HONDA CR750

Joep Kortekaas' Honda 750 CR kitted race bike

Restoration Process

General

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crkort2.jpg (140839 bytes)

 

GENERAL

A 1972 CB750R (also commonly known as CR750) was bought on 12 February 1999 including a number of spare parts.Frame # : CB750-1121394    Motor # : CB750E-2224691 

The bike had been allegedly built-up by a former mechanic of the Honda Australia racing team. As bought, the bike was equipped with standard pistons, a standard cylinder head, standard carburettors and battery ignition i.s.o. the factory racing magneto. However, a set of Kei-hin CR sand-cast 31 mm carburettors and a set of new racing pistons complete with piston rings came with the bike. The bike was not equipped with the oil cooler; oil cooler provided as separate part was not the original one.

The engine was entirely painted black, as were the lower fork legs. The original fairing stays were missing. The fairing was not original; it had the wrong shape and was much too thick and hence too heavy. The throttle was not the original one and provided only an opening cable, i.s.o. an opening and closing cable. Cable was approx. 50 cm too long, as was the rev. counter cable.

Spare parts include:

- Chain sprockets 17 and 18t.

- Chain wheels 44, 45, 46, 47, 48 and 49t

- 6 sets of main jets, # 110, 115, 120, 125, 130, 140.

- Set of clutch plates

- Two standard front brake discs.

- Various engine parts, such as 4 conrods, 2 camshaft holders, camchain tensioner and a cylinder block.

 

Started restoration: 20 February 1999.

 

 

ENGINE

Before taking the engine out of the frame, the oil was drained off.

After removing the magneto cover it transpired that the original builder had screwed in the magneto bolt, tightened it hard up and had then sawn-off the head! Bolt removed by Vintage and Modern (V&M). Crankshaft is original Honda CB750R with the polished conrods. The camshaft was the original Honda CB750R, but camchain and camchain tensioner were standard items (racing chain has straight links i.s.o. waisted ones, and racing tensioner is made of steel i.s.o. rubber). The camchain was replaced with one from a Honda CB750 F2, which is claimed to be as strong as the racing chain. The head breather cover had been drilled with two 6 mm holes, which had subsequently been filled with some plastic. Cover was replaced.

The original pistons had been shortened and one had some material from the lower bearing surface of the skirt removed by filing, for no apparent reason. Furthermore, since the high lift racing camshaft was used and the pockets for the valves had not been modified (!), the exhaust valves had hit the pistons. Fortunately, no damage had resulted from this.

Clutch and close-ratio gearbox were in excellent condition. The clutch cover was modified by taking off the three screw pillars for mounting the chromium plated cover which is no longer used. The contact breakers and the fixed racing contact breaker cam were in good order, but the mounting screws for the points plate were all badly damaged, as was to be expected. The chrome points cover was damaged and replaced. The spare cylinder block was bead blasted, bored for the racing pistons (clearance 0.05 mm), and sprayed silver.

The cylinder head had a broken-off cooling fin, and several bent cooling fins. Since the builder had done some "porting", the head was no longer considered to be serviceable, and was replaced. The inlet valves were replaced with the ones from the CB750 F2, which are 34 mm i.s.o. 32 mm, and which were turned down to 33.5 mm. The valves were worked on in the usual way by rounding off the sharp sides and polishing them. Inlet and exhaust ports were reshaped and roughly polished.

The kickstarter mechanism (spring, ratch) was removed, and the shaft (which carries the drive pinion for the oil pump) was shortened. The clutch cover kickstarter shaft hole was closed with an aluminium plug as on the works machines.

All unnecessary material was removed from the transmission cover over the gearchange mechanism. All engine covers were shot blasted and sprayed with one layer of etch primer and two layers of metallic silver. The ridges of the "box" for the starter motor were removed from the upper crankcase, and the hole for the starter motor filled with an aluminium plug 30 mm and 'O'-ring. The black paint was stripped from the crankcases by acid bath (V&M) and sprayed with one layer of etch primer, one layer of grey primer and two layers of metallic silver paint. The Phillips (crosshead) screws were replaced with stainless steel ones, obtained from Japan; all other screws, bolts and nuts were cadmium plated.

The neutral light switch locking bolt was drilled for locking wire, as were the two drain plugs for the crankshaft compartment. The oil filter bolt was modified by turning down the original 12 mm head (which always gives problems because it's too small for undoing the bolt, even if it's tightened correctly) and brazing-on a bored-out 17 mm nut, drilled for lockwiring and cadmium plated.

