Lotus Twin Cam Rebuild Part 1
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| Magazine Title: | Grassroots Motorsports |
|---|---|
| Published: | May 1990 |
| Author: | John Hagerman |
Contents |
Part 1 - The Short Block
It could be put off no longer. At 30% leakdown, the Europa's Twin Cam engine was a shadow of its former self. And besides, if we wanted to get somewhat serious about creating a nationally competitive A/Street Prepared car, a full blueprinting job was in order. So we yanked it.
Most car builders start with the suspension when preparing an autocross car; the sport of autocross places a major emphasis on handling and cornering, so the biggest gains are generally realized by tweaking the suspension. However, we began with the engine on our Europa project for one vitally important reason: oil pressure, or lack thereof.
When the Europa was being produced back in the early 1970s, street tires were far inferior in traction to the DOT-legal racing tires that are in use today, and cornering forces were therefore substantially lower. However, when shod with the ultra-sticky tires that are currently available, the Europa generates sufficient lateral acceleration to cause the oil to slosh away from the oil pump pickup during hard right turns. This problem is most apparent in skidpad-type situations, where the car corners at its limit for several seconds. More than one solution to this problem exists, and we'll be discussing them here.
Blueprinting is the process of optimizing the tolerances within an engine. When a mass-production engine is designed, relatively wide tolerances are used to avoid expensive, high-precision machining. As a result, many of the clearances within a production engine fall outside of the range that will produce the best performance. It therefore follows that optimizing each of the clearances will improve both power and reliability. Blueprinting is permitted by most stock class racing rules, and since it's the only method to legally increase power, it has become virtually mandatory for front-competitors.
Playing by the Rules
This article contains many tips from Dave Bean, who is one of the country's foremost experts on Lotus Twin Cam engines. Dave has built a countless number of Twin Cams, with power outputs ranging from stock to unreal. Since many of these tips were too good not to mention, they were included; however, before building an engine for your chosen category of competition, check the rulebook to insure these tweaks are allowed.
Crack Checking
While the bottom-end of a Twin Cam is quite strong, virtually all Twin Cam engines have been hammered on since they were new. Therefore, the first step in our engine building program was to have the critical components crack-checked. The crank and rods were inspected by Magnafluxing; fortunately, no defects were noted. If the flywheel and pistons had been reused, these also would have been checked; however, we chose to fit new oversized pistons and a Tilton aluminum flywheel to our project engine. Note that since Magnafluxing works only with iron and steel components. Aluminum parts are typically inspected with a dye penetrant; this technique would be required for the pistons and an aluminum flywheel.
Block and Pistons
A high-mileage engine block that has received no previous machining provides the best foundation for a blueprinted engine. When building racing engines, many professional engine builders prefer used engine blocks to new blocks. Blocks that have been in service for a few years have been heated and cooled innumerable times, and their castings have taken a final set. Once the block is fully re-machined, the newly machined surfaces will remain true, since no more shifting will occur in the casting.
After removing the jackshaft bearings, the oil gallery plugs, and the freeze plugs, our block was cleaned in a hot tank. With all of the plugs removed, the hot tank cleaning fluid could effectively clean the internal oil and water passages.
Following the cleaning operation, the line bore of the main bearing saddles was measured. The line bore is the relationship between the geometric centers of the main bearing saddles. When viewed along the longitudinal axis of the crankshaft, the centers of each of the saddles must all be in line. If the centers are not exactly in line, the crankshaft will be forced to bend into shape when the main bearing caps are tightened. Even a minor misalignment of the line bore will cause uneven loading of the main bearings and cyclic bending loads in the crank. This situation will lead to accelerated bearing wear, reduced power, and possibly a broken crankshaft.
The cure for this problem is fairly straightforward: a few thousandths of an inch of material is removed from the mating faces of each of the main bearing caps, the caps are bolted into place, and all of the saddles are rebored in line. Alternatively, the saddles can simply be rebored to a 0.015-inch larger diameter, and suitably oversized bearings can be fitted. Fortunately, our engine didn't require line boring.
The top deck of the block was checked with a machined straightedge to ensure that it was flat and would provide no head gasket sealing problems. Again, our engine checked out just fine.
The final machine work on the block consisted of reboring the cylinders to fit 0.040-inch larger pistons. SCCA Solo 11 Stock and Street Prepared rules allow this overbore, as long as the piston crown remains the original shape (i.e., no domed pistons). On the Twin Cam, a +0.040inch overbore gives a slight increase in displacement (36 cc), an increase in compression ratio of slightly less than one-quarter of a point, and restores the cylinder bore to perfectly round.
