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UNDER 1000cc
© 1995 Mahlon F. Craft. All rights reserved.

DISCLAIMER: The author has discussed herein, items he is personally most familiar with. Liability for inaccuracies, omissions, mistakes, and/or consequential damages arising from same is expressly denied and disclaimed, regardless of use or application. Use this information, or lack of it at your own risk.

From what I see and hear, not too many of us are running engines in our Abarths that are true to the original appearance of the car. I personally feel that far too much has been written about how to take the easy way out and stuff a hot Fiat 850 engine in your Abarth, and far too little about how to keep it more original. All around the world, Abarths are now receiving restorations that rival many other very expensive, faster cars. It may seem to be a small detail, but the way your engine looks, is just as important as how it performs. Right now, still exist for those with parts missing from their original engines, lacking them all together, or simply looking for a way to build a reliable vintage or historic race engine. Ten years from now these parts will be a lot harder to find. Anyone seriously contemplating a more original competition engine would do well to stock up on Fiat 850 cranks and rods, Fiat 600 633cc engines, and both Fiat 600 and 600D cylinder heads. Some of these parts will be required to assemble an engine, based on the information I will provide in this article.

The Fiat 600, 633cc block is the design Abarth used for nearly all of the under one liter engines. The 750 Abarths used it with only slight modifications. It is not clear to me if the 850 Abarth engines used a specially cast block, but all larger Abarth engines did. Except for the difference in bore spacing and crank journal bore size, they are dimensionally identical to the 633cc Fiat 600 block. Besides those modifications made by Abarth to the 600 block for the 750 engines, other modifications for longevity and reliability should be considered by anyone building one of these engines today.

The oiling system of the Fiat 600 block is with out a doubt the best for racing. All three main bearings can be supplied with oil directly from the main oil gallery, something not possible with the "D" block. Beginning with the 750 Bialbero, Abarth grooved the center main bearing journal saddle to improve oil supply to the critical center main bearing. Factory drawings of the 750 BA bearing shells show that these bearings were drilled with small oil supply holes at 4 points around the shells, in line with the groove. Later, Fiat themselves adopted a similar design on the 903cc engines in the 850 series, using grooved bearings to match. Still later, when Fiat built the A112 Abarth 70HP engines, all main bearing saddles and corresponding areas of the bearing shells were grooved. This modification was adapted from the 1000 Corsa block. With a high volume, high pressure racing oil system, this is probably a worthwhile modification to any block destined for racing. The oiling system of Abarth 1000cc & under racing engines was developed through many years of refinement.

Though not very original on 750 engines, an external, full flow oil filter is an excellent way to prolong engine life and add reliability. The oil pressure relief valve is moved to the bottom of the oil pump, where it drains excess pressure directly into the oil pan. Drilling an oil supply hole for the center main bearing is a must for all racing engines. On the Fiat 600, 633cc and Abarth 750 block this is easily accomplished by drilling a 3/16" hole straight up towards the center of the block to reach the oil gallery that originally fed oil to the bolt-on oil filter assy. The full flow oiling system also requires modifications to the oil passage leading from the oil pump. Look carefully at the oil hole in the block where it matches the oil pump. This hole feeds oil from the pump directly to the main oil gallery on a stock engine. A small hole on the side of this passage reaches the oil pressure relief valve bore, allowing the pressure relief valve to dump excess pressure back into the sump through another small hole in the bottom of it's bore. To modify the oiling system to the full flow type, the system must be changed so that it will operate as follows: All of the oil must flow out of the hole in the block where the oil pressure relief valve was and into a filter. From there, it will feed back to the main oil gallery through the tapping in the block where the stock Fiat 600 low oil pressure sending unit screws in. To do this, the following modifications to the block are required: Drill the oil feed hole in the block out to 3/8", to a depth just below the hole leading to the oil pressure relief valve bore. Tap the remainder of the hole between this area and the main oil gallery for an allen head pipe thread plug, and install it using Loc-tite bearing and sleeve retainer. No oil should be able to reach the main gallery with out going through the filter. The drain hole in the oil pressure relief valve bore must also be closed and the hole feeding into the pressure relief valve bore from the passage from the oil pump must also be enlarged to 3/8". Find or adapt high pressure oil line and fittings with an internal diameter of at least 3/8" to reach a remote filter assembly, leaving the block from the old oil pressure relief valve bore. The tapping for the oil line to reenter the block at the low oil pressure sending unit boss, must be enlarged considerably for a larger fitting so that no restriction of the oil flow will occur. When you have figured out how best to accomplish the above, all that remains is to move the oil pressure relief valve to the bottom of the oil pump, by brazing it onto the remains of a Fiat 850 or 600D oil supply tube, originally bolted to those pumps. The best way to do this is to cut off the tube altogether and silver solder the valve body directly to it. This will not clear a Fiat 850 aluminum oil pan. In that case. It must be attached to the tube at mid point. Be sure to gusset the tube to the mount plate. To prevent fatigue stress cracks caused by engine vibration.

