9 replies to this topic

[Jacks]

Hello what will i have to do and whats porting and skimming is it expensive and is it worth doing for performance

[Stiggytoo]

If you want performance I would just go for a 1275. Saying that I'm currently building a 998 with 12G940 head.

Ideally you will need to pocket block so the exhaust valves don't hit it. Also, one of the waterways on the block face will need tapping and re drilling to match up with the head. You will also need a small water pump pulley.

Edited by Stiggytoo, 26 December 2013 - 10:39 PM.

[Stiggytoo]

Skimming is when the face of the head is machined, both to make it flat, but more importantly to alter the compression ratio by reducing combustion chamber volume.

Porting is done to improve the ports in the head, by removing material and carefully reshaping; thus allowing the engine to breathe. If you didn't know what this was, it is probably a good idea to send the head to a specialist to do it. This = money.

Edited by Stiggytoo, 26 December 2013 - 10:44 PM.

[Cooperman]

The best suggestion before thinking about modifying your engine is to read up on how engines work, what is involved in modifying an A-Series unit and what needs measuring and subsequent machining to get it all correct.

This is called 'engineering' and it is not possible to just assemble an engine and expect it to work well.

Until one has a good understanding od=f how and why an engine works and what can be done to make it more efficient then best advice is to leave it all standard.

Improving engine performance is interesting and rewarding, but it must be done correctly. If one doesn't understand what to do then either learn or pay an expert a lot of money to do it correctly.

[scrog]

12g202 with 74thou skimmed off is a cheap option with good results paired with decent lcb and inlet manifold
old skool mod tried and tested by many :)

Edited by scrog, 27 December 2013 - 01:09 PM.

[Cooperman]

You can't just say "74 thou skimmed off" unless you have accurately measured everything. That could cause huge problems.

For example, some heads have had their chambers enlarged by gas-flowing and valve de-shrouding. Then some 998 engines had dished pistons and some have flat tops.

When building or modifying an engine, accurate measuring followed by machining as necessary to produce the CORRECT result is essential.

Ignore anyone who says differently as they are suggesting building by guessing rather than by engineering.

[scrog]

sorry. Always get advice on any mod's you do if your unsure.

[Cooperman]

One of the problems is that the 'Comics' (you know the magazines I mean!) bang on about engine specifications without fully explaining the engineering issues properly. They might say "Fit a 12G295 head with the compression ratio at 10:1" without explaining how to measure and check how to achieve this.

There is a vast difference between fitting a 12G295 or 12G202 head to a 998 with raised D-top Cooper pistons, flat top pistons or dished pistons. If a 12G295 were set for the correct CR on a 998 with dished pistons and then fitted onto a 998 with flat tops the CR would end up at a figure more suited to a diesel engine.

There is a lot involved in engine building and we on here are always ready to help with advice, checking calculations, etc., but it has to be pre-supposed that those doing their own engine work have a good basic idea of what is involved and how an engine works.

Just ask and an answer, or answers, will be forthcoming and the Mini Docs will be pleased to help 

[minimadles]

I must agree with cooperman on this a good basis for any project is to learn how the thing works before trying to improve or repair it especially with engines to understand how they work will stand you in good stead later on . Things don't always go to plan when modifying engines and having an understanding will help when trying to identify a problem although I am now in the fire service I have been in the motor trade for 35 years and the lessons learnt whilst an apprentice mechanic still hold true today always ask plenty of questions and you will always get an answer there are some very knowledgeable people on here who have done it all learnt valuable lessons on the way and are prepared to share that knowledge on here

[carcrazy]

did this years ago myself with a die grinder, plenty of silicon sealent and a burrette to check the pockets were all cut out

the same, it was a rough way of doing it i know!!

but it only cost me a peice of cardboard to put over the pistons, a tube of silicon sealent to plug the water and oil galeries

and seal she cardboard around the piston tops

use a 1275 head gasket was a peice of cake and well worth the effort

i had twin 1.25 carbs and lcb and rc40

it went like stink and was realy reliable

wish i could get it back!!  still on the road but can't trace it b380 mhf surf blue if anyone has any info?? 

steve