When reboring, the correct piston-to-wall clearance is determined by the construction of the piston to be used. Pistons are available in a few flavors, but the main ingredients that affect wall clearance are the skirt design and whether the pistons are of forged or cast construction. On a "tee" skirt piston, the piston crown is connected to the skirt only directly above the wrist pins; on a solid skirt piston, the crown is connected to the skirt over its entire circumference. A "tee" skirt piston tends to retain the combustion heat in the crown, since the heat transfer path to the skirt is limited. Having a much greater heat transfer path, a solid skirt piston dissipates considerably more heat to the skirt. With more heat being transferred to the skirt, the skirt will expand to a greater extent. Since piston-to-wall clearance is measured at the skirt, the piston-to-wall clearance must be greater when using solid-skirt pistons than when using "tee" skirt pistons. Piston-to-wall clearance must be further increased if the pistons are forged, as forged pistons tend to expand to a greater extent than do cast pistons.
For our project Twin Cam engine, we installed Hepolite cast pistons, and used a piston-to-wall clearance of 0.003 inches. To avoid interference between the oversize pistons and the head gasket, the outermost edges of the piston crowns were slightly chamfered. To measure piston ring end gap, each ring was placed in its appropriate cylinder, and squared within the bore by pressing it into the cylinder using an upside-down piston. The acceptable end gap for a Twin Cam is 0.012 to 0.018 inches.
Crankshaft
The key to making a high-revving engine live is to establish the proper main and rod bearing clearances. If the engine is to be consistently run at high revs, the bearing clearances should generally be increased. The increased clearance allows for a greater oil flow to the bearings. These clearances are set either by grinding the crank journals to the proper size, or by using special undersized bearings that provide additional clearance. We chose to use the standard bearings and turn the journals to the proper size.
We used a clearance of 0.0025 inches for the rod bearings, and 0.0020 for the main bearings. The main bearing clearance can be slightly tighter than the rod bearings, since the main bearing loads are less severe and more uniform. Since we knew the clearance we wanted, we chose to allow the machine shop to turn the journals as necessary, rather than specifying a journal diameter. We gave the machinist the main and rod bearings that we would be using, which allowed them to determine the proper journal diameters by measuring the inside diameter of the mounted bearings.
Connecting Rods
The late-model Lotus 125E connecting rods are strong units, and work well with even heavily modified Twin Cams. The earlier 116E connecting rods aren't up to the loads the engine will experience at high revs, and they should be replaced with the 125E units (be careful not to violate any preparation rules).
The connecting rods were first checked for straightness, and then the inside diameter of the big ends was recut to restore them to perfectly round. The recutting was accomplished in the same manner as would be done on a line boring job; a few thousandths of an inch was ground off the rod cap mating surfaces, the cap was bolted to the rod, and the big end was rebored.
The small end bushings were replaced with new bushings honed to a clearance of 0.0005 inches. Replacing the bushings allowed the effective length (i.e., the distance from the center of the big end to the center of the small end) of each connecting rod to be set identically. With each connecting rod having the same effective length, the piston heights were equalized, and therefore the combustion chamber volumes were more equal. Equalizing the combustion chamber volume results in a smoother running engine. Further, the top surface of the block can be milled to match the piston height, allowing the compression to be optimized for a given piston dome shape.
Since connecting rod bolts are typically the most highly stressed fasteners in an engine, it's wise to install new bolts as a part of every engine rebuild. The standard rod bolts are generally adequate for most applications, but for peace of mind we installed a set of Dave Bean's 12-point rod bolts. These bolts have a much higher tensile strength than the standard units, and while perhaps unnecessary for an autocross engine, they were cheap insurance.
Balancing
The next step in our engine rebuild was to have the moving parts balanced. Imbalances in an engine create internal stresses which can destroy the engine at high revs. In addition, the vibrations caused by the imbalance result in wasted power; all of that shaking is caused by power being fed into the engine assembly, rather than out through the driveline. Every manufacturer balances its engines to some extent, but very few mass-produced engines cannot be improved by a careful rebalancing.
Two balancing techniques are currently employed. The "Detroit" balancing technique balances the engine as an assembly, while the "zero" balancing technique balances each part individually. While a perfect balance is more readily achieved using the Detroit technique, the zero balance technique allows individual parts to be replaced without a complete engine rebalance. We chose the zero technique to allow flexibility in future engine work. The components balanced for our project engine were the crankshaft, pistons, connecting rods, front crank pulley, and the flywheel and clutch assembly.
Oiling System
As previously mentioned, the Twin Cam has an oil starvation problem during sustained hard cornering to the right; the oil sloshes away from the oil pump pickup. If nothing is done to stop this, the results are inevitable: the connecting rods will provide an unwanted form of positive crankcase ventilation! Fortunately, several cures for this malady are available.
The best solution to oil starvation in any engine is a dry sump system. Since the dry sump draws oil from a continuously-filled remote oil tank, the oil pump always has a ready supply of oil. Further, a dry sump system can gain one or two horsepower, since windage is reduced in the oil pan. However, a dry sump system is also the most expensive solution to the problem (approximately $600). If you decide to install a dry sump, Dave Bean has all the pieces you'll need.
