Hot head

[Trog]

I took a bit of a chance with this purchase as It seemed a good price for a refurbished 1275 head too plonk on my 1098

The seller had it on a 1275 mini sprite, can anyone confirm it's origin I've confirmed the casting number as 940g which on this head is next to the heater valve outlet. This brings me to the next question, the heater valve port is blanked or undrilled, can I just drill it out?

the head has double valve springs, presume this will up the pressure on the cam/timing chain?

Last one. no sign of the added 1275 head bolt holes, have a bought a door stop if I try and fit this head to my 1098 or did the gamble pay off? [frame][/frame][frame][/frame]

[chrisd87]

You definitely haven't bought a doorstop .

IIRC the Mini Sprite is an early 90s special edition, so this is probably an unleaded A+ head. This would tally with the lack of heater outlet, and probably means there is no bypass hose outlet either. None of which is a problem. The heater outlet can be drilled, although you'll note it's in a different position on the '940 head, which can foul the Minor battery tray.

Most 1275 heads are of the standard 9-stud variety, so no worries there. Double valve springs will increase friction (and therefore wear) on the camshaft. Best to replace them with standard springs, as double springs aren't necessary on a low-revving, road-going 1098.

[chrisryder]

I hope you're aware that you need to make sure the offset of the exhaust valve faces to the gasket mounting face needs to be 320 thou to ensure that the exhaust valves don't hit the block.

It's possible (just possible) that your head could have larger inlet valves than a normal 1275 head, which could be overkill on a 1098.

I'm not sure if huge inlets are a problem, or whether they're just of no benefit. You'll have to ask bmcecosse on that one

Regarding the heater tap fouling the battery tray, i found that if you drill the mounting holes in the heater tap out slightly larger, you can turn the tap away from the battery tray to allow sufficient clearance.

[Trog]

Thanks chaps I'm going to check the valve to gasket face clearance when I can find something better than my tape measure my next question was going to be valve seats (I'm planning on long distance runs ). the seats on this head seem set in, could be hardened inserts? [frame][/frame]

[chrisryder]

It would only be the exhaust seats that would be hardened. Those rings could well just be machining marks.

Here's my 12G940 head for reference (re: valve sizes)

[frame][/frame]

This head does have hardened inserts for the exhaust valves, but you can still see a ring around the inlet, much like you have.

The gap between the valves looks very similar. I'd be tempted to say your valve sizes are standard normal 1275.

If you remove an exhaust valve, you should be able to see if there is an insert fitted for the seat.

[Trog]

Just checked and valves are 33 and 29 mm as near as using a tape measure can give I'll drop out the valves and look at the seats when I change the springs back to standard.

I'll be rebuilding a standard short 1098 for this head in slow time and on the lookout for parts, any ideas on the best pistons/rods from mini/metro and will flat top pistons give too much compression, say over 9ish to 1?

[chrisryder]

The 1275 head will drop the compression already. not sure on the new ratio, but i can tell you that a standard 1098 head is 26.1cc capacity, and a 1275 head is 21.4cc. So that already drops the compression ratio.

[chrisd87]

Surely a smaller combustion chamber volume would raise the compression ratio?

[chrisryder]

sorry, i didn't mean 'drop the compression ratio', i meant drop the compressed volume, of course, upping the compression.

[bmcecosse]

Yes - dump the inner valve springs, and DO NOT use flat top pistons.........

[Trog]

Thanx again chaps just saved me a heep of time and cash. I'll stick with the standard 1098 internals, just check things are in good condition.

Do I bin just the inner spring or replace both with 1275 items?

Any tips on the type of gasket, copper/standard 1098/1275?

[bmcecosse]

Just remove the inners - and you must use a 1275 gasket. And 1275 rockers - or spend ages re-aligning the existing rockers on the shaft (which is probably worn and needing renewed..) so the pads act directly over the valve stem tips,,,,

[Trog]

Shopping list sorted

[linearaudio]

Payen BK450 is a good composite 1275 head gasket.

[bmcecosse]

Copper faced gasket only for me..........

[chrisryder]

...and be sure to remove a bit of material off the top of the waterpump, to let the head sit flat on the block:



An angle grinder makes light work of it, just make sure you cover the exposed block with duct tape or something, to keep the grinding dust out of the bores!

edit = spelling

[bmcecosse]

Few strokes of a handfile will be enough........

[chrisryder]

When I did it with the pulleys and alternator in place, I tried with a hand-file and there was nowhere to swing it.

When I finally found a direction where I could move it more than 1/2 an inch forward and back, the rate it was taking off material, I'd still be there now!

[bmcecosse]

Seconds to take the alternator off............

[chrisryder]

seconds to plug the grinder in