The set of Kei-hin CR carburettors had been modified for a Kawasaki, which has 15 mm more distance between the centre carburettors. The set was changed back again, and all steel parts cadmium plated. A number of M5 screw threads were damaged; repaired with heli-coils. The ignition coils seemed in good shape, but were only mounted with one M6 bolt; the clip that fits around the upper frame tube was missing and was obtained from a wreckers yard. The screw thread in the gearchange pedal had been stripped, and a nut put on a longer bolt. Because this nut has no square seating area, the bolt was bent. After careful straightening and removal, the screw thread was repaired with a heli-coil M6. During assembly of the engine all gaskets and 'O'-rings were replaced with new ones. Sealant used for crankcases : Permatex super blue. The upper, left top extension of the upper crankcase half, through which the M10 engine mounting bolt runs, had been sawn-off, and re-used as a spacer (?). However, because of the lost material in the saw cut and some subsequent filing, this spacer was too short. A new spacer was turned from aluminium.

 

CYCLE PARTS 

Frame

The frame was modified with the Honda-supplied brackets. A number of original, now redundant brackets had been left on, which were removed. The left hand bracket for the rear set footrest had been brazed on sticking out; this was rectified. The lower stay brackets for the fairing were partly tearing; this was rectified. Since the original builder had been welding on the lower part of the steering head to change the steering locks, the steering head had deformed slightly, causing the ball race to seize. After removal, the ball race chamber was ground to restore to original shape. The frame was shotblasted and powder coated black.

Swingarm

The redundant lugs for the chain guard were removed from the swingarm and the swingarm powder coated black. The dust seal caps and pivot bolt nut were chromium plated. A missing grease nipple in the pivot bolt was renewed. The chain adjuster stoppers were cadmium plated, the chain adjuster stopper bolts were hollowed down and cadmium plated. The bearing bushes (cast iron i.s.o. the usual plastic) were in good condition and were re-used.

Front forks

The steering stem nut had been tightened and loosened with a cold chisel or screwdriver and hammer (sic!), and the dust ring had come loose and was damaged, so the whole was renewed. The ball races had one ball missing. Between the springs and the upper tube bolts M10 nuts had been placed to increase pre-tension of the springs; these were removed. From the top stem nut the dome was removed as on the works racers and the nut chromium plated. Both upper tube bolts were turned flat and chromium plated. In future these will be made from aluminium (weight of two bolts in steel is 220 g). Some material, sticking out from the back of the top yoke was removed, and the top yoke re-sprayed black. The three M8 bolts of the top yoke were hollowed down and cadmium plated. The nuts were renewed. From the lower yoke the lock was removed by grinding. Yoke was newly powder coated. The two M10 bolts were hollowed down and cadmium plated, the bracket for the hydraulic steering damper was sprayed black.

The left fork tube (inner leg) showed a spot where the hard chromium layer had worn through. Furthermore both tubes showed some deep pits and scratches, caused by mechanical impact like hammering(?). The worn tube was bent 0.75 mm, the other tube was straight. The tube was straightened and both tubes were hard chromium plated and ground to original size. After removal of the black paint, the aluminium lower legs showed many damaged places, and a spot on the inside of the leg where somebody had been filing it (?). Both legs were cleaned up as much as possible and polished. Both oil seals were leaking and replaced with new ones. The dust seals were removed as being non-original equipment. Oil drain bolts and nuts to hold the axle holders were drilled for locking wire and cadmium plated.

Front wheel

The front wheel had been built-up with new spokes and was left as is. If a new front tyre of the right type (Dunlop KR84 3.00/3.25x18) can be acquired the existing Bridgestone front tyre will be replaced. The steel spacer on the front axle was replaced with an aluminium one, the speedometer drive (!) was replaced with an aluminium spacer. Axle and axle nut were cadmium plated.   

Front brake

The front wheel with the twin discs was difficult to remove from the forks, as the calipers are inside the width of the wheel, making it impossible to drop the wheel straight out. The joints for the caliper holders are handed, the right-hand one being a racing kit item, not for sale to the general public. Yet the original builder had seen fit to saw off the lugs for mounting the brake hose clips and the front mudguard! No alternative support for the brake hoses had been provided. A new left-hand joint was bought, and the right hand joint was repaired by welding on a new lug, taken from a left hand joint. Both brake hose clips and grommets were missing and were newly provided, as were the brake pipes.