Always use a Fiat 850, 903cc oil pump which has the largest gears. Another worthwhile modification, done to all later Abarth racing engines, is installing screw in plugs in all of the small oil passages. These are located in the bottom of the block, next to the main caps, and on the left side of the block, taping into and through the main oil gallery. This cannot be done to the plugs in the end of the main oil gallery due to their proximity to other oil passages. Most later Abarth racing engines were fitted with a special lubrication fitting to direct oil at the mating area of the cam gear and the oil pump/distributor drive gear. High lift cams, heavy duty valve springs and high RPM produce a lot of wear on these critical parts in a racing engine. Making up this fitting is very easy. Use a piece of flared 3/16" brake line and a brass flare nut fitting. Braze up the end of the tube, and then drill a very small hole in the tip, about .010" in diameter. Tap into the main oil gallery next to the dip stick hole. Install the tube using locktite bearing and sleeve retainer, and bend it so it's nozzle is about 1/2" from the gears. It is also a good idea to make up a support using one of the oil pump bolts as an anchor reduce the risk of metal fatigue failures.

The easiest engines to duplicate are the 750 and 850 Abarth engines. A 767cc replica which will appear totally correct and indistinguishable from an original on the outside can be made from any early FIAT 600, 633cc block using a Fiat 850 crank, 600D con rods and high compression 600D pistons. The Fiat 850 cranks used in this conversion are known for their reliability from their days in H-production SCCA racing.

The street 850 sport engines used as high as 9.5 : 1 with a redline of 6700 RPM. One vintage racer friend has used 12 : 1 compression for many seasons. He reports no appreciable wear on the bearings. He uses a 903cc Fiat 850 Crank and 66mm bore in an engine of 930cc displacement. Fiat 850, Abarth 1000OT or A112 70HP rods are a better choice for reliability, but require a minimum bores of 64mm and 66mm for the A112 1050. 600D wrist pins are the same diameter as Fiat 850, and as a result the "D" pistons can be fitted to them in the Fiat 850 manner by heating the rods and installing the pins as outlined in the Fiat 850 shop manual.

1000cc Engines are the toughest because the Fiat 600 block can't take a bore bigger than 63mm without getting the cylinder walls too thin. There is no good way around this other than using a late 600D block, which reportedly can be bored to 65mm. I placate


CYLINDER BLOCKS: All 750 Abarth use modified Fiat 600 blocks. All other Abarth blocks used this block design, and external appearance due to it's superior oiling capabilities. All of these blocks use the bolt-on oil filter housing, which is a partial flow unit, not suitable for racing. They also all have a large round boss with what appears to be a very large "freeze-out" plug protruding from the left side of the engine which is part of the crankcase ventilation system. All Fiat 600D blocks probably share the same lack of any easy way to oil the chain end crank bearing directly from the main oil gallery. This is because of the centrifugal oil filter on the crankshaft pulley. Because of this design change, the Fiat 600D block is second choice for racing.

BLOCK HEIGHT: All Fiat and Abarth blocks have the same height between the crank center and the top of the block, except, the Fiat 850/127/A112-903cc and A112 Abarth 1050cc, which are taller.

CON ROD LENGTH: All Fiat and Abarth con rods, including 1050cc are the same length except, Fiat 600-Abarth 750 which are shorter, and Fiat 850-903cc which are

CON ROD SIDE CLEARANCE: According to 1000 Corsa Homologation papers I have, the 1000s ran about .008" side clearance. On the street, the 1000 Monomille used Fiat 600D rods, which are thinner even than 1000 OT rods, which if Installed on a 229 series Bialbero or Corsa crank end up with about .025 side clearance. It is not clear to me how much clearance is too much, but my local machinist and engine builder says some race cars with forged aluminum rods run as much as .100" side clearance with no problems. It's probably best to ask your favorite expert for advice for your application.

