Constucting a Bullet Proof Engine: Part 1

[FRP]

Hi All.

This will be the beginning of a series on how to construct a completely bullet proof engine, regardless of make, type and number of cylinders. All engines respond favorably to the same basic preparations. These preparations involve component selection and compatibility, insuring dimensional consistency, adequacy and correctness of internal clearances, and the optimization of surface finishes within the engine.

With regard to component selection and compatibility ; it is hard to overstate, how truly dynamic a piston driven internal combustion engine is in its operation. You have heard it before "it's the combination that counts". You can't change anything within the operating system - bore, stroke, compression ratio, rod ratio, cam timing, cylinder head capacity, etc. that does not effect the way that all the other aspects of the operating paradigm behave. This is particularly true of the "hard" areas of rod ratio, compression ratio, and cam timing within the engine and with the "soft areas" of fuel type (and quality), fuel mixture and ignition timing, as well as intake and exhaust manifold design and for most of us, of course, turbo selection.

This series will focus first on the preparation of the short block and cylinder head and on the preparation of the component parts that make up these assemblies. I'll discuss the measuring and machining processes, how we alter and optimize internal clearances and why we need to do it.
I'll cover the important aspects of assembly techniques and materials and give you as many tricks of the trade as possible.

After we have our virtual engine constructed, I'll discuss how the dynamic areas effect each other and how component and specification selection can serve to optimize the engine for its intended use. A quick example of this inter relation is the one between exhaust event timing and turbine housing size on your turbo - these two dynamic areas are closely related with regard to spool efficiency at low and medium speeds, and, with the rate of back pressure accumulation within the hot side housing as rpms increase. An earlier exhaust valve opening point will help spool the turbo faster but will also make the engine "run out" of hot side breathing at a lower rpm. With this in mind you can see how camshaft selection must not be done without consideration of turbine size and A/R. and vice versa, and, how important it is to know what you expect of your engine before you build it. There are many such dynamic inter-relations effected by component selection and specification.

Engine Block Preparation:

Assuming the block is sound and has not had any trauma such as a spun main bearing or been ventilated by a blown rod, you can proceed with the prep. The first step is to get the block in and out of the hot tank so it can be thoroughly inspected. (If the block has piston oilers they must be removed first). The decision of whether or not to remove oil galley plugs is dependent on access (for brushes and compressed air) on a particular engine design. If the block is very rusty I put them through a shot blaster. It is important to cover the deck surface, or at least tape over the head bolt holes, and to bolt the main caps in place before shot blasting. (The shot will slightly abrade the bolt threads). * A note on freeze plug replacement; I assess freeze pug condition on an engine by engine basis. I don't just automatically knock them out. If they look at all iffy I replace them but if they are obviously not corroded and in good shape I leave them alone.

The block then goes onto an engine stand and the interior of the block is de-burred with a die grinder to remove casting flash. I bevel and blunt the top of each main bearing web to aid oil return and remove the sharp edges left from machining at the factory. The next step is to use a bottoming tap to clean and extend the threads to the bottom of the holes on all of the main cap and head bolt holes. This is time consuming but important since most aftermarket studs are of a design that assumes this step.

The next step is confirming the concentricity (roundness) of the block's main bearing bores. This is only applicable if the factory (not torque to yield style) main bearing cap bolts will be retained. If after- market main studs will be used the block must be lined honed as the studs nearly always distort the caps as they just seem to apply the clamping force in a different way than the bolts did when the bores were machined at the factory. If the stock bolts are to be retained they should be oiled and torqued to specs and a dial bore gage is used to confirm the roundness and saddle to saddle dimensions of the main bearing bores. If the main bores are of differing diameters they will be equalized during the line honing process by applying varying degrees of tightness to the caps when the honing is underway. Proper line honing technique assures that the bores are concentric, are of equal diameter and that the bearings will mount with their surfaces parallel to the crank journal surface.

* A note on German engine crankshaft main bearing clearances; Most all German engines come with factory main bearing clearances that are too small for proper high performance work, especially where increased turbo boost and drastically increased outputs are involved. I have seen many Audi, VW and BMW engines with 200K miles on them and close to zero main bearing clearance! They seem to get away with this at stock output levels but it does not work on high HP and high RPM motors. I mention this in context of line honing because we can safely add .001 of clearance with line honing without unduly effecting bearing crush within the cap and journal assembly.

The next step is to install the main bearings that will be used in the finished engine and torque the studs (or bolts) to specification. The main bearing journal sizes are taken from the crank via an outside micrometer. The micrometer is then used to set the dial bore gauge which is in turn used to determine the bearing clearance in each main bearing. Street engines should have a minimum main bearing clearance of .0022 to .003 (thousandths) of an inch. Very high output race engines should have a minimum of .0027 with .003 to .0032 being preferable. Generally, the clearance added by line honing when combined with aggressive polishing of the main bearing journals on the crankshaft is adequate on street engines. Extra clearance main bearings are available from ACL on certain applications. Where they are not, it is sometimes necessary to have the extra clearance added by grinding the main journals to add the needed clearance. This should only be attempted by highly competent crank grinders. Mile High Crankshaft in Denver is a good example of a shop with this level of competency.

In the next post I will go into boring, honing and cylinder wall finish, block decking and checking the crank for straightness.

[All_Euro]

Awesome - thanks for taking the time to put this together

[Mcstiff]

[scubadave]

Great to know Jeff. I was debating on weather I should have my machine shop deal with my bottom end as I was intending on replacing the rings, bearings (mains and rod) and ARP main studs. Is there anything that should/ought to be replaced like crankcase vent "while you're in there?" Also, are there any modifications that can be done that help with the higher HP environment such as, when is a girdle a necessity, and mods to the oil pan to help resistance to oil splash and whatnot with the higher inertia environments?

[ringbearer]

This is awesome Jeff, thanks for taking the time to write this and share some of your knowledge.

[speeding-g60]

it is nice to have something to read which gives the "behind the scenes" play by play.

[quattro87]

Nice write up Jeff. Thanks for your real world input. I for one enjoy reading about what has been done and for what reasons. I'm looking forward to more.