gasket sealing surface.
In most cases the top of the piston should be at or slightly below the
deck at Top Dead Center (TDC). The most common spec is at zero deck or
within .010? below the deck. This spec is based on an ideal squish area
(the space between the flat top areas on the piston and corresponding
head surface) of .040?, which is also the typical composition head
gasket compressed thickness.
It’s fairly rare to set up so the piston is above the deck at TDC
for several reasons. One is that this reduces the protection for the
top ring land of the piston and can make the piston ring more
vulnerable to damage especially if pre-ignition or detonation occurs,
but also for high-temp melt-down. The main reason is that you need at
least .035? of piston to head clearance to avoid contact between the
two under harsh conditions. Most layered compound head gaskets have a
compressed thickness of around .040?. That means more than .005? out of
the hole can be a problem.
So you’ll need to install a piston, rod, pin, and bearings on the crank
in the block and check the deck height. It can be done by independent
calculations, by measuring each part and adding up the dimensions, but
with small inconsistencies and tolerance stacking you will rarely come
out perfectly. I find that calculation generally gets you within about
.005? of the accurate measurement. By doing the pre-assembly, you can
measure it all in place.
Be careful, though. Pistons are made using CNC machining operations
that limit the pin height differences from one piston to the next to
near zero. However, you should check your rods and pistons to make sure
there are no differences in pin height or rod length over .005? total.
For a kit using full-floating pins, you can assemble the rod and
piston using the included pins. However, if you have pressed pins, it
requires using a modified pin. Shops that do custom work like Johnson
Machine will have pins that have been turned down a couple thousandths
so they will slide into both piston and rod rather than press.
Once installed you need to bring the piston up to TDC. However,
there is clearance between the bore and piston and you’ll see it rocks
in the bore. To prevent this and prevent inconsistent measurements,
Mike uses a couple of feeler gauge strips slid down on either side of
the piston. Then he can set up the dial bore gauge and set it so it
reads zero with the gauge pin resting on the block deck. Then, he
swings the gauge over the piston and checks the difference. An
alternative is to use a bridge micrometer or gauge.
In the old days and for custom strokers, one of the big concerns was
that the cam would come into contact with the rods or crank. I know of
no kit assemblies that have not been checked out before the first one
got shipped to make sure this was not a problem. It is one of the best
reasons to use stroker kits. In some cases there are machine operations
that can solve this, but none are easy.
An example might be that the side of the big end of the rods or the
rod bolt may contact the cam lobe. In some cases, you can shave the
bolt or remove a little material from the rod to get clearance. Another
might be where a counterweight on the crank gets into the cam. Here you
can chuck the crank into a lathe and turn it down to get clearance.
Remember that taking material off of any part of the rotating or
reciprocating mass means changes in balance. This has meant adding
Mallory metal and other balancing issues that can be pricey.
In any case, I’d make sure to ask before you lay down your cash if
the supplier knows of any interference problems between the crank and
block or rod to cam lobe. One answer would be that there are problems
and they should also know what it takes to correct them. Another would
be that they know there are no problems. The third is that they don’t
know or don’t even know what that means. You may want to go elsewhere.
Mike’s favorite tool for finding crank interference problems is a large
tie strap. These are about .050? thick and made from nylon so they are
both durable and will not damage parts as he checks them. He installs
the crank and then rotates it. If he finds a spot where it stops, he’ll
identify where that is and what is the problem.
In the case of the build I witnessed, two of the counterweights were a
little too large to clear the block. The crank would rotate, but the
weights were so close they would trap the tie strap between block and
crank. Mike figures if you don’t have at least .050? it’s too close.
As he works, Mike marks the areas where there is insufficient
clearance with a black marker so changes can be made in the right
places. Starting from one end and working step by step and inch by inch
to the other, he looks for any place where the crank gets close to
anything and identifies each.
To fix what he found in this instance, Mike asked Rex Eagleton to
put the crank in the lathe and turn .050? from the two counterweights.
Two areas, one large and one small, showed up when the crank was
turned. By doing this work on the lathe you can cut all the high
surfaces at once. By the way, it was also possible to have gone inside
the block to machine out the contact areas instead of turning the
crank, but that would have taken a lot more time, potentially weakening
the block, and that just didn’t make a lot of sense in this case.
Rod and Piston
Although not likely in a kit assembly, I’ve seen where the bottom of
the piston skirt would hit the crank. This can be fixed by milling the
skirt at the affected area or turning the crank the same way. If you
ever have to modify a piston, do so with care, remove only what’s
needed, and be as consistent from one piston to the next as possible.
Because most kits have been pre-engineered to spec out pistons that do
not have this problem, you may never need to deal with this, but it’s
important that you know it’s possible and take a good hard look at it
to make sure you don’t have something to fix. Mike again uses the tie
strap to check it.
Other places to worry about are where the big end of the rod gets
close to the bottom of the cylinder bore, where it gets close to the
cam, or in some cases at the pan rail. Here again the cam clearance
should have been verified or you should know any limits in lobe
heights. The cam must be installed and degreed in to check clearances
as small changes in rotation between the cam and other components can
dramatically change clearance. The bottom of the bores and pan rails
are routinely notched or relieved to get clearance. Another reason the
tie strap works well is that it’s often hard to see or get your hands
into these areas where the strap will slide right in. In other words,
some of this is sight-oriented, but other areas are identified by feel.
You have to be careful where you remove material. There are areas
where the casting is relatively thin and you can cut into the water
jacket and then you really have trouble on your hands. If you are not
absolutely sure of what is there to work with, you need to ask someone
who has more experience or knowledge with your specific engine before
you screw up.
Finally, each and every block, even the same size, year, and make,
is unique. Castings change, there is the problem of core shift, and the
original machining is not exactly the same from block to block. This
means it’s up to you to take the time and either identify and correct
or verify and pass each and every part of the short block. If you fail
to do this or miss something you can grenade the entire thing and
you’re gonna be left with a serious dent in your wallet. Obviously, the
alternative is to have a pro like Mike do the job for you. It’s going
to cost more that doing it yourself, but if you are not absolutely sure
that you have it done and done right, it may be the best investment in
your engine project you can make.
To download stroker kit charts, click here (Stroker Charts.pdf)
For a complete list of stroker kit suppliers, click here (Stroker suppliers.pdf)