6 replies to this topic

[daniel_johno]

im building up my 1275 A+ on a super budget build and was going to attempt skimming the head a little if it would actually increase performance.

I work for a large oil company so have people who can machine the head for me to very very very tight tolerances. I was going to get the head skimmed on a mill, cnc machine, surface grinder or a similar machine but dont know what is a safe limit to take it to.

Will i need to measure the chamber volume and piston dish or is there a rough set limit for a late 1275 A+?

the engine is a 1988 engine, 21962 hepolite pistons, engine number 12HE25143916 and the head is standard as is everything else on the engine. i will be building the engine back up with a minisport CA2 cam and matched valve springs but everything else will be left standard.

Thanks for looking.

Daniel

[Big_Adam]

Yo,

If you set the mill to a nice slow speed it should give a very nice finish but a grinder will be good. I was going to do this, I mean DIY skimming. The problem is holding the head down. If you look the only places to clamp the head are on the bottom of the thermostat bit and the exhaust ports. I e-mailed MED and found out they use a special clamp that uses the exhaust manifold studs and the heater tap studs.

As for skimming amount, take the head and find the oil hole for the rockers (least rocker pillar on one end or the other. little hole, like size of a 4-5mm bolt) stick the end of a vernier caliper down there to get the plung depth. Write that down then use the callipers on the head going from top to bottom (bit to be skimmed to top were rockers are) write that down. Subtrack the 1st measurement from the last and you get the total material thickness avalibe.

I would suggest a light skim at 1st as very heavy skims can require the rockers needing packing peaice to make up for the lost material thickeness. Beacuse the long push rods, can skim the push rods down if you like tho.

skimming the head will increase the compression ration which means a bi more oompf. But I'd do it when I was going for all the power I can squeeze out of the A-series.

edit

Also buy the David Vizard book, if you got a mill and a lathe theres lots you can do.

edit

..hmm....actualy, might be slightly wrong. Somthing about heads cracking at too hevey a skim. I'll get back to you unelss someone else does

Edited by Big_Adam, 10 January 2007 - 01:29 AM.

[fikus01]

i was told anything over 60 thou with a std head is absolute tops before u get problems!!
trouble is how do u know if the heads been skimmed before!

[Dan]

It's always best to measure your own actual engine and work out the CR you have currently and what you will arrive at given a certain amount of material skimmed off. As said above you don't know if any work has been done already and production engines can vary greatly from one to the next in the detail.

[MRclubman1100cc]

im sure the new laser machines would be able to tell you if the head has been skimmed before...

im getting a 12g295 head for my 1098 and i was going to just bolt it on. but im going for the full hog now. everything that can be done to the head is going to be done, that should give a few more ponnies. and at the end of the yr im wanting to get a SW5 cam, should see about 60HP when finished..

[Dan]

Good job as if you just bolted a 12G295 on it would destroy your compression and the engine would probably barely run. They have a massive 28.3cc chamber as standard whereas the Mini 1100 engine (assuming the Antipodean version is similar to ours) had either a 26.1cc or 25.5cc I think. The 295 is built for special high crown pistons as fitted to the MG1100 and Cooper 998 which protrude up into the chamber to keep the CR up and do magical things to the squish band. A properly modified 12G295 is a great thing though, that's what I'm using although it may be getting past its best now. The only drawback to them is they are all very old lumps of iron now and getting a bit tired as mine is. I don't think mine will last much longer, there's not much original meat left to work with.

Edited by Dan, 10 January 2007 - 03:23 AM.

[Guess-Works.com]

Personally, I would hang fire until you've found out if your block needs boring ( based on the fact you've just got the new rings )...

If you need to rebore the block then that will automatically increase CR, also will your choice of pistons... it's quite viable that you'll be getting some where near 10:1 cr without any head mods.

And sometimes it's actually better to deck the block rather than skim the head..