500cc DomiRacer

Hello,

I am planning to build a 500cc Domi Racer engine to compete in classic races in Japan.
I plan to use parts from JS Jim for the pistons, connecting rods, valves, and cams.

After careful consideration, and taking into account the compatibility of the ignition system and various other parts, I concluded that using the crankcase from a Commando 750 would offer the most benefits.
I will use 500cc parts for the crankshaft, connecting rods, cylinder, and piston to create a 500cc engine.

So, I have a question for everyone: has anyone ever built a 500cc engine using a 750cc crankcase?
As shown in the photo, I did a test assembly, and it seems to be working fine.

In Japanese classic racing regulations, as long as the displacement meets the requirements, other factors aren’t usually an issue.

I cannot answer your question, but I’m very interested in your build! Please do a ‘build thread’ …

An excellent project but it would be better to use later 88ss/650 crankcases because the 88 barrels bolt straight on. The 750 cases require work to line up the barrel flange to crankcase stud holes. You can fit the early points in timing cover parts to the Domi crankcases with a little work (I did it on mine).

If unlimited funds are available go with late 750 Commando cases, alloy 750 barrel, a one piece 59mm stroke crankshaft, special lightweight cam followers, carbon fibre push rods, lightweight rockers and valves etc. Tune for up to 10,000 rpm. (That is probably just me dreaming!)

Whatever you do steer clear of 72 750 cases, ie any set of cases without the large sump plug. The cases you currently have do have the large sump plug, this has a filter so if you do have a blow-up large debris will not get to the oil pump.

It's my understanding that Domiracers used pivoting finger cam followers, which I toyed with for my 650ss. You're not considering going that way?
Cheers

Thank you, everyone.



I did a test assembly of the case and tried fitting the 500cc barrel, but as expected, the stud positions were off by a few millimeters.


When using the 500cc case, there are still issues to resolve regarding the use of the JS cam (Commando type) and the ignition system (planned to be Tri-Spark).

As shown in the photo, the cams differ between the Domy and Commando types, and it looks like I won’t be able to use the Domy cam sprocket if I use the Commando cam.

I’m also considering whether I can replace the timing side parts and the timing cover with Commando parts.


I’ll keep you updated.

Probably telling you what you already know, but the optimal timing for a Commando and a 88 are a bit different.

Also, a high lift Commando cam might be a bit aggressive for an 88. You may end up with quite a bit of flutter/bounce with those steep ramps designed around such a long stroke motor. Jim will have good advice though. A super light valve train will be a must.

Is it the cam tunnel/trough in the cases being too narrow for the high lift cam that you're running into?

@hybridracer, I'd be very interested to see your ignition set up on the domi cases.

t ingermanson said:

Probably telling you what you already know, but the optimal timing for a Commando and a 88 are a bit different.

Also, a high lift Commando cam might be a bit aggressive for an 88. You may end up with quite a bit of flutter/bounce with those steep ramps designed around such a long stroke motor. Jim will have good advice though. A super light valve train will be a must.

Is it the cam tunnel/trough in the cases being too narrow for the high lift cam that you're running into?

@hybridracer, I'd be very interested to see your ignition set up on the domi cases.

Click to expand...

I used a battery powered mk1 Boyer electronic ignition unit in the Commando timing cover, driven by a 4S cam. I can’t do photos because I stripped the 88 engine in 1984 to put the parts back on the 750 Combat race engine for a final few races.

In order to fit the 4S cam and Commando timing cover the following was done -
increase clearance in the cases and the bottom of the cam follower tunnels for the cam lobes,
fit a thrust washer to the cam (on the Domi cam this is an integral part of the cam),
fit Commando intermediate gear/sprocket, cam sprocket and nut,
Grind clearance for the chain on the tensioner mounting boss, the tensioner will not fit behind the chain so I reversed it and made a spacer and longer studs so it fits in front of the the chain.
There didn’t seem to be any oilway issues in the Commando timing cover so that bolted straight on.
I think that covers it.

I only used it for 3 race meetings because of a fuel starvation problem with the 32mm Amal concentrics. They had worked fine on the 750 but on the 88 the fuel stopped flowing above about 6000rpm. Prior to cutting out, the potential to thrash all but the fastest Commandos was clearly there but I had already started loosing interest and didn’t have the time or facilities to sort it out.

I heard back from Jim.

He mentioned that there’s a 500cc version of the JS cam, so it looks like I’ll be able to use the 500cc cam sprocket and intermediate gear.
Next, I plan to order the following kit and sprocket to install the Tri-spark.

Just to summarise where I am now:

I am building a 500 Norton twin using:

• 500 cases, crank (72.6mm), JS rods and pistons (66mm)
• Standard 500 iron cylinder (spigot type)
• 650 head (non-spigot, #25319)

The issue is the spigot mismatch.