The M10x1.25 socket head screws, holding the calipers together, were all damaged due to using the wrong size spanner; one had been attacked with a cold chisel. They were replaced with new ones. One brake pad had seized solid in the caliper, and was removed with great difficulty. All brake pads were renewed. Both bleeder valves were damaged (six sides rounded) and were replaced. Bleeder valve caps were missing and were renewed. Both caliper holding pins were cadmium plated, and both caliper holders were shot blasted and sprayed silver metal. Mounting bolts for the joint were hollowed down and cadmium plated.

All oil bolts were hollowed down and cadmium plated. The holder for the distributor T-piece was straightened and cadmium plated. Both spare discs were turned down from the standard 7 mm thickness to the kit thickness of 4.5 mm, the centre of both discs was resprayed black. These discs were used, rather than the mounted ones, which were of two different types, and had several damaged spots on the braking area. Disc mounting M8 bolts were hollowed down, cadmium plated and mounted with new locking tabs. A spacer of 2.5 mm thickness (original kit item) was added to correctly position the R/H disc.

Front mudguard

The font mudguard had only one stay, held with one M6 bolt on each side, making the construction prone to tipping. A new mudguard stay will be made up.

Rear wheel

The wheel was dismantled and cleaned. The rear brake was bead blasted and resprayed silver metal. All steel parts were cadmium plated, including the set of sprockets. The brake linings were in good condition and were left as is. The wheel was rebuilt with stainless steel single butted spokes. The original spokes were cadmium plated, the original spoke nipples gold cadmium plated to be kept with the bike as original parts. The rear axle head was damaged with some dents/scratches. After repair the axle, axle nut and washer were chromium plated. Both spacers were replaced with aluminium ones. The chain tensioner bolts were cadmium plated, and the locknuts were drilled for locking wire and cadmium plated. The torque arm stay was cadmium plated, the mounting bolts were hollowed down and cadmium plated; it is intended to replace the steel torque arm stay with an aluminium one as on the works Hondas. If the opportunity arises, the rear tyre (Dunlop KR83 3.50x18) will be replaced with a new one.

Tank

The filler cap was stripped of paint. Filler cap transpired to be magnesium. Fuel tap was cleaned of paint. No more work on the tank foreseen. The steel part of the rubber and steel mounting strap, holding the tank at the back, was cadmium plated.

Oil tank

The oil tank was badly damaged; various dents caused by the bike stand, a large dent where the oil drain plug was bashed in, and various smaller dents wherever previous generations saw fit to put dents. Tank repaired by V&M and resprayed. Oil drain plug hollowed down and cadmium plated. The oil filler cap was crudely provided with a ring to ease removal; ring removed and cap chromium plated. The oil hoses to and from the oil cooler were missing; new ones with stainless steel braiding were made up by Enzed by using the fittings and replated nuts of old standard hoses.

Oil cooler

The oil cooler provided was not the original one, and had to be modified by changing the mounting lugs. Two banjos and banjo bolts were acquired, and the balnjos modified to screw onto the oil hoses.  

Footrests

Both footrests were bent, and the rubbers damaged. New aluminium footrests were obtained. These are very close to the original ones, only the thin rubber covering is missing. Mounting bolts were hollowed down and cadmium plated.

Brake pedal

The brake pedal was cleaned. The pivot hole, which is lined with a nylon bush, was worn out, and will be very difficult to repair. The pivot bracket was turned new from aluminium, the mounting bolt hollowed down and cadmium plated. The pedal stopper bolt was cadmium plated. Rear brake cable was in serviceable condition, but adjusters were replated. 

Fairing stays

The front fairing stay was newly made, and powder coated black. Pinch bolt was cadmium plated. The rear fairing stays will be made when a new fairing is available.

Rev. counter/oil gauge stay

The rev. counter and oil gauge stay showed various cracks, and abundant evidence of (bad) repair welding. Welds were ground out, holes were filled by new welding, and cracks were repaired by soldering in a pair of reinforcing rings. Resprayed satin black. The chromium plated, dented oil pressure gauge bezel was replaced with the correct black one. A speedometer cable was used as tachometer cable. New tachometer cable installed; oil pressure hose shortened and provided with a metal braid and clear plastic sheath.

Clip-ons

Both clip-ons had been extended approximately 45 mm. The left hand side clip-on had a rounded nylon insert; the right hand side one was provided with a cork! The right hand side clip-on was damaged during a fall and badly straightened/repaired. The clip-on was repaired by de-soldering the join between tube and pivoting clip-on bracket, shortening the tube 45 mm to get rid of the buckles, and re-soldering to the stub on the bracket. The left hand clip-on was shortened 45 mm. Both clip-ons were provided with rounded aluminium inserts on the ends and sprayed black. The mounting bolts, already hollowed down and drilled for locking wire, were cadmium plated. Original Honda grips were used.