CON ROD BEARINGS: All Fiat and most Abarth con rods use Fiat 600D-850 bearings. Fiat 600/Abarth 750 use smaller "big-end" bearings. Some racing rods for 1000 Corsa and TCR used special narrow bearings, but were same ID as Fiat 600D-850.

PISTON PINS: There are various types:
Fiat 600-Abarth 750: 18mm free floating, bronze bush in rod. Fiat 600D: 20mm free floating, bronze bush in rod. Fiat 850: 20mm fixed pin, no bush in rod. Heat rod to install.
Abarth 1000 TC, Corsa, OT and A112 1050cc: Use 18mm free floating pins, bronze bush in con rod.

CRANK SHAFT BEARINGS: All Fiat and Abarth engines use main journal bearings of the same ID, except all 1000cc, 1050cc and some 850 Abarth, which use larger ID "big mains" bearings. A112 Abarth bearings make good substitutes for older "big mains" Abarth engines, but require minor modifications for use. When installing a Fiat 850 series crank in a Fiat 600 or 600D block, a minor problem arises with the chain end bearing. You will see that the groove in the bearing does not line up correctly with the oil hole in the crank. The bearing assembly should be moved towards the rear of the block by cutting new locating tab notches in the opposite side of the main journal saddle and cap for this journal only. Good bearings to use for this application are 2 Vandervell VP2775 for the center main and 4 VP2759 for the other two journals. One hole should be drilled in one of the VP2775 bearings aligning with the oil supply hole in the block Minor oiling differences between engine series result in some variations in bearing type, however, generally speaking, Fiat 850 bearings can be used in all engines, except Abarth mentioned above.

Sizes still available ( as of 1995) which will work in Fiat 600/600D blocks or Abarth 750 as of 12/16/87: (I think)
1. Fiat 850-843cc, 817cc flattop 64mm & 65mm plus over sizes, various compressions. High compression versions of these pistons will not work with Fiat 600 or 600D heads.
2. Fiat 600D 62mm, 63mm, and 64mm, original and aftermarket in various compressions
3. Abarth 850 63mm, 64mm- may still be available from Bayless Racing. Deck block .020 to use with 68mm Fiat 850-903cc crank.
4. Abarth 1000. Hard to find in all sizes, except A112 Abarth 1050cc
5. 63.5mm 12 : 1 forged, slipper style racing pistons with 18mm pins for use with 69mm crank and "D" head.
6. 65mm 12: 1 forged, slipper style racing pistons with 18mm pins for use with 69mm crank and "D" head. (Both 5 & 6 can be used with a 68mm Fiat 850 crank by decking the top of the cylinder block .05mm - about .020"

Fiat 600 D rods will fit all bore sizes. Can be bushed in the small end to fit any piston pin. Due to their odd construction, I believe it is not possible to recondition rods
which are out of round. New rod bolts not available.

Fiat 850-843cc and 817cc rods will only fit engines with a bore of at least 64mm. Use A112 Abarth rod bolts for replacement or even better PBS Engineering makes special racing bolts available, claimed to be reliable to 8,500 RPM.

A112 Abarth 1050cc rods can be used to replace any Fiat 600D or 850 rod and most Abarth rods, but require a 66mm bore to be used as is. I believe it is possible to modify them to fit a 65mm bore by removing a small amount of material from the con rod with out weakening it. They are probably the strongest rods available, short of having a set of custom made racing rods made.

Safe OVERBORE: 600 block 62.5mm max., some late 600D block 65mm. Ref: "Tuning and Modifying the Fiat 600 Engine" by John Rich.

CYLINDER HEADS: Fiat 600 and 600D heads will fit either block, but the 600 head has a shallow combustion chamber and will give more compression. According to Rich motors, the installation of 1 1/8" inlet valves in a "D" head was good for a 20% performance increase. Fiat 850 heads may also work, but are not very original in appearance.

CAMSHAFTS: These are a bit of a problem. There are only two profiles currently available in reproduction form in Italy for these engines. The 30/70/70/30 and the 083 45/85/85/45 racing cam, which comes in at about 5,000 RPM. Both require notching the center cam journal for installation, per instructions in the 750 Abarth owners guide.

