15 replies to this topic

[sam138]

Could someone please advise me the best method to measure the piston to deck height on my 1380 engine build ? I’m using a dished piston ( power max P20659) and currently trying to calculate the compression ratio of my engine.

[Spider]

You need to bring the piston to absolute TDC. You won't do this by eyeballing or trying to feel it. You need to measure for it.

Set up a degree wheel like you would for dialing in your cam, set it at your best guess to TDC, then set up a dial gauge to the piston crown. It is important that you do that in the right place. The best place is on the flat edge of the crown, right over the gudgon pin. Wind the engine over a few times, then bring it over TDC, make a mental note of the gauge reading as it clocks over that TDC, say that's 0.020 (rising past 0.010) to get there. Then on an upward stroke, stop at 0.015 and read off the angle on your degree wheel, write that down. Go over TDC to say 1/4 - 1/2 way down the bore, then turn the crank in the opposite direction so the piston again is rising, stop at the 0.015 before it gets to TDC, read off & write down that angle.

Taking the two angles, average them and that's then TDC. So lets say you have 5⁰ BTDC as the first number and 9⁰ ATDC as your second number, total, that's 14 degrees, the average of that is 7 degrees. Take that 7 and add it to the first number, that puts you on 2⁰ ATDC (Keep in mind the first number was BTDC, so this moves up past 0⁰ and 2⁰ more after). Wind the engine over until it's dead on 2⁰ ATDC then move your degree wheel pointer to 0⁰. You now have the engine at absolute TDC.

 

Another method that's faster of finding TDC is to use a Piston Stop, but you'd need to make something to use for that.

Once you have the engine at TDC, using a depth Micrometer, measure to the flat land of piston crown over the gudgon pin. That is your Piston to Deck height. It might be tempting to use something like the depth gauge of a vernier to measure this but it's a bit hit and miss as unless you have it dead square, it won't be an accurate reading. Check both sides on the piston over the pin. If you have a variation here either the pistons are rubbish and / or the rod(s) are not straight, and / or the deck is not parallel to the crank line.

[sam138]

Thank you much appreciated

[sam138]

I've started to measure up the piston to deck height and would like to seek some help on how you use a depth micrometre please ? 

As you can see from pic below I've set it up to measure each side of the piston using a 25mm metal insert supplied with Micrometre. 

 

The measurement looks to show 50 ( gone past 45 back to 0)  and mark 1 on the downward markings please could someone help to confirm what this measurement would be as I'm confused ? 

 

 

 

 1234556.jpg   56.93K   7 downloads

Edited by sam138, 08 June 2022 - 08:56 AM.

[GraemeC]

Hard to tell from that camera angle, but that looks like 1.00mm to me.

 

You have the 25mm insert in place, so the scale is starting from 0 (without the insert you would be starting at 25mm) = 0

You have passed the first mark on the top side of the main scale so you are between 0.5 and 1.0mm = 0+0.5

You have wound the micrometer section a full revolution such that the the 0 (or 50) line now coincides with the main scale centre line = 0+0.5+0.50 = 1.00

Edited by GraemeC, 08 June 2022 - 01:23 PM.

[Tones61]

Should be decking block for pistons flush,
Minimum / zero squish band for minimum det/ pinking
My 1293 omegas are 5 thou out of block

[Cooperman]

Since a couple of thou either way is not critical, I use a straight edge and a set of feeler gauges once the piston is at TDC.

It works fine.

[Spider]

I'll refer you to a couple of youtube clips.

 

The first is a basic one on reading Micrometers in general;-

 

 

The second refers specifically to depth Micrometers, you maybe able to skip this one, just keep in mind from an Outside Micrometer, the numbers on the barrel run the other way;-

 

 

[sam138]

Thanks for the reply’s, working out my compression ratio it comes to 10.1 which is ideal for my road engine. Will the pistons down the bore cause an issue ? Taking an average it’s 36.5 thou

[timmy850]

On a bore of 73.5mm (1380):

Piston 36.5 thou down the bore (0.93mm)

The volume of the "empty space" is about 4.0cc. This is enough to have a significant difference on compression ratio unless it's taken into account. 

 

This will give you a rough idea:

https://www.classicm...cal/compression

[GraemeC]

36.5 thou is quite a long way down the bore.  However having the pistons down the bore, whilst not the perfect solution, will unlikely have too much impact on the performance of the engine unless you are looking for the last fraction of power and torque.

The alternative would be to get the block decked down and the head chambers enlarged to accommodate, or deck the block and find some pistons with a larger dish.  With modern fuels I wouldn't go with any higher compression ratio - in fact I would say 10:1 is probably already quite high for a road motor.

[sam138]

Thanks for the info, using that compression ratio calculator I’ve got the following 10:01 compression.

Do I need to also take into account the 4cc empty space as noted above or do these calculators take into account ?

Head 21cc
9cc piston dish
4 cc head gasket ( Af470)
Standard crank
73.5mm pistons


Engine spec is
1380 engine
Ac Dodd RS road cam
MSE 4 minispares head
Hif44
Med Stage 2 ultralight flywheel/ clutch

Edited by sam138, 09 June 2022 - 01:50 PM.

[imack]

You need to account for the piston crown to deck measurement and include it in your calculations.

[mini13]

Yeah, I agree that allthough flushing stuff is optimum, most A series run the pistons a fair bit down the block, so unless your chasing optimum efficiency, or building a forced induction motor i wouldnt worry overly,

 

of course if you did flush the pistons to the block, another way of finding a bit of volume is to get the piston dishes taken out a bit,

 

its also worth mentioning that on large overbores, ( 73.5 +) the head gasket can start overhang the bore depending on alignment, gasket etc... so if you flush the pistons they can start to stick out and touch the gasket at high rpms.

 

 

 

36.5 thou is quite a long way down the bore.  However having the pistons down the bore, whilst not the perfect solution, will unlikely have too much impact on the performance of the engine unless you are looking for the last fraction of power and torque.

The alternative would be to get the block decked down and the head chambers enlarged to accommodate, or deck the block and find some pistons with a larger dish.  With modern fuels I wouldn't go with any higher compression ratio - in fact I would say 10:1 is probably already quite high for a road motor.

 

[timmy850]

Thanks for the info, using that compression ratio calculator I’ve got the following 10:01 compression.

Do I need to also take into account the 4cc empty space as noted above or do these calculators take into account ?

Head 21cc
9cc piston dish
4 cc head gasket ( Af470)
Standard crank
73.5mm pistons


Engine spec is
1380 engine
Ac Dodd RS road cam
MSE 4 minispares head
Hif44
Med Stage 2 ultralight flywheel/ clutch

Use that calculator and put in the piston deck height as 37 thou. That will include the 4cc

Edited by timmy850, 09 June 2022 - 11:59 PM.