Throttle

The throttle was replaced with a Honda throttle using an opening and closing cable; the throttle's black anodising layer was removed and the throttle polished. Throttle cables were missing and were renewed, using original CB750 ones, which were modified to fit the CR carburettors.

Clutch lever

The clutch lever was cleaned-up, the pivot bolt chromium plated and the clamp bolt cadmium plated. The bracket was cleaned up and sprayed black. Clutch cable renewed; the adjuster replaced with the race adjuster.

Front brake lever

The lever was cleaned-up, in the centre of the bracket a 10 mm hole was drilled and the bracket was resprayed black. Mounting bolts cadmium plated.

Steering damper

The original hydraulic CR type steering damper was corroded and the rubber sleeve was partly missing. Cadmium or zinc plating is impossible, since the steering damper cannot be taken apart. The steering damper body was polished using steelwool, and was then sprayed with clear lacquer. The rose joint was cadmium plated.  

Fairing

The fairing provided is the wrong shape, is too short and is much too heavy. Fairing will be replaced with one from Airtech in the States.

SPECIFICATIONS

Engine: Four stroke, air cooled four cylinder in-line engine, with single overhead camshaft and two valves per cylinder, in unit construction with the gear box.

Performance: Power 96 hp @ 9,500 rpm

Bore x stroke: 61.5 x 63 mm

                                        Capacity: 748.6 cc

                                        Comp. ratio: 11 : 1

                                        Pistons: Slipper type, one compression and one oil scraper ring

Crankshaft: Honda CR racing type, lighter than standard and specially balanced, with polished conrods.

All crankcase main bearing bores are size B.

                                        Camshaft: Honda CB750R, valve lift 8.5 mm for both inlet and exhaust

                                        Timing: i.o.: 20 BTDC

i.c.: 45 ABDC

e.o.: 45 BBDC

e.c.: 20 ATDC

Valve timing at 1 mm clearance

Valves: Inlet 33.5 mm, seat width 0.8 mm

Exhaust 27.4 mm, seat width 1.0 mm

Valve clear.: Inlet 0.15 mm, exhaust 0.15 mm

Carburettors: Four sand cast Kei-Hin CR 31 mm with desmodromic operation.

Exhaust

system: Four separate pipes with megaphones, downpipe 32 mm x 530 mm long, megaphones 32 mm x 72 mm, 850 mm long.

Ignition: Crankshaft mounted low-tension magneto with two separate, double outlet high-tension coils, timed by two sets of crankshaft mounted breaker points.

Ignition advance 35 fixed.

Point gap 0.35 mm + 0.1 mm

Spark plugs: NGK D9ES or D10ES

Transmission: Primary two chains, ratio 0.588 (1 : 1.7)

Wet multi plate clutch

Five speed close-ratio gear box, with ratios:

1st: 24/41 = 0.585

2nd: 27/38 = 0.710

3rd: 30/36 = 0.833

4th: 31/33 = 0.94

5th: 34/31 = 1.1

 

Gear box sprocket 17t and 18t

Rear wheel sprocket steel, 44t - 49t, #530 chain

Possible

secundary

ratios: 17/44 = 0.386 18/44 = 0.409

17/45 = 0.378 18/45 = 0.400

17/46 = 0.369 18/46 = 0.391

17/47 = 0.362 18/47 = 0.383

17/48 = 0.354 18/48 = 0.375

17/49 = 0.347 18/49 = 0.367

Cycle parts

Frame: Tubular steel, duplex cradle frame, wheelbase 1473 mm, steering head angle 27, trail 95 mm, seat height 630 mm

Front forks: Telescopic, Honda CB750R

Fork tubes 35 mm

Rear shocks: Twin, telescopic, Honda CB750R

Steering

damper: Hydraulic, Honda CR

Front wheel: 18" DID WM3 (2.15) high flange aluminium rim on Honda hub, 40 spokes,

double disc brake

Discs 300 mm x 4.5 mm thick

Single piston, standard Honda calipers

Front tyre Bridgestone, 3.00 x 18

Rear wheel: Honda magnesium alloy 205 mm tls brake in 18" DID WM3 (2.15) high flange aluminium rim, 40 stainless steel single butted spokes

Rear tyre Dunlop KR83 3.50 x 18.

Fuel tank: Aluminium, Honda CB750R, 27 l. Magnesium filler cap, special twin

outlet (8 mm) fuel tap.