An alternative method of dealing with the starvation problem is to install an Accusump. The Accusump is a hydraulic accumulator which stores a reserve supply of oil that is discharged when the engine oil pressure drops; when the oil pump resumes pumping, the Accusump is recharged. The volume of oil stored in the Accusump is dependent upon the normal operating oil pressure of the engine; for the Twin Cam with a standard oil pump, the volume is slightly over two quarts, while a high-pressure oil pump increases the stored volume to approximately three quarts. We have not tried an Accusump on our Twin Cam, but have had good luck with it on road racing Triumph engines; if we were to use one, we would also install a high-pressure oil pump to increase its stored oil capacity.
Oil pan baffles can be made, which successfully stop oil starvation; however, developing an effective baffle is often a trial- process, and we chose not to follow this route.
We solved the oil starvation problem in our Europa by installing one of Dave Bean's trick swinging oil pump pickups. This clever little device has a pickup that is free to swing around the oil pan, chasing the oil as it sloshes. With the swinging pickup always immersed in oil, the starvation problem is eliminated. There's only one problem with the swinging pickup-Bean doesn't have any left in stock. He does have plans to produce a few more, however, and hopefully will have a fresh batch of them completed by the time you read this. Give him a call.
We also installed a high-pressure oil pump on our project engine to help the lubrication at high revs and high oil temperatures. This pump raised the normal operating oil pressure from 45 to approximately 65 psi.
Bean also recommends an additional modification to improve the performance of the oiling system. A restrictive plug with a 0.090-inch-diameter hole can be installed in the block to reduce the flow of oil to the head. In a Twin Cam, the valve train receives a disproportionate flow of oil when the engine runs at high revs, causing a build-up of oil in the head. Reducing the oil flow to the head allows more oil to be retained in the sump, and also provides additional flow to the crank and rods.
Water Pump
The water pump should be rebuilt, whether it appears to be in good shape or not. The water pump is built into the front timing chain cover, which is wedged between the head and the oil pan. If a pump failure occurs after the engine is installed, replacement is a murderous process due to all of the parts which must be removed and properly realigned.
Engine Assembly
We used Vandervell VP-2 bearings for both the rods and mains. This particular bearing type is used in the Cosworth DFX racing engines; obviously, it's okay for our autocross Twin Cam. Prior to assembly, we coated each of the bearings and journals with molybdenum disulfide (MoS2) "moly" lubricant. Forget about all of the other assembly lube concoctions-moly is simply the best. It provides proper lubrication during the critical initial start-up of the engine, when no oil is in the galleries. Moly is available at any automotive speed shop. All gaskets were sealed using Permatex Hylomar sealant. This compound can be used on virtually all gaskets, including the head and exhaust manifold gaskets. Hylomar remains resilient after it sets and, unlike silicon sealer, it doesn't ball up and clog oil passages. The crank end float was measured using a dial indicator. The correct end float for the Twin Cam engine is 0.005 to 0.010 inches; if the end float is found to be excessive, thicker thrust washers are available to bring it to within specs. The rod-to-crank side clearance was measured using feeler gauges; the correct clearance is 0.005 to 0.010 inches. The torque settings specified by Lotus are adequate for all of the fasteners used in the engine. These settings should be used to avoid over- stressing threads and/or distorting components. With the bottom end of the engine complete, it was time to move on to the head-a subject we'll cover in the next article.
Further Reading
If you own a Lotus of any description, the most valuable reading available on the subject is Dave Bean's Lotus parts catalog. This half-inch-thick book is much more than simply a listing of parts-it contains extremely valuable assembly recommendations for every subsystem on a Lotus. It's also quite fun to read, being filled with clever tricks learned by Dave during his 15+ years of experience with Loti. The catalog is available for $6 (the best money you'll ever spend on your Lotus) from Dave Bean Engineering
Twin Cam Blueprinting Specifications
(courtesy of Dave Bean Engineering)
| Main bearing bore dimension (standard) | 2.2711 - 2.2716 inches | |
| Main journal diameter (standard) | 2.1251 - 2.1258 | |
| Rod journal diameter (standard) | 1,9367 - 1.9374 | |
| Crank end float | 0.005 - 0.010 | |
| Rod big end inside diameter | 2.0827 - 2.0832 | |
| Rod side clearance | 0.005 - 0.010 | |
| Piston-to-wall clearance | ||
| Cast piston, "tee" skirt | 0.0020 - 0.0030 | |
| Cast piston, solid skirt | 0.0030 - 0.0040 | |
| Forged piston, solid skirt | 0.0035 - 0.0040 | |
| Piston ring end gap | 0.012 - 0.018 | |
| Piston ring to groove clearance | ||
| New | 0.0015 - 0.0020 | |
| Wear limit | 0.0040 | |
| Wrist pin nominal diameter | 0.8120 | |
| Wrist pin to rod clearance | 0.0005 - 0.0009 | |
| Wrist pin to piston clearance | 0.0004 - 0.0008 | |