The piston protrudes about 4 mm above the cylinder at TDC, so removing the spigot seems risky (top ring too close to the edge).

Instead, I am considering machining the 650 head to accept the spigot.

Approx. measurements:

• 500 spigot: 72.3 mm OD / 3.7 mm deep
• 650 head recess: 69.4 mm OD / 3.2 mm deep

So about +3 mm diameter and +0.5 mm depth needed.

Questions:

1. Has anyone machined a non-spigot head like this?
2. Any concerns with ~1 cc increase in chamber volume?
3. More importantly — what happens to squish?
4. Is it reasonable to adjust piston position using head gasket thickness, or is that a bad approach?

The 500 and 650 chambers look quite similar, and since most capacity difference is stroke, I assume the chamber difference is small — but I may be wrong.

Any advice appreciated.

Hal said:

Just to summarise where I am now:

I am building a 500 Norton twin using:

• 500 cases, crank (72.6mm), JS rods and pistons (66mm)
• Standard 500 iron cylinder (spigot type)
• 650 head (non-spigot, #25319)

The issue is the spigot mismatch.

The piston protrudes about 4 mm above the cylinder at TDC, so removing the spigot seems risky (top ring too close to the edge).

Instead, I am considering machining the 650 head to accept the spigot.

Approx. measurements:

• 500 spigot: 72.3 mm OD / 3.7 mm deep
• 650 head recess: 69.4 mm OD / 3.2 mm deep

So about +3 mm diameter and +0.5 mm depth needed.

Questions:

1. Has anyone machined a non-spigot head like this?
2. Any concerns with ~1 cc increase in chamber volume?
3. More importantly — what happens to squish?
4. Is it reasonable to adjust piston position using head gasket thickness, or is that a bad approach?

The 500 and 650 chambers look quite similar, and since most capacity difference is stroke, I assume the chamber difference is small — but I may be wrong.

Any advice appreciated.

Click to expand...

I have checked my barrel & head dimensions and all you should do is increase the diameter of the head recess from 69.4mm to about .05mm greater than the barrel spigot. There is no need to increase the depth as yours is already slightly deeper than mine. The apparent interference goes away when you fit the gasket which on mine is .9mm thick (an original copper/asbestos sandwich gasket).
One other dimension you can check is the depth of the combustion chamber from the cylinder head face. It should be 22.1mm.

Hal said:

Just to summarise where I am now:

I am building a 500 Norton twin using:

• 500 cases, crank (72.6mm), JS rods and pistons (66mm)
• Standard 500 iron cylinder (spigot type)
• 650 head (non-spigot, #25319)

The issue is the spigot mismatch.

The piston protrudes about 4 mm above the cylinder at TDC, so removing the spigot seems risky (top ring too close to the edge).

Instead, I am considering machining the 650 head to accept the spigot.

Approx. measurements:

• 500 spigot: 72.3 mm OD / 3.7 mm deep
• 650 head recess: 69.4 mm OD / 3.2 mm deep

So about +3 mm diameter and +0.5 mm depth needed.

Questions:

1. Has anyone machined a non-spigot head like this?
2. Any concerns with ~1 cc increase in chamber volume?
3. More importantly — what happens to squish?
4. Is it reasonable to adjust piston position using head gasket thickness, or is that a bad approach?

The 500 and 650 chambers look quite similar, and since most capacity difference is stroke, I assume the chamber difference is small — but I may be wrong.

Any advice appreciated.

Click to expand...

Hello The cylinder head for 500ss/650ss is reported as the same casting but the spiggot recess in the cyl. head would be machined to suit whichever liner .When liners cracked they were machined off and a metal ring inserted into the spiggot recess. I believe this was a common problem with production models so the factory did away with the spiggot altogether which I think was a retrograde step . If you have the non spiggot head it may be an earlier 650 version i.e pre downdraught head and the exhaust ports I believe were closer. My 500SS is from a '63 build I'm told by the factory at the time. The last ones were the downdraught inlet ports and the wider apart exhaust ports. Have you considered Andy Molnar alloy cylinders here in U.K. iron liners or nickasil extra cost. From the photo the crankcases shown appear to be from the 1950's . The casting across the mouth suggests this is so. They are not suitable for ss crankshafts. SS crankcases bulge are the rear just below the cylinder flange for SS crankshaft flywheel clearance. Going back in time 1970, Gus Kuhn London did have a 500 racer with Commando crankcases and some imported U.S.A made con rods and pistons but no other info as to B. x S. The works Domiracer were modified that nothing was interchangable with prod. models. Might be worth a try if you can trace a Rob North/Norton framed racer . SHINJIRO NADA Black Gold Racing Team .Tokyo. but I'm talking that was 25 years ago Long shot I know! best of luck with project