27 replies to this topic

[Anthony30]

Cheers for the offer, Cooperman. Still up in the air what I'm going to do yet, but I do remember envying this build.

http://www.theminifo...275-eating-998/

I remember reading that engine build thoroughly. In the end he said the Kent 276 cam was too much for a road car, I believe. He only lived in Widnes/Cheshire, not far from me.

[Mini Cheddars]

Although he did like how much the Megajolt improved things, which may be a further addition.

What started out as a simple head swap seems to be turning into something a great deal more.

Edited by Mini Cheddars, 03 February 2015 - 05:54 PM.

[Spitz]

Use the 295 head! ( after skimming and checking)

 

I've got two running....one on a 1098(1130) and one on a 998.  Both have dished pistons.

 

It is important though that it be skimmed to bring it down to 24cc(?) from it's stock 28cc(?)

You have to measure this when you get the head.  You can simply use a small piece of clear plexi glass, have two small holes drilled in it, clamp it over one of the chambers and use a medical type syringe to measure of the cc and squeeze some fluid ( I use ATF ) into one of the holes (the other hole lets air escape).  Keep track of the cc's used and that is your chamber volume.

 

I skimmed .080" off both my heads to reach the 24cc mark.

[Mini Cheddars]

So you recommend using it as is.

 

One thing that's just crossed my mind after searching for cams... I believe I have a non-A+ engine and the Kent 266 I see on Minispares says for A+.

 

Would a '76 engine be A+ or does it not make much of a difference?

[Cooperman]

Is 24 cc small enough with dished pistons? The dish is something like 5 cc on the 998 (guessing a bit here) so 24 cc might still give a low figure.  It also depends how far up the bore the piston tops are at TDC. For every 0.010" down the bore the top of the piston comes to there is about 0.9cc of additional volume.

If the dish is 5 cc, the pistons come right to the top, the gasket volume is 3 cc and the chamber is 24 cc, the C.R. will be ((998/4) +32) / 32. This is 8.78:1 which is a bit low really. Ideally you need around 9.75:1 to 10.2:1. If the dish is 6 cc the C.R. would be down at 8.56:1 with a 24 cc chamber. If the piston tops are 0.020" down the bores with a 6 cc dish, the CR would be 8.17:1 - far too low.

If not skimmed at all and with 28 cc chambers, the CR would be 7.9:1, or less when the pistons are not right to the top at TDC and that is assuming a 5 cc dish although it might be 6 cc, so the CR with no skimming could be as low as 7.6:1.

Even with a new 295 head you can only skim around 0.080" from the face as the oil hole comes to within 0.125" from the head face leaving only 0.045" thickness after skimming.

Edited by Cooperman, 03 February 2015 - 08:46 PM.

[Mini Cheddars]

I did ask for British Racing Green, but nevermind!

 

 

 

 

 

 

 

[Spitz]

Is 24 cc small enough with dished pistons? The dish is something like 5 cc on the 998 (guessing a bit here) so 24 cc might still give a low figure.  It also depends how far up the bore the piston tops are at TDC. For every 0.010" down the bore the top of the piston comes to there is about 0.9cc of additional volume.

If the dish is 5 cc, the pistons come right to the top, the gasket volume is 3 cc and the chamber is 24 cc, the C.R. will be ((998/4) +32) / 32. This is 8.78:1 which is a bit low really. Ideally you need around 9.75:1 to 10.2:1. If the dish is 6 cc the C.R. would be down at 8.56:1 with a 24 cc chamber. If the piston tops are 0.020" down the bores with a 6 cc dish, the CR would be 8.17:1 - far too low.

If not skimmed at all and with 28 cc chambers, the CR would be 7.9:1, or less when the pistons are not right to the top at TDC and that is assuming a 5 cc dish although it might be 6 cc, so the CR with no skimming could be as low as 7.6:1.

Even with a new 295 head you can only skim around 0.080" from the face as the oil hole comes to within 0.125" from the head face leaving only 0.045" thickness after skimming.

 

ug.....I have a headache now...lol  ( all good info though ).  I may well still have low comp. ( I'm certain the p[istons were dished )....either way....it's all for naught(almost) unless you do it right with a better cam and better fuel/exhaust to match better head

 

That head looks good...hardened seats in place....Why would you want a BRG head though?  Maybe MOWOG green....... unless you're trying to match your cars exterior colour, although I think engines should be a contrasting colour to the rest

[carbon]

Mini Cheddars - have you had a chance to measure the depth of the 295 head yet?

[Mini Cheddars]

I've not, no. Would I just put a ruler against it?

[Anthony30]

No aftermarket oversized d-top  pistons available?

[Anthony30]

No aftermarket oversized d-top  pistons available?

998 Cooper parts are sometimes hard to come by, we had to source the pistons from Australia. They are Kolbenschmidt, a very good quality German piston and have the impossible to find proper D tops. 

 Found this info on another website. There must be a company producing oversized d-top pistons somewhere?

[Spitz]

I've not, no. Would I just put a ruler against it?

Sure...put a ruler against it..... 2.75" stock.....but measuring the volume is accurate

[Cooperman]

It's not the thickness of the head which determines the chamber volume, as the chambers could have been modified to improve gas flow. The critical dimension is how close the bottom of the oil feed hole comes from the face of the head. it is nominally 0.125" from the head face and there needs to be, ideally, 0.050" remaining metal thickness after skimming. You can go to 0.045" but less than that is not ideal.

 

I've never fitted anything but either flat-top pistons or the Cooper D-top variety with a 295 head. I guess with a 1098 engine it is easier to get the CR correct even with dished pistons.