Milling a head on a Stock 1275 for compression?

[Timmy1959]

I'm restoring a bugeye and I bought a 1973 Midget as a donor car. The 1275 engine is in such great condition I don't really want to pull the pistons out, bore it and replace the pistons.
I understand this motor has 8:1 compression and I'd like to gain what I can by milling the head. Does anyone know how much I can mill the head before the valves hit the pistons and what compression gain I can expect?

 

[BlueMax]

Theirs no simple rule regarding your question. Take this web link plug in all the information working the math where by you know how much you have to play with. Remember 93 octane fuel is generally what you have to deal with today so your limited on CR ratio to 10 to 10.5 to 1 staying in the safe zone. https://www.rbracing-rsr.com/


You will be looking for "Static Compression Ratio Calculator"



HTH

 

[nomad]

Enjoy it as is and burn the cheap fuel!!

Kurt.

 

[Gerard]

There are 8.0 (LC) and 8.8 (HC) pistons. Both were stock.

I'm not sure removing head material is the way to go just for the sake of increasing CR.

If you have LC pistons, you can probably swap for the 8.8 ones and there are also likely flat top pistons that will go to around 9.5 or even 10.5 CR. I prefer to save that for making sure the head needs correction for any distortion. There are other things to do that will help much more that cutting the head. If you're going to pull the head, consider porting and some better valves. David at APT can build you a very nice street head. Add one of his cams and an RC40 exhaust and you'll find a big difference in performance. While you are at it, make sure your cam is dialed in accurately too.

 

[Timmy1959]

Thanks for the good advice. How can I tell which pistons I have?
Is there a stamp on the piston or do I have to measure them?
The bore. compression and pistons are in such good condition, either way I'll gain a few horsepower by working on the head, valves and cam as you suggest. I'd like to end up in the 65 hp range if I can. Time to take the head to a head shop.

 

[Timmy1959]

What a great link. I found the compression calculator. Lots of other interesting projects on that site too!

 

[Gerard]

You shouldn't have any problem attaining 65 HP... that's what they were stock. :thumbsup:

The pistons should have a part number visible, usually around the edge of the crown. The difference if probably in how many CC's the dish is.

Vizard lists the following pistons for 1275 to 1310:*

Hepolite 20395/V1 - 16cc dish-8.0 CR
Hepolite 20755 - 12cc dish-9.4 CR
Leyland - 8cc dish - 10.0

Hepolite 20394/V1 - 10cc dish-8.8 CR
Hepolite 20747/KR - 6cc dish-9.7 CR
Leyland Cooper S - 6cc dish-9.7 CR

Howley Omega - flat top - 11.0 CR
Hepolite 20395/V1 - 16cc dish-8.0 CR
Longman 20395/V1 - .6.5cc offset dish-9.6 CR

Probably not a complete list or necessarily up to date. I listed these just as examples. You'll have to research what's available today. Much more detail available in his publications.

 

[HealeyRick]

If you can't find an ID # and have dished pistons, try this. Cut a piece of plexiglass to completely cover the top of the piston. Drill a small hole in the center. Get some ATF or Marvel Mystery Oil and a medicine dropper graduated in cc's. Drip the fluid into the hole until the fluid fills the dish. Calculate the amount of missing fluid from the medicine dropper and you'll have figured the cc's of your dish. Then use Gerard's figures above to figure CR. Don't forget, if the pistons are already oversized, it will change the mathematics of the equation.

 

[Timmy1959]

Thanks for all the great advice. It looks like I'm going to step up and replace the pistons, rework the head and probably replace the cam. Of course this is always what happens when you dive into a project car. Fortunately, my brother gave me a set of pistons so i'll just have to pay for the 3 over bore job. I'll like the car a lot more if I build some horsepower into it but I don't want a lumpy idle. Any recommendations for a cam? Looks like I'll be getting out the checkbook. So much for the budget restore...

 

[Gerard]

You'll want a 266 or 276 cam for the street. Both 7Ent and APTFast have highly recommended cams for the A-series.

 

[Timmy1959]

Gerard, thanks for the info. I just got my hands on David Vizards tuning an A series motor book. There's a whole lot to this. After a brief scan of the book, it looks like I should invest in the head,cam and overall airflow. Just as you recommended in one of your earlier posts. I wonder what the best bang for the buck is on an Aluminum head?

 

[Spridget64SC]

At this point in the game, the best bang for the bucks spent are;
Compression - up to 9.0/1
Spot on carburetion - Good jets and needles and right Air/Fuel ratio.
Better inlet air flow - K&N Style filtration
Exhaust flow - headers and upsized exhaust.
Better head - Good exhaust flow and better valve seats
Mild cam - something with ~.300" lift/275ish duration & 106 lobe centers.
Displacement - up to +040

All this assumes an engine in good condition. Rings in these long stroke engines wear a good bit. Remember, unless the engine has been rebuilt, a 1973 is almost 40 years old.

M2210 or M2211 or M2213 are likely stamped into the top of the stock pistons. If the flat portion of the top of the piston is about 1/4" wide or wider, then these are the higher compression pistons. If the flat part between the dish and the edge of the piston is 1/8" or less, then these are the low compression pistons. High piston dish is about 11.2cc and the low is about 15.4cc.

FWIW: All the BLMC Special tuning literature related that a nominal 0.012" surfaced off the head resulted in a 1.0cc reduction in head volume. To get to 9.0/1, you'll need to cut about 0.070" (oops fat fingered a 9 in there) off the head. Way too much if you plan to use the head in the future with a +.040 overbore and smaller dish. Best to cut about 0.025" and go with that for now.

HTH
Mike Miller
Comptune

 

[Gerard]

I wonder what the best bang for the buck is on an Aluminum head?

Timmy,

I suggest you consider a ported head from David Anton at APTFast. David has done flow bench comparisons between his ported heads and the aluminum heads and claims his modified head flows better than the aluminum heads. I his used his ported heads on complete rebuilds. The results have always been great. That, along with Mikes's recommendations should give you great results.

 

[Hap Waldrop]

Most of the aftermarket heads, aluminum, cast iron, MGB, or Midget flow worse than the stock heads due to sloppy casting, they can be greatly improved with porting, but out of the box, they normally flow worse than stock.