Oil tank: Centrally mounted, aluminium, Honda CB750R

Oil cooler: Aluminium, unknown origin

Clip-ons: Honda CB750R, hinged, steel

Rev. counter: Honda CB750R, 4,000-12,000 rpm, with angle drive

Oil pressure

gauge: Smith, 0 - 160 psi

Seat: Honda CB750R, polyester

Weight:

Frame: 16 kg

Swingarm: 4 kg

Total, dry: kg (front ......kg, rear ........kg)

 

PARTS RENEWED OR REPLACED

Front wheel spacers:

One was speedo drive, the other steel. Both replaced with aluminium ones

Lock tabs for disc mounting bolts Missing

Discs Two different ones, and damaged

Disc spacer r/h disc 2.5 mm thick Missing

Brake pads Damaged

Caliper mounting socket head bolts Damaged

Bleeder valves Damaged

Bleeder valve caps Missing

L/H caliper joint Damaged

Hydraulic oil pipes Damaged

Hydraulic oil hoses Old; one was too long

Hydraulic oil hose clips Missing

Hydraulic oil hose grommets Missing

Front fork oil seals Leaking

Front fork stem nut Damaged

One steel ball Missing

Fairing stay Missing

Fairing Wrong type

Throttle Wrong type

Throttle cables Wrong type, only one i.s.o. two

Rubber grips Wrong type

Clutch cable Damaged

Ignition coil clamp Missing

Breather cover Damaged

Rev. counter cable Wrong type; speedo cable was used

Cylinder head Damaged

Inlet valves Wrong type

Camshaft chain Wrong type

Camshaft chain tensioner Worn

Pistons and piston rings Wrong type

Piston pins and clips To go with new pistons

Output shaft oil drip bolt & locktab Damaged

Oil gallery cap Damaged

Breaker points cover Damaged

Engine spacer Too short

Carburettor set Wrong type

Fuel hose Wrong type

Oil hoses Wrong type, missing

Swing arm bolt grease nipple Missing

Brake pedal pivot Damaged

Rear wheel spacers Steel, replaced with aluminium

Rear wheel spokes Replaced with stainless steel

All gaskets, 'O'-rings and oil seals

A number of bolts and nuts Damaged

 

PARTS REPAIRED OR MODIFIED

R/H caliper joint Damaged

Fork tubes hard chromium plated Damaged; one was bent

Fork legs polished Damaged

Upper and lower fork yoke Superfluous material removed

Stem nut Dome removed as works bikes

Tube nuts Turned flat as works bikes

Meter mounting plate Damaged

Oil pressure gauge bezel Wrong type; damaged

Clip-ons One damaged by fall; both too long

Frame Superfluous lugs removed; footrest stay straightened

Oil tank Damaged

Oil tank cap Modified item

Oil cooler Wrong type

Gear change pedal Screw thread damaged

Transmission cover Superfluous material removed

Crankcases Superfluous material removed, starter hole blanked

Clutch cover Pillars removed, kickstarter shaft hole blanked

Oil filter bolt Bigger head

Kickstarter shaft Shortened

Carburettor set Damaged

Footrests Damaged

A large number of bolts and nuts Damaged

Bolts hollowed down As works bikes

Bolts and nuts provided with locking wire holes

All parts eligible were replated or painted

 

CONTRIBUTING COMPANIES

General restoration work, blasting, painting and powder coating

Vintage & Modern

Tony

Unit 4, 34 Davison Street

Maddington, WA 6109

 

Wheel building, spokes, tyres

Spoke Wheel Services

Brian Morgan

Unit 6, 34 Davison Street

Maddington, WA 6109

Tel./fax: 9459 2035

 

Plating, polishing

Ultrachrome Pty Ltd

49 Gordon Road (East)

Osborne Park, WA

Tel.: 9444 5060

Fax: 9444 5155

 

Honda parts

The Honda Shop

Eddie Peters, Sean Peters

Cnr. Great Eastern Highway & Sayer Street

Midland WA 6056

Tel.: 9274 3555

Fax: 9274 1978

Two Wheel Wreckers

7 Neil Street

Osborne Park WA

Tel.: 9444 1484, 9443 2770

Fax: 9443 8213

 

Hard chromium plating, grinding

M.J.S Precision Grinding Pty Ltd

Mike Smith

Unit 4, 73 Buckingham Drive

Wangara, WA 6065

Tel.: 9409 1777

Fax: 93095160

Engine Analysis

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Copyright: Murray Barnard/Joep Kortekaas - 1999