My Gearbox Problem
Discussion
As I posted recently, I had a transmission failure whilst lying 4th overall and 1st Historic on the recent Carpetbagger Rally.
The engine and box came out yesterday and I stripped everything down.It turns out the transfer gear has stripped, the first time I've ever had this happen.,Fortunately no damage to housing, but I think a few bits of gear have gone through the box so I'm going to fit a new gear set, syncromhubs and bearings, etc, jus to be on the safe side.
Also, the engine has done 28 rallies now so I think it's time for a rebore and new pistons. The engine has Karl Schmidt pistons, but this time I think I'll go for Hepolite 21253 as theyt are 1/3rd the price and I can change them after, say, 12 evfents and still be in pocket cost-wise. Anyone happen to have the data of gudgeon pin to crown dimensions for the Schmidt and the Hepolite pistons so I can calculate what will need machining to maintain the c.r?
The engine and box came out yesterday and I stripped everything down.It turns out the transfer gear has stripped, the first time I've ever had this happen.,Fortunately no damage to housing, but I think a few bits of gear have gone through the box so I'm going to fit a new gear set, syncromhubs and bearings, etc, jus to be on the safe side.
Also, the engine has done 28 rallies now so I think it's time for a rebore and new pistons. The engine has Karl Schmidt pistons, but this time I think I'll go for Hepolite 21253 as theyt are 1/3rd the price and I can change them after, say, 12 evfents and still be in pocket cost-wise. Anyone happen to have the data of gudgeon pin to crown dimensions for the Schmidt and the Hepolite pistons so I can calculate what will need machining to maintain the c.r?
coop'
the 21253s are a fine piston (and cheap) but do have a bit of a reputation for breaking ring lands on high compression engines - though to be fair to them they are the control piston in mighty minis and seem to put up with a fair ammount of abuse!
why not consider the minispares 'mega' pistons. a better spec material than omegas and avalible in both flat and dished sizes, they arnt particually dear (220 a set) and i use them in both my engines with no problems.
as with 21253s they are avalible individually.
a few years ago a mate of mine had a screamer with scmidt pistons in it, he still talks about how good it was (and how much one piston was going to cost when it blew up!!)
the 21253s are a fine piston (and cheap) but do have a bit of a reputation for breaking ring lands on high compression engines - though to be fair to them they are the control piston in mighty minis and seem to put up with a fair ammount of abuse!
why not consider the minispares 'mega' pistons. a better spec material than omegas and avalible in both flat and dished sizes, they arnt particually dear (220 a set) and i use them in both my engines with no problems.
as with 21253s they are avalible individually.
a few years ago a mate of mine had a screamer with scmidt pistons in it, he still talks about how good it was (and how much one piston was going to cost when it blew up!!)
What annoyed me with the Schmidt was that after 15 rallies I decided to change the rings and was told I could not buy separate ring sets. In the end I use the 'Total Seal' rings, the ones with the gapless 2nd ring, and they were fantastic. However, if you pay £400 for a set of pistons, you don't expect to have to buy non-OE rings at £125 a set later. This is why I am keen on the Hepolites. The 21253 is a piston I've used in my Enduro 1400 Formula car and they seem great. My c.r on my '64 car is about 11:1, so it's not too high.
What are the dish volumes with the Mega Pistons? I need about 8 cc for the right c.r, although I guess I could open up the combustion chambers a bit more, even though my head is very de-masked already.
How do the compression heights of the Megas compare with the Hepolite 21253's?
What are the dish volumes with the Mega Pistons? I need about 8 cc for the right c.r, although I guess I could open up the combustion chambers a bit more, even though my head is very de-masked already.
How do the compression heights of the Megas compare with the Hepolite 21253's?
guru_1071 said:
6.5cc
the deck height is the same
Could you do me a favour. What's the difference in deck height between a 21251 and a 21253? I have measured a Schmidt piston and a 21251 I happen to have lying around. If I knew the difference between the two Hepolites I could work out what will be involved in changing to a 21253 or a Mega Piston. It may be that 6.5 cc is not enough as I'm also having to go out an extra .020" on the bore which will push the c.r. up even further and I think I might be looking to reduce it a bit. I don't want to change the head as mine is so well-machined and the Schmidt pistons are, I think, 6.5cc. For me I do think a 21253 at about 10.8 c.r would be fine.
