4 replies to this topic

[clubby crazy]

Hi,

I am currently rebuilding an engine here are the specifications as I am guessing they will be needed for my questions.

1275 + 60 thou and skimmed flush with pistons
Flat top pistons + 60 thou fully floating pin
Standard rods lightened and balanced
Standard CAM 6232 crank with - 10 thou mains and big ends balanced
Megadyne 276 cam
Standard head with rimflo valves and the springs that cam with the cam kit Head has also been skimmed 1.5-2 thou to clean the face.
1.5 roller rockers
Vernier Timing Gear

The problem I have is since the pistons are flush with the deck and using a fairly high lift cam and made even higher with 1.5 rockers, how much valve to piston clearance should I be looking for to make sure that the valves don't hit the pistons at high revs say 5500?

I understand timing does play a part in this as obviously the maximum lift of the valve can be altered in relation to top dead centre. What would be suggested as the best timing for this set up so that I can measure the clearance of the valve to piston?

Also what is the best method of measuring this height? The only method I can find is to use modellers clay on the piston crown and then measure its lowest point if the valve hits.

My last question would be if I find that the valve and piston hit or the clearance suggests that the piston and valve will hit what is the best course of action, would it be recess the valves deeper into the head or machine the piston with pockets?

Thanks In advance for any help

[mini13]

you will be fine with the 276,

Its rare to run into issues of valve to piston clearance on the A series, you'd need to be running a very very agressive race cam. essentially you can prety much forget about it unless you are designing your own cams.

[bmcecosse]

As long as the timing chain doesn't break - they won't hit each other..... Use a duplex chain.....

Edited by bmcecosse, 09 September 2011 - 10:53 PM.

[clubby crazy]

Thanks for the quick replys, my panic is now over!!

I am using a duplex chain so should be ok in that respect

thanks again means I can carry on the build in the morning.

[Cooperman]

By the way, 5500 rpm is not a high figure with a 276 as peak power won't be until about 5900 rpm and will start to fall off above that figure.