16 replies to this topic

[BaronVonchesto]

So I'm rebuilding my engine and unfortunately the compression ring clearance is quite a bit out of what is specified in the haynes manual.

 

I'm considering putting in +20 flat top pistons for my 998 (minispares ADU3490-20) these incidentally happen to be the cheapest option sold on minispares as far as I can tell.

 

Engine came stock with dished pistons. Piston 1 has the following written on the top: 21965  68 and below that PR0M1 S15

 

Am aware that changing to flat top will raise the compression ratio. Minispares says CR becomes 10.3. My head is the standard A+ 998 head. The number cast on the centre of the head (top side) is 20B0.

 

Are there any concerns with switching to the flat tops? Im not porting or modifying my head in any way for the time being.

[unburntfuelinthemorning]

As long as the fuel isn't low octane it should be fine, what fuel can you get there?  You'll need a touch less total ignition timing.

 

Unless you know the history of the engine make sure you measure the bore sizes before you commit to new pistons so you don't buy the wrong size,  you may have oversized pistons already.

[nicklouse]

calculate the new compression ratio. you may need to spend on a new head sooner than you thought.

[nicklouse]

some reading

https://www.minispar...es.aspx?1~2~103

and

http://www.minispare...tons.aspx?1~2~8

 

so as standard your engine had an 8.3:1 CR and the flat tops had a 10.3:1 CR on standard bore. you are going up in size so the CR is going to get higher....

[BaronVonchesto]

some reading

https://www.minispar...es.aspx?1~2~103

and

http://www.minispare...tons.aspx?1~2~8

 

so as standard your engine had an 8.3:1 CR and the flat tops had a 10.3:1 CR on standard bore. you are going up in size so the CR is going to get higher....

 

So reading the Haynes manual, I'm thinking my garage has given me wrong advice vis a vis changing the pistons. He didn't measure the piston diameter and the bore to get the piston to bore clearance, instead all he did was measure the end gap of the compression ring at the top of the bore for cylinder 1. It came out to 0.321mm if i recall correctly whereas the haynes manual says the max is 0.305mm for a 99H engine.

 

Having done that he was like "yea you need to upsize your pistons to +20 and rehone"

is it possible for the piston ring end gap to be so far off but the piston to bore clearance be within spec? 

[Spider]

Using the Ring Gap for checking for Bore Wear is fraut with a lot of unknowns and errors.

 

Two basic checks that can be done to see if in fact you need a re-bore;-

 

 Feel for a lip at the top of the bores where the top ring runs to, this is usually about 3/8" down the bore. Need to check every bore and right the way around.

 

 Measure the Bore diameter.

 

Using the Ring Gaps, even if it was known from when the engine was built isn't at all accurate. For one thing, the Gaps specified in the Haynes (or any other manual) will be for Factory Rings. Replacements will have a different spec and you don't know what they where when the engine was built. Also, as the Ring runs in and then wears. the Gap will increase.

 

If you are looking to replace your Pistons, you'll find high quality types at very fair prices here  https://www.precisio...D=1290&EID=3432

[BaronVonchesto]

Using the Ring Gap for checking for Bore Wear is fraut with a lot of unknowns and errors.

 

Two basic checks that can be done to see if in fact you need a re-bore;-

 

 Feel for a lip at the top of the bores where the top ring runs to, this is usually about 3/8" down the bore. Need to check every bore and right the way around.

 

 Measure the Bore diameter.

 

Using the Ring Gaps, even if it was known from when the engine was built isn't at all accurate. For one thing, the Gaps specified in the Haynes (or any other manual) will be for Factory Rings. Replacements will have a different spec and you don't know what they where when the engine was built. Also, as the Ring runs in and then wears. the Gap will increase.

 

If you are looking to replace your Pistons, you'll find high quality types at very fair prices here  https://www.precisio...D=1290&EID=3432

 

thanks Moke. I think I'll take the block and pistons to a machine shop to properly measure the bore and piston diameter instead. Haynes don't specify a bore tolerance but do specify piston to bore clearance range and only state that the pistons should be renewed if the piston ring to piston groove clearance is out of spec. Think i'll stick with that as frankly I'm not looking to change pistons unless I really have to.

[Spider]

 

thanks Moke. I think I'll take the block and pistons to a machine shop to properly measure the bore and piston diameter instead. Haynes don't specify a bore tolerance but do specify piston to bore clearance range and only state that the pistons should be renewed if the piston ring to piston groove clearance is out of spec. Think i'll stick with that as frankly I'm not looking to change pistons unless I really have to.

 

 

Sounds like a good plan.

 

As the Pistons you have are not originals, you really need to establish what brand and type they are as the Piston to Bore clearance varies because the Pistons are often made from different alloys.

 

Eg, the original 1275 Pistons had a 0.0025" Clearance, where as the type I now use only have a 0.0015" clearance. 0.001" may not sound like a lot but it's an over 30% change !

[BaronVonchesto]

 

 

thanks Moke. I think I'll take the block and pistons to a machine shop to properly measure the bore and piston diameter instead. Haynes don't specify a bore tolerance but do specify piston to bore clearance range and only state that the pistons should be renewed if the piston ring to piston groove clearance is out of spec. Think i'll stick with that as frankly I'm not looking to change pistons unless I really have to.

 

 

Sounds like a good plan.

 

As the Pistons you have are not originals, you really need to establish what brand and type they are as the Piston to Bore clearance varies because the Pistons are often made from different alloys.

 

Eg, the original 1275 Pistons had a 0.0025" Clearance, where as the type I now use only have a 0.0015" clearance. 0.001" may not sound like a lot but it's an over 30% change !