I just got that data from your web site . It seems as though the 21253 sits 0.0587" higher than the 21251 and the Karl Schmidts sit 0.033" higher than a 21251. Thus I shall need to machine 0.026" off the top of the 21253 to maintain the correct deck height, maybe a bit more to compensate for the consequently smaller dish volume. I'll do the calcs.
guru_1071 said:
{puts ruler away}
I need to machine about 0.026" off rthe piston crown which will mean a reduction in the dish volume by a small amount.
AS I'm already on over 11:1 c.r I don't want it any higher. The Karl Schmidt pistons have about 6.5cc dish volume, similar to the Megas, so unless I machimne a bit more off and have the crowns sitting some way down the bores, the c.r. will be a bit too high, especailly as the bore will increase the swewpt vol and push it up a little as well. Therefore I think I'll go tfor the 21253'3 and just machine off the 0.026". Then the c.r will drop to about 10.8:1, which is fine. With the low cost of the 21253'3 I can change them after about 12 rallies and it won't cost much more than a ring change on the Megas. I don't think they'll break at up to 6500 rpm in 12 events and the quality of the Hepolite range is excellent. Trip to Potters Bar next week then!
id still go for the megas.
the 21253s have a huge slot in them to allow oil to get to the oil control ring. the megas have a much better (and stronger) set of small holes drilled.
the 21253s are also a cock hair taller than the megas and although they have a thicker crown to top ring (about 3mm) the crown is thinner than that on the mega. this of course reduces the chance to get a bit more cc out the bowl.
the 21253s have a huge slot in them to allow oil to get to the oil control ring. the megas have a much better (and stronger) set of small holes drilled.
the 21253s are also a cock hair taller than the megas and although they have a thicker crown to top ring (about 3mm) the crown is thinner than that on the mega. this of course reduces the chance to get a bit more cc out the bowl.
I think you'll find the 21253 has drilled holes in the oil control ring slot, whereas the 21250 & 21251 have slots. That's what the 21253's I just fitted in my 1990 Enduro car have. They look quite strong actually.
The big problem I have is that I don't want the c.r to 'creep' up any further. Really I want to reduce it a bit below 11:1. Using the 21253 with the 8.3 cc dish, even though I'll be machining 25 thou off the top, will still reduce the c.r from that with the Schmidt ones at 6.5cc dish. If I machine the Mega's with only about 6 cc to start with I'll be putting my c.r up to, I reckon, about 11.2 :1 which is a bit high for a rally car. I strip the engine after every 5 events or so, so I can always fit a complete new piston set then if they look at all 'iffy'.
Ideally what I need is a Mega with a 9 cc dish, but I guess I won't get one.
The big problem I have is that I don't want the c.r to 'creep' up any further. Really I want to reduce it a bit below 11:1. Using the 21253 with the 8.3 cc dish, even though I'll be machining 25 thou off the top, will still reduce the c.r from that with the Schmidt ones at 6.5cc dish. If I machine the Mega's with only about 6 cc to start with I'll be putting my c.r up to, I reckon, about 11.2 :1 which is a bit high for a rally car. I strip the engine after every 5 events or so, so I can always fit a complete new piston set then if they look at all 'iffy'.
Ideally what I need is a Mega with a 9 cc dish, but I guess I won't get one.
guru_1071 said:
newer 21253s have a slot, which whilst not as big as the one on the lesser pistons is definatley a slot (ive just opened two sets to make sure im not going mad!!!)
its gonna be one of those choices, choices!!!
They must have just changed the design quite recently then.
So the choices I have to keep the c.r. at a reasonable level for a rally car are to machine quite a lot from the top of a Mega piston to drop the crown a bit further down the bore or to use a 21253 with slots, machine just 0.025" off the top and get the c.r. right with optimum 'squish' as well. Difficult decision. Unless I could find someone holding older stocks of 21253 at +0.040". Any ideas?
I suppose I could find a Karl Schmidt dealer asnd see what they could do for me.
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