VALVE TRAIN PARTS: Rocker arm screw breakage in Fiat 600 and 600D heads was a common problem with high-lift cams. The Fiat 850 series engines use a rocker arm with a larger diameter screw. They can be used on the Fiat 600D rocker arm assemblies with no modification when using these assemblies on 600D heads. When using them on Fiat 600 assemblies to be used on Fiat 600/Abarth 750 heads, they must be machined narrower in order for the rocker arms to line up correctly with the push rods and push rod holes in the head. You can also do away with the small tubing that separates the Fiat 600/600D rocker arms and replace it with Fiat 850 spring separators. Careful attention to rocker arm geometry and the clearance between the heel of the rocker arms and the push rods should be given. If it appears too close, shimming or cutting of the rocker shaft towers may be required.

Fiat 600, 600D and 850-843cc and 817cc all use valves with square cut keeper grooves. Fiat 903cc uses valves with rounded grooves. Valves from 903 Fiat will work in 843cc and 817cc heads if all springs, retainers, keepers and seats are transferred. Fiat 850 series valves will not fit Fiat 600 series heads as the stems differ in length. Fiat 850 double valve springs can probably be adapted to Fiat 600 series heads by machining the seat area out to a larger diameter and perhaps a greater depth, but some interference between the rocker arm and the spring retainer may be encountered. VALVE LIFTERS: Fiat 850 series use a slightly different valve lifter than Fiat 600 and 600D engines. The 600 series lifter has a smaller head, and an oil hole drilled in the shaft of the lifter which I believe aids cam lobe lubrication. I believe the 850 series lifters will fit 750, 850 and 1000cc Abarth engines, but unless the heads are reduced in diameter, may not clear the counter bore Abarth provided for them in the block.

1. 64mm Fiat Abarth 750 are hard to find in good condition. The small con rod journal size does not make the bearings long lasting and they cannot take much abuse. One vintage racer however, revs his genuine 750 Bialbero to 7800 RPM regularly. He is reportedly very careful about maintenance, and the 750 Bialbero rods, which appear to be a much re- worked Fiat 600 rod, may be the secret to longevity. (His rods are also modified to take 8mm rod bolts) The moderate 9.7 : 1 compression ratio of this engine probably also contributes to long life.

2. 63.5mm Fiat 600D, in my opinion are not as strong as Fiat 850. They can be distinguished by several ugly casting bosses on the sides of the crank throws.

3. 63.5mm and 68mm Fiat 850 cranks were widely used for racing in H production Fiat 850 Spyders. Al Cosentino claims to have used 8,700 RPM with 13:1 pistons. Many vintage racers use the Fiat 903cc 68mm crank with near dead reliability to 8,000 RPM.

4. 69mm Abarth cranks, some made with "big mains" such as the 214 Record Monza S engines, were used in the 850 TC Nurburgring. Also a high revving engine, and the first of the "Berlina Corsas". There were several types, including special racing cranks: 214D 850TC street sedan 214D/A 850TC Nurburgring 214D/C 850TC Nurburgring Corsa (racing) 214D/D 850TC Nuovo Nurburgring Corsa (racing)

5. 70mm Nardi cranks. There are quite a few of these cranks around. I do not have access to information on their reliability.

6. 74mm Abarth one liter cranks. These cranks, in many different variations found their way into all Abarth 1000cc engines, beginning with the 1000TC, through the 1000
TCR "Radiale", 1000 OT and Autobianchi A112 1050cc. Putting these cranks in a Fiat 600/Abarth 750 block requires line boring for larger main bearings. All except A112 Abarth 1050cc and 1000OT are stamped 229. Racing cranks may be stamped 229/A.

Fiat 600D, 850 series and some Abarth cranks designed for these blocks have a hole in the timing chain end of the crank for the centrifugal filter. For use in an older engine, this hole must be tapped and plugged. 68mm and larger stroke cranks may require clearance work in the block, such as notching the cylinders at the bottom for rod clearance. All 1000cc cranks will interfere with stock Fiat 850 camshafts and derivatives. Abarth cams for engines from which these cranks derive used small shaft diameters and narrow lobes in order to eliminate this problem. A good idea is to check in any case.

OTHER READING...all out of print:

"Tuning and Modifying the Fiat 600 Engine" Don Rosendale and John Rich Published by Rich Motors, Glendale. CA (out of print)

"Fiat 850 Competition guide." FAZA / Al Cosentino. Interesting, but occasionally confusing booklet. Some engine preparation information applies. Nice photos of parts you can't get anymore.

Fiat 600 derivation 750 Abarth owners manual. (long out of print.)

Greg Schmidt's "Fiat and Abarth Tricks." Tons of useful information on engine swaps and other fun stuff.