 

 

hmm how do i know if my pistons are original or not? first step is to clean off all the carbon on them anyway. AFAIK this engine has never been opened.

[Spider]

 

 

 

thanks Moke. I think I'll take the block and pistons to a machine shop to properly measure the bore and piston diameter instead. Haynes don't specify a bore tolerance but do specify piston to bore clearance range and only state that the pistons should be renewed if the piston ring to piston groove clearance is out of spec. Think i'll stick with that as frankly I'm not looking to change pistons unless I really have to.

 

 

Sounds like a good plan.

 

As the Pistons you have are not originals, you really need to establish what brand and type they are as the Piston to Bore clearance varies because the Pistons are often made from different alloys.

 

Eg, the original 1275 Pistons had a 0.0025" Clearance, where as the type I now use only have a 0.0015" clearance. 0.001" may not sound like a lot but it's an over 30% change !

 

 

hmm how do i know if my pistons are original or not? first step is to clean off all the carbon on them anyway. AFAIK this engine has never been opened.

 

 

Sorry, I've just re-read your opening post. I was of the impression that it was on +0.020" (and so overhauled at some point), but now I realise that is not the case.

 

My bad !

 

They do appear to be the original pistons.

 

 

Just also going back to the notion of flat top pistons. Even if the same CR is maintained with flat tops vs dish pistons, usually better power can be gained from the flat tops as they offer an improved squish area when at TDC. There are a few factors involved with this and it is only 'in general'.

[BaronVonchesto]

Sorry, I've just re-read your opening post. I was of the impression that it was on +0.020" (and so overhauled at some point), but now I realise that is not the case.

My bad !

They do appear to be the original pistons.

Just also going back to the notion of flat top pistons. Even if the same CR is maintained with flat tops vs dish pistons, usually better power can be gained from the flat tops as they offer an improved squish area when at TDC. There are a few factors involved with this and it is only 'in general'.

I dont think I have the budget to change pistons, cam and modify the head, not am I keen to modify the combustion chamber even if I were to mod the head.

How would I know what the new CR would be were I to move to +20 flat top pistons, and how would I know if my head can handle the increased compression.

My current CR should be 8.4 and going by minispares the flat tops would raise the CR to 10.3, a huge bump.

Edited by BaronVonchesto, 22 March 2019 - 02:58 PM.

[BaronVonchesto]

 

 

 

 

thanks Moke. I think I'll take the block and pistons to a machine shop to properly measure the bore and piston diameter instead. Haynes don't specify a bore tolerance but do specify piston to bore clearance range and only state that the pistons should be renewed if the piston ring to piston groove clearance is out of spec. Think i'll stick with that as frankly I'm not looking to change pistons unless I really have to.

 

 

Sounds like a good plan.

 

As the Pistons you have are not originals, you really need to establish what brand and type they are as the Piston to Bore clearance varies because the Pistons are often made from different alloys.

 

Eg, the original 1275 Pistons had a 0.0025" Clearance, where as the type I now use only have a 0.0015" clearance. 0.001" may not sound like a lot but it's an over 30% change !

 

 

hmm how do i know if my pistons are original or not? first step is to clean off all the carbon on them anyway. AFAIK this engine has never been opened.

 

 

Sorry, I've just re-read your opening post. I was of the impression that it was on +0.020" (and so overhauled at some point), but now I realise that is not the case.

 

My bad !

 

They do appear to be the original pistons.

 

 

Just also going back to the notion of flat top pistons. Even if the same CR is maintained with flat tops vs dish pistons, usually better power can be gained from the flat tops as they offer an improved squish area when at TDC. There are a few factors involved with this and it is only 'in general'.

 

 

So it's confirmed that i need to reboer my block as the cylinder bores are unevenly worn (not circular) and tapered towards the top 

 

i still like the idea of raising my CR by using flat top pistons but want some confirmatory advice if it would be okay or if I would likely run into problems.

 

Thank's to Moke's link, the price difference between flat top and dished pistons is gone (unlike minispares), meaning its just a question of sticking with the standard compression ratio or raising it.

 

Have decided to go with Cooperman's EVO01 cam suggestion, and am currently using an electronic distributor very similar to minispares 59D-MS version. I'd like to continue using 95 RON fuel, but am alright with 50-50 mix with 97 RON fuel as well (have been doing this all the time any way).

 

Not modifying my standard A+ head in any way for now. Unfortunately I can't accurately calculate the current CR as the engine has been fully dismantled so i cant measure the full piston travel. Though going by the haynes manual the current CR should be 8.3:1.

 

Anyone want to tell me I should stick with the dished pistons and not switch to flat tops?

Edited by BaronVonchesto, 28 March 2019 - 09:47 AM.

[DeadSquare]

If you are going to retain the standard unmodified head, go with the dished pistons because it is the simple option that will work.

 

If you intend to get your head modified quite soon, go for the flat tops and use 2 or 3 head gaskets until one of them blows and then get the head done.

[BaronVonchesto]

If you are going to retain the standard unmodified head, go with the dished pistons because it is the simple option that will work.

 

If you intend to get your head modified quite soon, go for the flat tops and use 2 or 3 head gaskets until one of them blows and then get the head done.

 

i intend to modify the head (was originally budgeted as part of this rebuild) but now that i have to rebore im temporarily shelving that.

 

I don't like the idea of stacking multiple gaskets. sounds like it'll blow at any moment. Are there thicker head gaskets available? i can't find any.

[DeadSquare]

Yes, there are.  They are made for lowering the compression when supercharging, but they are expensive.